8 insects are phat but not fat - johns hopkins university ...organic- based creatures, living or...

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8 Insects Are Phat but Not Fat

Diet, Nutrition, and Food Assimilation

Throughout the animal kingdom the se lection of diet has become most highly

developed among mobile species. It has attained fi nest discrimination among parasitic

forms and, of free- living species, among insects which feed upon plants.

Dr. V. G. Dethier (1954)*

Insects are incredible eating machines. From the moment of egg hatch, juveniles raven-

ously consume food, appearing zombie- like in their incessant eating. But their feeding

is not really mindless, as insects obey neural commands, feeding to meet nutritional

needs and stopping when the need has been met. Many will not eat if the proper diet

is not available. To the casual observer, it may seem like insects eat anything and

every thing, all the time. In real ity, they can be quite selective in their dining habits, fi n-

icky even. Taste receptors are located in multiple locations on the body— feet, anten-

nae, abdomen, and mouthparts— but all are external to the mouth. That’s right— located

exposed to the outside environment. The advantages are obvious, aff ording the abil-

ity to avoid toxic foods and eat only what is needed. In other words, insects do not get

fat. What self- control! Here we dive into the world of insect nutrition, food pro cessing,

and assimilation of digested foodstuff s, making comparisons to other animals, includ-

ing humans, to showcase the incredible effi ciency displayed by insects in terms of

food acquisition, dietary intake, digestive effi ciency, and ultimate conversion of food

into body mass.

Key Concepts

d What’s on the menu? Nutrient requirements of insects

d Tools of the trade: Structures used for food collection

d Why insects don’t get fat but people do

d Eating “crap” makes sense! Food pro cessing depends on what is eaten

d It is only effi cient if you can use it: Food assimilation

What’s on the menu? Nutrient requirements of insects

What do insects eat? Of course the knee- jerk response is “Every thing!” Or, “It would

be easier to discuss what they don’t eat.” Are these fair and accurate assessments? From

a global perspective, discussing insects as a whole, there are few naturally occurring

*From “Evolution of feeding preferences in phytophagous insects,” Evolution: 8(1): 33–54.

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194 Insects

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gestion is tailored to food of par tic u lar sizes, shapes, and

chemical composition. Technically, so is ours. So why,

then, are insects considered better or more effi cient than

humans? Easy— they eat what they are supposed to. In

other words, insects respond to neural signals that sig-

nify specifi c nutritional needs, they seek out such foods,

taste them before ingestion, and then consume them if

the appropriate sensory feedback is received (fi gure 8.1).

This means that consumption occurs only, at least in the-

ory, if the nutritional need will be met by the discovered

food. Once the need is met, the hunger drive is turned

off , and in turn, the insect no longer forages. There are

exceptions to the feedback loop, such as honey bees for-

aging for the entire colony, but for individual insects, the

digestive system is driven by neural centers of hunger

and satiety. Gluttony, or eating to excess, is absent from

the world of insects. For clear contrast, take the cele-

bration of the Thanksgiving holiday in North Amer i ca.

In the battle for planetary supremacy between mankind

and insect, the clear winner in the category of self- control

goes to creatures with six legs. We will further explore

the topic of neuroregulation of nutrient intake later in the

chapter, but fi rst we turn our attention to the nutrients

needed by insects.

As a starting point, the basic question that can be

asked is, do insects have diff er ent nutritional require-

ments than most other animals, including us? The

logic behind this question stems from at least two ob-

servations. The fi rst is that, because insects outnumber

all other animals by such a wide margin, it would seem

reasonable to assume that perhaps they are able to eat

foods that others cannot, thereby acquiring unique nu-

trients. The second observation is one we just discussed:

insects do not overeat. Consequently, we can rule out

the notion that insects simply out- consume, pound- per-

pound, other animals. That’s not to say they don’t eat a

lot, because indeed they do. But again, overeating or glut-

tony just simply does not happen with insects. So the

idea that insects need dif fer ent or unique nutrients is

a logical premise. But in fact, insects require the same

basic nutrients that we do, though not always in the

same amount (usually less) or necessarily used in the same

ways that other animals use them. Some nutriment needs

are unique among members of the class Insecta, but not

enough so to account for their extreme evolutionary

organic- based creatures, living or dead, terrestrial or

aquatic, that are not consumed by something with six

legs. For one million species to not just survive but also

thrive, the dietary menu must be quite large. So yes, in-

sects appear to eat every thing. However, individual spe-

cies of insects have relatively selective diets. To argue

that they are fi nicky eaters is perhaps not accurate, but it

is true that for many species, dietary intake is limited.

This can be attributed to the morphology of the mouth-

parts, taste receptor selectivity, and design of the gut

tube itself. The net eff ect is that specifi c insects gener-

ally feed on specifi c types of food.

For example, the majority of insects “restrict” their di-

ets to vegetative or plant matter and are classifi ed as

phytophagous. Those that consume animals are termed

carnivorous. Several species may be grouped as true par-

asites (of either plants or animals) or the as the more

specialized parasitoids. Some insects are fl uid- feeders and

specialize on liquid diets derived from another animal or

a plant. And still others feed on decaying organic matter

originating from any type of organism. These insects

may be termed detritivorous or saprophagous, the latter

referred to as necrophagous when feeding on dead ani-

mals exclusively. The point is that diff er ent insect species

tend to feed on specifi c types or kinds of foods, and as

we will soon see, the design of the digestive system re-

fl ects the food type.

Regardless of how each species is classifi ed in terms

of feeding or food types, insects share several unifying

features of nutrition and digestion, all linked to their in-

credible effi ciency.

1. Finding food (sensing and foraging)

2. Manipulating and consuming food (ingestion with

specialized mouthparts)

3. Breaking down complex food into useable forms

(chemical and mechanical digestion)

4. Absorbing nutrients and converting them to body

mass (food assimilation)

What makes insects so good at these pro cesses? So

much of the effi ciency of the class Insecta relates to the

simple idea of form meeting function. Morphologically

speaking, the design of the insect digestive system— from

mouth to anus—is adapted for eating, pro cessing, and us-

ing specifi c types of food. This literally means that di-


Insects Are Phat but Not Fat 195

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insect hunger drive is activated in response to nutritional

needs, foraging and subsequent ingestion generally leads

them to foods that contain the essential nutrients. Many

of the needed nutrients can be synthesized from precur-

sors found in ingested foodstuff s or derived from break-

down products. Such nutrients are called nonessential

nutrients. Here is where insects stray from the path of

many other animals: what is essential or nonessential

does diff er substantially from the human condition. The

basic nutrient classes, however, are the same, regardless

of whether a backbone is pres ent or not, or how many

legs an animal possesses.

Nutrient classes

1. Amino acids

2. Carbohydrates

3. Lipids

4. Vitamins

5. Inorganic compounds

success story. What will become clear as we explore

insect- food interactions is that insects are incredibly ef-

fi cient at all phases of digestion. Insects eat for need,

show extraordinary effi ciency in breaking down food

into useable forms, and then make excellent use of what

they have consumed. However, it all begins with eating

the right foods in the fi rst place. Sound familiar?

Nutritional needs

Insects as a whole are chemically quite similar to each

other in terms of the composition of their tissues and

bodily fl uids (intra- and extracellular). Not surprisingly,

then, the nutrients needed to maintain homeostasis

are also very similar among diff er ent insect species.

Although some species do indeed display very unique

diets, as a whole, the basic nutritional requirements of

insects inhabit common ground. As with most animals,

certain nutrients must be obtained in the diet of an in-

sect, and those are termed essential nutrients. Since the

Food source

IngestionDigestion

+Absorption

Hungerinhibited

Satietyactivated

(----)

Hungeractivated

Satietyinhibited

Foragingcontinues

taste receptors do notdetect needed nutrient

taste receptorsdetect needed nutrient

Food isrejected

Figure 8.1. Negative feedback loop associated with insect feeding.


196 Insects

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to producing amino acids in this fashion is that the

biochemical reactions are typically energy demanding

and can generate byproducts that require detoxifi cation

(which requires energy) or immediate excretion from the

body. The amount of each type of amino acid needed by

insects varies greatly between species and developmen-

tal stages.

Carbohydrates

Most insects do not have an absolute carbohydrate re-

quirement in their diet yet do use simple sugars or

monosaccharides as the primary cellular energy source.

The above statement might seem to confl ict with itself,

but actually it does not, for at least two reasons. First, a

par tic u lar macromolecule consumed for energetic needs

is often not considered an essential nutrient. Classify-

ing a macromolecule as a nutrient versus energy source

comes down to deciding whether the consumed macro-

molecule simply sustains life (an energy source) or

promotes growth and development (a nutrient). Second,

carbohydrates are commonly synthesized from amino

acids or fats found in food and therefore fall into the cat-

egory of nonessential nutrients. This does not diminish

the importance of ingested or synthesized carbohydrates:

several monosaccharides function as primary cellular

fuel; some have structural roles, particularly in the syn-

thesis of chitin; and many can serve as the structural

foundation to produce amino acids and fats. As with pro-

teins, the digestibility of complex carbohydrates dictates

the utility of these macromolecules in meeting nutritional

needs. Enzymatic hydrolysis of polysaccharides and di-

saccharides generally liberates monosaccharides that

are either absorbed or converted to other macromole-

cules. Insects do not directly digest large polymers pro-

duced by plants, such as starch and cellulose, since they

lack the necessary digestive enzymes. In these instances,

microorganisms residing in the digestive tract of phy-

tophagous insects function as symbionts by synthesiz-

ing the appropriate enzymes and thus benefi ting both the

insect host and the microorganisms.

Social insects, particularly colonizing bees, represent

unique exceptions to the dietary rules of carbohydrates

in that they do have an absolute need for these macro-

molecules. Colonies demonstrate a division of labor

based on reproductive status (i.e., queen versus workers)

Now, before we dive into the nutrients needed by an

insect, let’s address one question that may be in the back

of your mind: How do you know what insects actually

need nutritionally? It is not as if you can get them to com-

plete a survey indicating likes, dislikes, and needs versus

wants. Three common approaches for assessing dietary

needs of insects and other organisms are (1) the deletion

method, in which a specifi c component of a chemically

defi ned diet is removed and the eff ects of that on growth

and development are mea sured; (2) substitution method,

in which an analog of a specifi c nutrient is used to re-

place the nutrient in question; and (3) radiolabeled track-

ing, which relies on tagging precursor molecules with

radioactive labels so that biosynthesis using the precur-

sors can be mea sured. All three methods essentially re-

quire being able to raise the insect in question in the

laboratory on an artifi cial diet. Not only is this very ex-

pensive, but it is also labor intensive. Consequently, only

a fraction of all insects have been tested by any of these

methods. The discussion of insect nutrition that follows

is based on a limited number of species, so undoubtedly

many exceptions exist.

Amino acids

Amino acids are derived from protein digestion so that

they can be used in turn to synthesize new proteins. In-

sects, like other animals, need to produce proteins for

structural purposes, to serve as enzymes, for functional

roles in membranes, and to circulate in hemolymph as

transport proteins. Specifi c amino acids have roles in de-

pen dent of protein synthesis, including as energy sources

(e.g., proline for fl ight), cryo- or stress protectants (ala-

nine and proline), during sclerotization (tyrosine), as eye

pigments (tryptophan), or to function as neurotransmit-

ters (glutamate or γ- aminobutyric acid) ( table 8.1). Gluta-

mate takes on a special role in helping to synthesize other

amino acids through a series of transamination reactions.

Generally most amino acids are obtained from dietary

protein, and the ability of an insect to meet an amino acid

need is dependent on the digestibility of these proteins.

Insects also use several nonessential amino acids, and

though they are not required (i.e., essential), they can fa-

cilitate more rapid rates of growth and development.

Nonessential amino acids are commonly synthesized

from essential amino acids in the fat body. The downside



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For several insect groups, polyunsaturated fatty acids

are essential dietary requirements. The nutritional re-

quirement is generally met by consumption of linoleic

or linolenic acids (fi gure 8.2). In the case of Diptera, the

vast majority of species examined have no growth de pen-

dency on polyunsaturated fatty acids, yet all mosquitoes

( family Culicidae) do. For some other orders (e.g., Lepi-

doptera and Hymenoptera), the fatty acid need is stage-

specifi c: completion of adult development depends on

linolenic acid, but a similar requirement is absent during

larval stages. Some cockroach species (order Blattodea)

require dietary polyunsaturated fatty acids for reproduc-

tive success, specifi cally needing the lipids to produce

normal ootheca, the protective coating surrounding de-

veloping embryos (fi gure 8.3).

Nearly all other types of fatty acids and phospholipids

needed by insects for constructing membranes or other

structures can be synthesized from precursors in the diet.

Therefore these lipid molecules are nonessential nutrients.

The major exceptions are certain fat- soluble vitamins,

discussed in the next section. It is also impor tant to note

that some insects demonstrate an entire lifestyle change—

into that of a parasite—in which dietary lipids are ob-

tained exclusively from the host and usually in a form that

allows the parasitic insect to immediately assimilate the

fats directly into its own biomass. In fact the lipid profi le of

the parasitic insect shows an incredibly high homology—

greater than 90% in some cases— with the host.

Vitamins

Vitamins are a chemical hodgepodge, in that vitamin

molecules share little in common structurally other than

being organic substances. Most are needed only in small

quantities, and each is typically considered essential

for growth and development. The general view is that

several vitamins must be obtained in dietary sources

because insects cannot synthesize them. But for some

insects, symbiotic microorganisms living within the

digestive tract synthesize the required vitamins. Tech-

nically for these latter insects, the vitamins are essen-

tial but not obtained directly from the food.

Vitamins are classifi ed based on their solubility prop-

erties: fat- soluble or water- soluble.

Fat- soluble vitamins. Carotenoids are the primary

fat- soluble vitamin required by all insects. Perhaps most

and also an age- based division of labor, all determined

by the diet of young larvae. Larvae fed nutrient- rich,

carbohydrate- laden royal jelly become queens, whereas

larvae fed a less rich diet become workers. Newly

emerged worker bees are “assigned” a variety of tasks in-

side the hive including tending eggs and larvae and hon-

eycomb construction and food handling, while older

workers forage for nectar and pollen. The point is that

dietary carbohydrate, and not ge ne tic diff erence, has a

profound infl uence on caste determination and behav-

ioral patterns in some social insects.

Lipids (fats)

Insects diff er substantially in their dietary requirements

for lipids. To a degree, the nutrient needs align with tax-

onomic relationships and feeding be hav ior. All insect

orders studied to date display an absolute need for ste-

rols (ste roids), which has led to a broad generalization for

all insects. Cholesterol is the most impor tant dietary ste-

rol needed by insects. Most species can synthesize other

sterols from a cholesterol backbone, including the major

molting hormones, ecdysteroids (discussed in chapter 6).

Carnivorous, omnivorous, and hematophagous (blood-

feeding) species generally can obtain cholesterol in their

diets, whereas phytophagous insects cannot and must

convert plant- specifi c sterols into a useable form.

Essential Amino Acids

ColeopteraArginineHistidineIsoleucineLeucineLysine

MethioninePhenylalanine

ThreonineTryptophan

Valine

BlattodeaArginineHistidineIsoleucineLeucineLysine

ThreonineTryptophan

Valine

Hemiptera

HistidineIsoleucine

LysineMethionine

Table 8.1. Comparison of essential amino acids in three orders of insects. Note that while insects require the same basic essential amino acids as most other animals, all Insects do not have the same requirements. Data from Chapman (1998).


198 Insects

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and ele ments, in par tic u lar sodium, potassium, cal-

cium, chloride, phosphate, and magnesium. The pre-

cise amounts needed by diff er ent insects vary greatly

among species and also between diff er ent developmen-

tal stages. Iron, zinc, and manganese are considered es-

sential for all insects and must be obtained from dietary

or microorganism sources. Several other inorganic com-

pounds are known to dramatically accelerate the rate of

growth of many freshwater species. This is especially

evident in the larval stages of several mosquito species,

in which the size and shape of anal papillae are modu-

lated by availability of chloride, potassium, and sodium.

Beyond the text

Why is cholesterol not directly available in the diet of

phytophagous insects?

Tools of the trade: Structures used for food collection

The primary tools for collecting and ingesting food are

the mouthparts. As we discussed in chapter 5, insect

mouthparts are quite varied. In fact, the structure of the

mouthparts directly refl ects dietary tendencies, particu-

larly in terms of the physical form of the food. Mouth-

part type, and hence diet, is also correlated with insect

groups or specifi c orders. For example, liquid diets de-

rived from plants are the norm for moths and butterfl ies

(order Lepidoptera), and consequently adults of this or-

der rely on siphoning mouthparts to partake of scrump-

tious carbohydrate- rich off erings from plants. In contrast,

the larval forms of moths and butterfl ies are typically

voracious plant feeders, utilizing mandibles with sharp

and fl at surfaces to chew and grind vegetative materials.

Similarly, nearly all feeding stages of beetles (order Cole-

optera) depend on mandibles for chewing, whether on

animal or plant diets, a combination of the two, or dedi-

cated to dining as saprophagous feeders. Beetles using

mandibular mouthparts consume a wide range of foods.

In fact, the majority of insect species and groups use

mandibles for food acquisition and manipulation. Is there

any special reason(s) for this trend? Well, actually, yes,

there is! For one, mandibular mouthparts are quite adapt-

able, off ering utility in obtaining, pro cessing, and even

tasting many types of food diff ering in physical compo-

common among these is β- carotene or pro- vitamin A,

which is used by insects in the formation of eye pig-

ments and also in pigment deposition of the integument.

Only microorganisms and plants synthesize these com-

pounds. The non- carotenoid vitamin E appears to be es-

sential to the reproductive success of some, but not all,

insects and enhances the fecundity of several species of

insects.

Water- soluble vitamins. At least seven water- soluble vi-

tamins are essential for all insects. These include biotin,

thiamine, ribofl avin, folic acid, pyridoxine, nicotinic

acid, and pantothenic acid. In essence, the class Insecta

requires all members of the vitamin B complex.

Water- soluble vitamins function as cofactors for numer-

ous enzymes and also have structural roles.

Inorganic compounds

In the older lit er a ture, “salts” refers to nutrients critical

for certain insect species, in par tic u lar aquatic insects.

Practically, salts refer to several inorganic compounds

Figure 8.3. The ootheca of a hissing cockroach. Photo by Whit-ney Cranshaw available at http:// bit . ly / 1iBZqpx.

CH3

OH

O

12

9

1

Figure 8.2. Chemical structure of linoleic acid. Image by Ju available at http:// bit . ly / 1FqfR2Q.



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(mandibles and maxillae) and antennae, with each pow-

ered by the vestiges of the ancestral ventral appendage’s

muscles, meaning leg muscles. Unlike humans or other

animals, when an insect eats, the mouthparts com-

mand the force or strength of legs, not of jaw! This may

provide a new perspective as to why the bite of a tiny ant

hurts so much when applied to the tender fl esh of a

human foot or leg.

Adding to the versatility of the mandibular mouthpart

is the complex yet effi cient design of the accompanying

structures. A pair of large, relatively fl at accessory jaws

(maxillae) is pres ent that allows the insect to manipulate

food, along with leg- like palps that occur in pairs on max-

illae as well as on the lower lip, called the labium (fi g-

ure 8.5). The palps are also covered by thousands of taste

receptors, as are the mandibles and maxillae, permitting

insects the ability to “taste it before they eat it.” The pres-

ence of lips (labrum, or front lip, and labium, or rear

lip) is a remarkable evolutionary advancement in it-

self, as these structures allow insects to close their

mouths once food is ingested (fi gure 8.6). By doing so,

ingested food does not roll back out of the mouth, and a

sition and structure. More impor tant, though, mandibles

are the ancestral mouthpart type of the class Insecta, the

original mouthpart type that all others are derived from.

So whether an insect crunches on a leaf, laps up nectar

from a fl ower, or sucks blood from the butt of a host, the

structure used is modifi ed from the chewing mandible

type (fi gure 8.4).

The way mandibles evolved— really, the entire head,

for that matter— accounts for much of the remarkable

effi ciency that insects display in food acquisition and

manipulation. The short story goes something like this:

Insects evolved from a wormlike ancestor that was seg-

mented and lacked a true head but did have a simple

acron with an opening anteriorly located for food to en-

ter and possessed several ventral appendages that aided

in locomotion. The fi rst six trunk segments are believed

to have fused with the acron to form a complex head re-

sembling that of modern- day insects, particularly that of

a cockroach (order Blattodea) or grasshopper (order Or-

thoptera). Im por tant in this evolutionary transition is

that the pair of ventral appendages located on each trunk

segment became head structures, including mouthparts

Figure 8.4. Photo of a house cricket head with the mandibles on display. Photo by USGS Native Bee Inventory and Monitoring Laboratory available at http:// bit . ly / 1OYJGtx.


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Mandibles are good, in fact very good, in functions as-

sociated with collecting and pro cessing several types of

foods used by insects. But for some species, mandibles

are not the right tools for the job. Mandibles are well

suited for solid foods, whether derived from plants or

animals. However, for those species that feed exclusively

on liquid or semiliquid diets, the mandibular design

needed to be adapted into something more functional.

The evolved design for liquid feeding occurs in modern

insects in three basic forms: as siphoning (order Lepidop-

tera), piercing- sucking (Hemiptera, some Diptera), and

sponging mouthparts (most Diptera). Why so many

mouthpart adaptions? Simple— liquid diets are not all the

same. Such diets diff er in source, composition, and vis-

cosity, necessitating structures that maximize acqui-

sition. Generally some sort of muscular pump is also

required to promote intake, and as we will see later in

this chapter, gut morphology for the liquid diet is also

distinct from that used for pro cessing solid diets. The

take- home message is that all insects do not eat the same

foods, and their mouthparts refl ect this diversity in diet.

Bug bytes

Butterfl ies feeding

https:// www . youtube . com / watch ? v = MYWPWTme _ YI

Why insects don’t get fat but people do

Have you ever seen a fat insect? I mean fat! In other

words, one that is overweight or obese. If being totally

honest, the answer should be an unequivocal no. Insects

in nature do not overeat, and hence do not get fat. This

is not to say that it is impossible for them to become

overweight. It is theoretically pos si ble under unnatural

conditions. For example, in the laboratory it has been

demonstrated that if neural regulatory mechanisms that

control food intake are inhibited or severed, hyperpha-

gia or elevated appetite is induced, which may lead to a

state of overeating. So a fat insect could occur, but only

if the ventral nerve cord or recurrent nerve* is ablated

or cut in front of or behind the brain. Either situation

*The recurrent nerve links the frontal ganglion (FG) of the stomatogastric ner vous system with the hypoce re bral ganglion, which lies posterior to the FG.

unique internal environment can be maintained that

facilitates the initiation of chemical digestion. Keep

in mind that most animals do not have lips! Within this

internal space is a tongue or glossa (or hypoglossa) that

does function somewhat similarly to that of humans:

it is covered with thousands of taste receptors and

can also be used to lap up liquids. Mandibles and the

associated mouthpart structures work very well under

most situations, which is why so many insect species

still use them.

Figure 8.5. Cartoon of a grasshopper’s mouthparts (lr = labrum, md = mandibles, mx = maxillae, hp = hypoglossa, and lb = labium). Image by Heds available at http:// bit . ly / 1KX5OTl.



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mastered? Do they possess unique mechanisms of self-

control that other animals lack? Most research examin-

ing insect nutrition, specifi cally those aspects associated

with hunger and satiety, suggests that the mechanisms

employed by insects are quite similar to vertebrate ani-

mals. Broadly speaking, diet is regulated throughout the

insect digestive system and can be observed from the

time of food ingestion, through food movement from

one location to another (gut motility), to the release of di-

gestive enzymes (in terms of both type and amount), and

to the pro cesses of absorption and assimilation. However,

when discussing overeating, we tend to direct our atten-

tion to food intake. After all, the secret is eating right in

the fi rst place. So how do insects achieve this goal?

For one, unlike us, insects do not eat for plea sure.

Rather, insects eat for need; insects eat when a nutritional

need arises. How does an insect perceive this? Theoreti-

cally the same way we do, or should: via activation of

the hunger drive, leading to the sensation of hunger. In

humans, neural centers located in the hypothalamus

serve to create the hunger drive after pro cessing sensory

information sent from throughout the body. An oppos-

ing neural center, the satiety center, is also located there,

to essentially turn off the hunger drive. You have per-

sonal experience in knowing that these signals can be

ignored. How many times have you been at a restaurant

and ordered dessert even though you are completely

stuff ed? I know, that choco late- caramel- covered cheese-

cake was calling you, so what choice did you have? You

prob ably should even be commended because you ate it

is likely encountered in nature only if a predator con-

sumes an insect’s head, a condition that can’t commonly

be corrected nor one in which obesity would any longer

be of much concern.

Domestication can have this eff ect as well— that is, the

overeating, not the beheading. Certain species of insects,

as with pet dogs and cats, can become conditioned to eat-

ing whenever and what ever is supplied by the human

caregiver. They can eat foods that they never would in

the wild and likely have a surplus of food available 24/7.

Several years ago in my General Entomology class, one

of my students raised an adult female Madagascar hiss-

ing cockroach, Gromphadorhina portentosa, exclusively on

water and Ritz Bits peanut butter crackers. After twelve

weeks, that girl was fat (the cockroach)! Her abdomen

was an almost perfectly round cylinder, her legs barely

touched the ground since the abdomen was nearly as

wide as the legs were long, and she made no attempt to

run during the annual Madagascar Madness races that

highlight the last day of class. She was overweight and

Jim (her caregiver) was depressed (Dr. Phil* was not a

tele vi sion icon back then, so there was no appropriate

counseling available for the unhappy couple).

Under natu ral conditions, however, insects do not

overeat. How do they pull off what humans have never

*Dr. Phil is an American tele vi sion show starring Dr. Phillip McGraw, who provides psychological counseling to guests. To date, he has yet to provide counseling to any distressed insect, and it remains to be determined if this is simply a blatant disregard for patients with more than two legs.

Figure 8.6. Maxillary and labial palps on display on this adult beetle. Photo by Siga available at http:// bit . ly / 1R92Mwc.


202 Insects

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However, for simplicity, we will still rely on the termi-

nology of taste receptors, since our focus here is on nu-

trition and regulation of food intake. The basic feature

that stands out is that hundreds to thousands of taste re-

ceptors are located outside the mouth, so any insect can

taste its food before ingestion (fi gure 8.7). This permits

detection of food composition so that an assessment of

whether the nutritional need will be met can be made,

and also whether the food item is potentially toxic. Sen-

sory feedback from any of the taste receptors can inhibit

the hunger center located in the frontal ganglion, activate

satiety neurons, and/or result in hunger center neurons

remaining active (fi gure 8.8).

Chemical modulation. Chemical regulation of hunger or

satiety can result from both endogenous and exogenous

chemical signals. Several biogenic amines, neuropep-

tides, and hormones have been identifi ed from an array

of insects that are capable of stimulating or inhibiting the

hunger drive or satiety (fi gure 8.9). Biogenic amines, in-

cluding octopamine, target octopaminergic receptors

to modulate the insect hunger drive, particularly with

to prevent someone else from doing so, who, no doubt

would have faced an embarrassing pants explosion right

in front of every one. Thank you for your good deeds! In-

sects do not face such dilemmas.

In the world of insects, their sleek, trim fi gures are

maintained through the same type of hunger and satiety

controls. Only, they obey the messages! Basically, hun-

ger and satiety are regulated through a series of intercon-

nected mechanisms:

1. Taste receptors

2. Chemical modulation

3. Neural mechanisms

Taste receptors. As we have discussed earlier, taste re-

ceptors are located in multiple locations on the insect

body, including on the vari ous mouthpart structures, on

tarsi, on antennae, and on the abdomen. In insects, it is

more correct to refer to such receptors as contact chemo-

receptors, in that foods can be tasted via sensing chemi-

cal signals in liquid or dry form, and the same receptors

may detect chemicals in de pen dent of feeding activities.

Figure 8.7. Scanning electron micrograph of the head of a pyralid moth (Lepidoptera). The coiled siphon mouthparts are covered with taste receptors, or chemoreceptors. Image by Svdmolen available at http:// bit . ly / 1G9lRYE.



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matophagous species; once a threshold “stretch” is

reached, satiety neurons are activated and the hunger

drive suppressed. Several studies suggest that a neuro-

modulator, termed cholecystokinin- like peptide, is re-

leased once the threshold is achieved, which in turn

binds to neurons in the stomatogastric ner vous system

to trigger the onset of satiety. For most insects studied,

stretch receptors positioned along the foregut modulate

long- term feeding requirements, while those associ-

ated with the abdomen provide short- term regulation

(fi gure 8.10). Nerves from the central ner vous system

and stomatogastric ner vous system also innervate the

gut tube and mouthparts. Consequently, sensory feed-

back from taste receptors, associated muscles, and those

monitoring the internal gut environment contribute to

modulating appetite and fullness.

Just how eff ective are these dietary control mea sures?

Several studies have shown that if insects are not off ered

the right food, they will not eat. Starvation and death will

result, rather than insects eating nonnutritious food.

Some rely on complex foraging be hav iors before even

making an attempt to taste a potential food source. For

any insect species, obeying the neural and chemical sig-

nals from the hunger and satiety neurons results in food

intake that meets specifi c nutritional needs. Under nor-

mal or natu ral conditions, insects cannot ignore these

signals, and consequently, they do not overeat or get fat.

This is almost unimaginable for me, the proud owner of

two bichon frise* dogs that proudly eat animal excrement

out of the yard despite being well fed, with one showing

all the morphological characteristics of a cracker- fed

cockroach or a taxidermied sheep (fi gure 8.11).

Eating “crap” makes sense! Food pro cessing depends on what is eaten

For an insect to take advantage of its ability to eat the

right foods, it must have the capacity to release the valu-

able nutrients trapped inside ingested foods, through the

pro cess of digestion. Digestion relies on physical manip-

ulation or breakdown of foodstuff s, termed mechanical

digestion or mastication, and on chemical digestion, a

*Bichon frise is a breed of lap dogs reputed to be quite intelligent. In my experience, however, they rank just below freshwater sponges and slightly above river rocks.

regard to meeting protein needs. In contrast, compounds

like sulfakinins and corticotropin- releasing factor- related

diuretic hormone regulate the satiety neurons. In the

case of sulfakinins and related kinin- like compounds, the

signaling molecules are released from insulin- producing

cells in the frontal ganglion and then bind specifi cally to

G- protein coupled receptors in brain neurons to induce

satiety. Neuropeptide F and short neuropeptide F both

appear to stimulate appetite in some insects. Several

other neuropeptides and hormones indirectly infl uence

appetite and satiety by regulating muscle movement

along the entire length of the gut tube, modulating diges-

tive enzyme release and controlling the rate of crop

emptying.

Neural mechanisms. Neural regulation of hunger and sa-

tiety relies on stretch receptors positioned critically

along the digestive tract. For example, crop emptying

stimulates activation of hunger in several insect species,

generally to meet a carbohydrate need. Stretch receptors

along the midgut infl uence feeding be hav ior of many he-

Figure 8.8. Light micrograph of an antenna of the vinegar fl y, Drosophila melanogaster. The antenna is covered with a forest of chemoreceptors used for smelling (olfaction) food. Photo by AlphaPsi available at http:// bit . ly / 22gdre8.


204 Insects

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food that does not make sense to you, but it prob ably

does to the insect; other wise, it cannot extract nutrients

from what is eaten. This means that those fl y maggots

that you may have seen dining on cow manure actually

know what they are doing; their gut tubes were designed

to, literally, eat crap. We explore these ideas in more de-

tail by examining topics focused on the design of the in-

sect digestive tract and the pro cesses by which chemical

digestion occurs primarily in the midgut, fl uid fl ow in the

ecto- and endoperitrophic spaces facilitates chemical di-

gestion, and microbial symbionts aid digestive pro cesses.

Insect digestive tract

As mentioned, the insect digestive tract is designed as

a tube. The tube is portioned into three sections, each

evolved for a specialized function: the foregut or stomo-

deum, the midgut or mesenteron, and the hindgut or

proctodeum. Both the foregut and hindgut are derived

from ectoderm and also lined with cuticle. Yes, the same

cuticle discussed in chapter 6 as a major component of

the insect exoskeleton. Cuticle, regardless of location, is

shed during molting. So the lining of two- thirds of the

gut tube is ripped out with each molt and replaced with

pro cess that relies on enzymatic digestion and the chem-

ical environment (i.e., pH and ions) in which the food is

contained. Believe it or not, these are hallmarks of so-

phisticated digestive pro cesses, and also typify animals

that possess a gut tube. The insect gut tube is open at two

ends (mouth and anus), permitting one- way fl ow of food.

It is also designed for specialized functions to be carried

out in diff er ent locations, thereby permitting multiple di-

gestive events to occur si mul ta neously. While these

may sound like unique adaptations, there are not. Com-

partmentalization of digestive functions is the norm with

vertebrate animals and is also observed in the higher in-

vertebrate phyla. With that said, however, some aspects

of chemical and mechanical digestion are truly insect-

owned, which will become apparent as we delve further

into the topic of digestive functions.

The pro cess of digestion is a mechanism that also en-

sures insects eat the right foods. This can be seen through

the design of the gut tube, which has evolved to pro cess

foods of specifi c chemical and physical composition.

The same can be said for the types and amounts of diges-

tive enzymes produced by a given species; they match the

food that should be eaten. An insect may be consuming

O

F

HO

O

HN

OH

NH2

FOH

NH2

OH

NH2HO

O

NH2

ONH2

HN

O

HN OH

F HN

O

OH

F

HN

NH2

O

HO

NH2

OHNH2

OHNH2

OH

F

F NH 2

4-fluoro

4-fluoro

4-fluoro

3-fluoro

3-fluoro

p-methylp-

met

hyl

beta-ketone

beta-ketone

3-fluoro

p-methyl be

ta-k

eton

e

p-m

ethy

l+b-

keto

ne

p-methyl+b-ketone

p-methyl+b-ketone

phen

ethylamine alpha-methyl N,alpha-dimethyl

OH

HO

NH2

Figure 8.9. Octopamine and several of its analogues that are capable of modulating the insect hunger center. Image by Ezekiel.golan available at http:// bit . ly / 1YHDZ7z.


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Protein

Chemical

modulation

Neural

regulation

Octopaminergic

receptorsHunger

activation

biogenic amines increases

Foregut

stretch

receptorssatiety

activated

long-term

short-term

Abdominal

stretch

receptors

Figure 8.10. Relationship between chemical modulation and neural regulation of protein consumption.

Madagascarhissing

cockroach

—eats for need

TaxidermiedSheep

—domesticated,eats what is

fed

Bichon Frise

—appears to eatanything andeverything,

24/7

Figure 8.11. Feeding relationship among three animals of questionable intelligence (questionable actually only for two). Photo of cockroach by Alambes (http:// bit . ly / 1LdYk0h), Dolly the sheep by Toni Barros (http:// bit . ly / 1FA9nOO), and Bichon Frise by Aka-porn Bhothisuwan (http:// bit . ly / 1OYLFhe).


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the young caterpillar is a codling

moth, Cydia pomonella (Lepidoptera:

Tortricidae). However, this implica-

tion is quickly dropped when we

learn that, on day two (Tuesday), our

young six- legged adventurer

partakes of two fully mature pears,

then three plums (Wednesday),

followed by strawberries (Thurs-

day), and on Friday consumes fi ve

oranges. Even if we were to believe

that the hunger center was driving

this young caterpillar to eat such a

diverse range of fruits, it is highly

unlikely that the gut tube design or

digestive enzymes produced by one

species of moth could accommodate

this unique fi ve- day smorgasbord, let

alone the fact that these plants do

not occur in the same geo graph i cal

regions nor is the fruit of each ripe

during the same growing seasons.

Let’s say for the sake of argument,

however, that this is pos si ble—

which it isn’t— because as scientists,

open- mindedness is a must. Our

young caterpillar would have

consumed a diet capable of support-

ing the development of nearly one

hundred individuals, assuming that

each reaches an average weight of

150–200 milligrams prior to becom-

ing a pupa. What does this mean? It

should be obvious that Mr. Carle has

implied that the young caterpillar

practices gluttony, which we know

insects simply do not do under

normal conditions. And, since poor

parenting is in fact the norm in the

world of insects, we know it isn’t

that. Our trust of Mr. Carle is

fl eeting, and all compassion for the

hungry caterpillar is completely lost,

as Saturday’s gustatory barbarity is

fi nally unveiled. The young caterpil-

lar is purported to consume a piece

of choco late cake, one ice cream

cone, a pickle (presumed dill), a slice

of cheese and a slice of salami, one

lollipop (apparently just lying

unwrapped next to the salami), a

piece of cherry pie, one sausage (a

carnivorous caterpillar to boot!), one

cupcake, and one slice of water-

melon. Utter dietary chaos.

Saturday’s menu has too many

issues to address them all, so let’s go

straight for the entomological

jugular: to consume this gluttonous

meal the hungry caterpillar would

have had to ignore its taste recep-

tors, the signals from its hunger and

satiety centers, and the neural

feedback from stretch receptors.

None of these scenarios would make

any sense. Insects eat for need, not

for plea sure. There is no empirical

evidence that any insects have a

need for a piece of cake or an ice

cream cone, or anything else on

Saturday’s menu, with the pos si ble

exception of watermelon. Of course,

no explanation is provided for how

this caterpillar came upon just a slice

of each food item; no doubt it

simply cut off the slices itself,

despite lacking opposable thumbs

(insert sarcastic face here). The truth

of The Very Hungry Caterpillar is that

the book is fi lled with impossibili-

ties. Undoubtedly, countless

numbers of children have been

fraught with uncertainties about the

dining preferences of caterpillars

throughout their lives, and have not

had the good fortune to seek

counseling from a professional

entomologist to alleviate the

intellectual suff ering. For future

generations, we can only hope that

children ignore the inaccurate

portrayals of feeding caterpillars,

and consult entomological texts at a

very early age!

iThe Very Hungry

Caterpillar is a liar!

The classic tale The Very Hungry

Caterpillar by Eric Carle has prob ably

been read to nearly every

elementary- school- age child in the

United States. The quaint yarn

follows the ontogeny of an un-

named caterpillar from egg hatch to

adult emergence. Most of the story

focuses on the voracious appetite of

the young caterpillar. We are told

that on Sunday, the neonate

caterpillar hatches from the egg

with the rising of the sun, a very

plausible scenario considering that

for many insects, egg hatch as well

as adult emergence from the pupal

stage are circadian events timed

with daybreak. From this point on,

however, the biology of The Very

Hungry Caterpillar, specifi cally the

description of feeding be hav ior,

raises all kinds of red fl ags. For

instance, the mother apparently did

not lay her eggs (I am presuming

mom laid a clutch and not just a

single egg, yet there is no mention

of siblings; maybe they’re dead) on a

host plant suitable for her young to

eat, as the poor juvenile does not

fi nd food until Monday. Few newly

hatched insects could not survive

for even a few hours without food,

let alone a day, and the requirement

for a journey from the site of egg

hatch to an apple tree would surely

be a death sentence in almost all

cases. Mr. Carle depicts Monday’s

feast as a mature red apple, suggest-

ing a late summer to early fall

season. This implies that perhaps



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for the bulk of chemical digestion. Some insect

species regurgitate portions of the food, principally

water, as a means to concentrate the food material to

maximize enzymatic digestion.

Midgut. The basic design of the midgut, or mesenteron,

is a thin tube lined by a single layer of epithelial

cells. Extensive microvilli extend from the surface of

the cells toward the lumen of the midgut, serving as

sites for digestive enzyme release and also providing

enormous surface area for absorption of released

nutrients. The area between the surfaces of the

epithelial cells to the tips of the microvilli is charac-

terized by a pH gradient, which facilitates compart-

mentalized chemical digestion under diff ering

conditions contained within microenvironments in

the midgut. The anterior end of the midgut displays

fi n ger- like projections called gastric cecae, which

functionally produce digestive enzymes and peri-

trophic membrane and represent maximized surface

area for nutrient absorption. The entire length of the

midgut is lined with a noncellular peritrophic

membrane. This structure functions as a frictionless

surface for food movement and protects the delicate

epithelial cells from potentially abrasive food

molecules. Posteriorly, the mesenteron terminates

at a muscular pyloric valve.

Hindgut. A cuticular lining protects the hindgut, or

proctodeum, but does not prevent absorption of

a new one. That just sounds painful! The midgut is pro-

duced from endodermal tissue and does not possess a

cuticular lining. Instead, epithelial cells synthesize a

protective membranous tube (peritrophic membrane)

that fi lls the inside of the midgut. The functions and as-

sociated structures are discussed for each gut region

separately (fi gure 8.12).

Foregut. The fi rst region of the gut tube, the foregut or

stomodeum, is composed of the mouth, pharynx,

esophagus, and crop. The region terminates

posteriorly with a muscular valve, the proventricu-

lus, which often possesses a series of cuticular

teeth- like structures called dendricles. Contraction

of the proventricular muscles can then drive the

dendricles into the food emptying from the crop,

resulting in partial mechanical digestion of solid

food and cells. The mandibles and maxillae actually

begin the pro cess of mastication. Salivary glands

(usually paired) reside at the anterior end of the

foregut and release saliva that functions to lubricate

mouthparts, and moisten food so that ingested

materials form a bolus, and contains a few digestive

enzymes that initiate chemical digestion. Enzymatic

digestion targets chiefl y carbohydrates, but some

protein digestion occurs as well. The salivary en-

zymes work primarily in the crop, as ingested food is

retained in this region until emptied into the midgut

Figure 8.12. Internal anatomy of an adult insect digestive system. Image by Bugboy52.40 available at http:// bit . ly / 1KFzzU1.


208 Insects

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Chemical digestion occurs primarily in the midgut

As with the design of the gut tube, the type of digestive

enzymes found in any given insect refl ects the chemical

composition of the primary food source. Most insects

must digest the major nutrient macromolecules, meaning

protein, carbohydrates, and lipids. Therefore, the same

basic types of digestive enzymes are pres ent in all in-

sects. What diff ers is the exact form of the foodstuff s

ingested. Insects with a solid diet of either plant or ani-

mal material will ingest predominantly proteins of dif-

fer ent sizes, polysaccharides, and lipids in the form of

glycerides, phospholipids, and glycolipids. By contrast,

fl uid- feeders benefi t from tapping into the circulating

f luids of their hosts, meaning that the dirty work of

digestion has already occurred, so much less invest-

ment in chemical digestion is needed. When digestion

does occur, it happens primarily in the midgut, regard-

less of the feeding habits of the insects in question. Epi-

thelial cells lining the midgut synthesize the bulk of

the digestive enzymes, although some are produced

by the salivary glands and travel with the food bolus to

the midgut. In most instances, the enzymes are produced

in response to food arrival into the mesenteron. The

value of this approach to individual insects is that en-

zymes do not accumulate over time and are synthesized

in response to the chemical composition of the food.

Released digestive enzymes must cross the peritrophic

membrane to reach the food bolus. Continued digestion

occurs outside of the peritrophic membrane, in the ec-

toperitrophic space, including along the surface of the

epithelial cells.

What types of digestive enzymes do insects produce?

For the most part, the same types that we make. There

are subtle diff erences, such as molecular sizes, precise

target sites on substrates, and of course enzyme names,

but the basic enzyme types are very similar to ours. For

instance, proteases are synthesized to digest proteins,

and they tend to fall into two categories, endopeptidases

and exopeptidases. They can of course be further char-

acterized, but knowing the names of all the enzymes an

insect uses really does not help much to understand what

they eat and how they use food to meet their nutritional

needs.

water, ions, amino acids, and other solutes. The

hindgut tube is functionally separated into the

anterior ileum, followed by the rectum, which

terminates as the anus, an opening to the outside of

the body. A series of long, thin tubelike projections

called Malpighian tubules extend from the anterior

portion of the ileum, fl oating freely in the hemo-

lymph or becoming embedded in tissues throughout

the hemocoel. The hindgut and associated struc-

tures take on numerous roles, ranging from diges-

tion and absorption of nutrients to ion and water

balance to removal of undigested food materials

(defecation) and metabolic wastes (excretion) to serv-

ing as host to a wide range of microorganisms that

aid in chemical digestion and synthesis of valuable

nutrients.

As with so many aspects of the insect condition,

there are exceptions to every rule and, in this case, to

the basic design of the gut tube. For example, some

species, particularly those that feed on a liquid diet,

lack a distinct crop and may instead possess blind

end pouches, or diverticuli, as extensions from the

esophagus. Liquids like nectar or blood, even plant

resins, can be stored in the pouches until food is

moved by muscular contractions (peristalsis) into

the midgut. Other species may have either Malpi-

ghian tubules (cryptonephridial arrangement of

Lepidoptera) or portions of the hindgut (fi lter

chamber of Hemiptera) in intimate association with

the midgut as a means to quickly and effi ciently

remove water from a liquid meal so that the solutes

are concentrated before enzyme secretion. The

hindgut is modifi ed in some insects to facilitate

storage of undigested food material (paunch of

Isoptera) and/or to promote fermentation (fermenta-

tion chamber of Coleoptera). More subtle anatomical

adaptations include the absence of salivary glands in

some fl uid- feeders, elaborate pumps in the pharynx

of species that “suck,” and elongate microvilli in the

rectum of insects that absorb water through the

anus. Now, that last statement deserves a moment of

pause. . . . Adaptation to maximize digestion is the

norm in insects, and what has been described here is

just the basic design that all other designs have

evolved from.



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Fluid fl ow in the ecto- and endoperitrophic spaces facilitates chemical digestion

The presence of the peritrophic membrane in theory

poses a potential prob lem to several aspects of digestion

and absorption. The prob lem can be summed up suc-

cinctly as diff usion. Diff usion is the mechanism that ac-

counts for digestive enzymes making contact with

solutes— that is, the food particles to be digested. It is the

same mechanism used for liberated nutrients to cross the

peritrophic membrane into the ectoperitrophic space and

then migrate to the epithelial cell surfaces for absorption.

In an environment in which the fl uids are not stirred or

in motion, the concentrations of enzymes and substrates

are the most impor tant factors infl uencing the rate of dif-

fusion, followed next by temperature.

Since insects are poikilothermic, the midgut temper-

ature refl ects ambient air or water conditions, which are

ordinarily not predictable and therefore not dependable

for increasing diff usional fl ux. What ever is an insect to

do, then, to help itself overcome the barrier of the peri-

trophic membrane? Amazingly, the solution is to stir the

gut fl uids. Any agitation, no matter how slight, of an

unstirred fl uid (or air) can greatly increase the rate of

diff usion. Though there are likely several mechanisms

employed by insects, one characterized for some Diptera

With that said, it is worth mentioning that a few in-

sects, namely some species of Diptera, possess cathepsin

D- like proteinase, which is functionally similar to verte-

brate pepsin but structurally nearly identical to cathep-

sin D. Pepsin- like enzymes have been found only in

animals with an incredibly acidic stomach (midgut) to

facilitate consumption of foods laden with microorgan-

isms. In turn, carbohydrases are used to digest polysac-

charides to liberate monosaccharides, the nutrient form

that is absorbed. Several types of carbohydrases are pro-

duced by insects; the exact type depends on the source

of the carbohydrate, that is, whether plant or animal. In

some cases the primary polysaccharide is cellulose; in-

sects do not directly synthesize cellulase, the enzyme

needed for digestion of this large polymer and rely in-

stead on endosymbiotic microorganisms to produce the

necessary enzyme. Far less is known about the lipid-

digesting enzymes in insects, largely because the di-

etary need for fat is miniscule in comparison to protein

and carbohydrate. Lipases or esterases are the broad class

of enzymes that target lipids. Most dietary lipid is in the

form of neutral lipids, principally triacylglycerol. Midgut

lipases release monoacylglycerol and fatty acids through

the hydrolysis of triacylglycerol. However, in phytopha-

gous insects with a highly alkaline gut pH, the digestion

products are more apt to be free glycerol and fatty acids.

Food bolus

Midgut

Ectoperitrophic space

Endoperitrophic space

ProventriculusPyloricvalve

Figure 8.13. Gut fl uid current in the endo- and ectoperitrophic spaces of the midgut. Illustration based on Terra (1988).


210 Insects

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zymes to digest or producing a nutrient needed by the

host. In turn, the insect provides habitation and nutrients

needed by the microorganism. The relationship is endo-

symbiotic, as both members of the association benefi t.

Generally, insects with simple straight gut tubes contain

very few microorganisms, largely because their diet is

not complex and therefore fairly straightforward to di-

gest and then absorb. Insects with a complex, highly con-

voluted gut tube are more apt to harbor a wide range of

microorganisms, and often rely on endosymbiotic organ-

isms to fulfi ll certain aspects of digestion and nutrition

(fi gure 8.14).

The types of endosymbionts residing in insects can be

broadly classifi ed as those that live in the digestive tract

in or along the lumen, known as extracellular endosym-

bionts, and those that physically reside within cells of the

digestive tract, termed intracellular endosymbionts. Re-

gardless of classifi cation, most of these organisms exist

in the hindgut, most likely to avoid the milieu of diges-

tive enzymes pres ent in the midgut and to benefi t from

predigested food materials moving into their environ-

ment. Extracellular symbionts in the form of bacteria

and protozoa are known to release digestive enzymes

that break down plant polysaccharides like cellulose, pec-

tins, and gums, which the insect host is not capable of

doing itself. Some also fi x atmospheric nitrogen, which

the insect, in turn, can directly assimilate into its own tis-

sues. Termites actually harbor protozoa that host bacte-

ria at the cell surface that cooperatively ingest and digest

cellulose for the benefi t of all three organisms. The roles

of intracellular endosymbionts are quite varied, with

some functioning to convert stored uric acid into amino

acids or to recycle nitrogen pres ent in excretory prod-

ucts, others transforming nonessential amino acids into

essential, and still others synthesizing vitamins that the

insect host cannot acquire in its diet. In most cases, the

endosymbionts are transferred from one generation to

the next, often in specialized cells called mycetocytes.

It is only effi cient if you can use it: Food assimilation

Insects demonstrate extraordinary regulation over what

foodstuff s are consumed and then exceptional effi ciency

in digesting what has been ingested. Of course, none of

these accolades matter if the highly sought after nutrients

and Orthoptera involves midgut cells in the posterior

portion of the midgut releasing water into the ectoperi-

trophic space. The fl uid fl ows anteriorly, to be absorbed

by gastric cecae and then returned to the hemolymph. A

unidirectional fl ow could be deleterious, in that food and

nutrients would accumulate anteriorly, yielding limited

absorption and an issue with removal of undigested and

nonabsorbed food particles. Never fear, there is a solu-

tion. Cells located adjacent to the gastric cecae are also

able to release water into the endoperitrophic space,

creating a countercurrent to the one fl owing in the ante-

rior direction (fi gure 8.13). The eff ect is that the fl uids are

stirred on both sides of the peritrophic membrane, pro-

moting enhanced rates of diff usion resulting in greater

opportunities for enzymes to interact with food sub-

strates, and, in turn, increased chance for liberated nutri-

ents to contact epithelial surfaces for absorption to occur.

Exchanges also occur between the endo- and ecto-

peritrophic spaces, creating solute concentration shifts

across the peritrophic membrane in both directions.

Water will follow solute concentration shifts, inducing

further movement of gut fl uids. The solution, countercur-

rent fl ow in the midgut, is reminiscent of the same type

of mechanism in the human kidney (loop of Henle).

There, as in the insect midgut, water and solutes are ab-

sorbed along the entire length of exposed epithelial cells.

Absorption of solutes further facilitates uptake of water,

augmenting, though quite subtly, the fl uid fl ow created

by water secretion by midgut cells. It is quite likely that

in some insects, the absorption of water and ions by Mal-

pighian tubules accounts for all or some of the posterior

movement of midgut fl uid.

Quick check

Where does initiation of carbohydrate digestion begin

in an insect’s digestive tract?

Microbial symbionts aid digestive pro cesses

Though it is sometimes diffi cult to admit (okay, maybe

only for me), insects cannot always do it alone. “It,” in

this case, being digestion. Aid commonly comes in the

form of microorganisms that take up residence within

the gut tube of a host insect and take on the role of either

digesting foodstuff that the insect lacks the necessary en-



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diff usion, and as we discussed earlier, the mechanism for

diff usion is concentration diff erences. Generally, absorp-

tion by passive transport relies on the solute in question

moving down a concentration gradient: that is, from a re-

gion of higher concentration (the midgut lumen) to a

region of lower concentration (intracellular environ-

ment). Obviously, under normal conditions, the cellular

membrane serves as barrier to solutes moving freely

back and forth, so a channel, gate, or transport receptor

is required to allow a nutrient entry into the cell via dif-

fusion. When operating properly, nutrients absorbed by

passive transport move for free, meaning cellular en-

ergy is not required (fi gure 8.15).

Movement of solutes against a concentration or elec-

trochemical gradient requires energy or active mecha-

nisms for transport. It appears that ATP is the energy

currency used by most insects to drive pumps (vacuolar

type ATPase) located in apical membranes ( those facing

the lumen) of midgut epithelial cells to transport protons

into the lumen in exchange for potassium and possibly

calcium. Macromolecules in the gut must bind to mem-

brane or transport proteins to be “pulled” into the cell

against a concentration gradient. Likewise, ions moving

never reach their targets. That is to say, highly effi cient

digestion is pointless if the nutrients are not absorbed

with the same type of effi ciency. Likewise, the absorbed

molecules only become nutriment if they reach the tis-

sues in need. In other words, the nutrients liberated by

digestion cross the epithelial cells to enter the hemo-

lymph, so that they eventually reach target tissues to

become assimilated. The absorbed nutrients are used

(assimilated) in cellular functions such as metabolism

and the assembly of molecules or structures, or as fuels.

Effi ciency in food assimilation has two components:

nutrient absorption by midgut cells and uptake and us-

age (assimilation) by cells and tissues throughout the

body.

Absorption

Food is digested in the midgut to free or liberate nutri-

ents that can be absorbed predominantly by midgut cells.

This is a testament to the versatility of epithelial cells of

the midgut, as many of these same cells produce diges-

tive enzymes and function in creating the countercurrent

fl uid fl ow. Two main mechanisms drive absorption in in-

sects: passive and active transport. Passive transport is

Figure 8.14. The endosymbiotic bacterium Buchnera aphidicola resides in aphid hosts within specialized cells (bacteriocytes) like the one shown here. Photo by J. White and N. Moran available at http:// bit . ly / 1FBK9PY.


212 Insects

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across the cells into the hemolymph to fat body. Inor-

ganic molecules are more varied, as potassium and water

move into midgut cells via passive transport, whereas

chloride, sodium, and calcium move against electro-

chemical gradients by active transport. All these inor-

ganic substances, with the exception of water, are also

absorbed across the ileum and rectum of the hindgut,

and the mechanisms alternate between active and pas-

sive transport depending on the concentration and

electrochemical gradients encountered. Water movement

is always passive and occurs throughout the hindgut,

including along the entire length of the Malpighian tu-

bules.

Assimilation

Absorption is not the fi nal step for macromolecules, for

to truly be a nutrient, the substance must be used by a

cell or tissue in need. Absorbed molecules circulate via

hemolymph and move across target cells by active and

passive mechanisms. Utilization of the nutrients occurs

against an electrochemical gradient are pumped into the

intracellular environment at the expense of ATP.

Most critical nutrients released in the gut are absorbed

via passive transport. For instance, amino acids are usu-

ally in higher concentration within the gut lumen than

in the intracellular environment of epithelial cells. Mem-

brane proteins or symports facilitate the movement by

fi rst binding to specifi c amino acids, and then undergo-

ing a conformational change that transports the amino

acid into the cell. Carbohydrates also move by diff usion,

and this transport is greatly enhanced by rapid conver-

sion of glucose into trehalose within the fat body. The net

eff ect is that a concentration gradient is maintained from

gut to hemolymph, since the concentration of free glu-

cose in the hemocoel is homeostatically low. Lipids are

somewhat diff er ent than the other macromolecules in

that they generally cross into epithelial cells by passive

diff usion, often get modifi ed within the cells, and then

require active transport to be exchanged into the hemo-

lymph. The exceptions are sterols, which move freely

Figure 8.15. Passive transport, or diff usion, is the primary mechanism used to absorb nutrients across the midgut epithelial cells. Image by BruceBlaus available at http:// bit . ly / 1YIPuf1.



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matic digestion as well as the rate of absorption. As long

as the maggot temperatures are not deleterious to fl y de-

velopment and survival, the rate of food assimilation in-

creases with maggot mass temperatures. As a result,

assimilation efficiencies typically exceed 75% and for

some species, approach 90%. These values far surpass

other saprophagous insects, highlighting the impor-

tance of self- produced temperatures in enhancing diges-

tive effi ciency of insects.

chapter review

d What’s on the menu? Nutrient requirements

of insects

■ What do insects eat? Insects generally feed on

specifi c types of food. The majority of insects

“restrict” their diets to vegetative or plant matter

and are classifi ed as phytophagous. Those that

consume animals are termed carnivorous. Several

species may be grouped as true parasites or as the

more specialized parasitoids. Some insects are

fl uid- feeders and specialize on liquid diets derived

from another animal or a plant. Still others feed

on decaying organic matter originating from any

type of organism and are called detritivores, or

saprophagous.

■ Regardless of how each species is classifi ed in

terms of feeding or food types, insects share

several unifying features of nutrition and diges-

tion. Among the most signifi cant are that insects

are among the most effi cient of all animals with

re spect to fi nding food, manipulating and

consuming food, breaking down complex food

into useable forms, and absorbing nutrients and

converting them to body mass. What makes

insects so good at these pro cesses? So much of

the effi ciency of the class Insecta relates to the

simple idea of form meeting function. Morpho-

logically speaking, the design of the insect

digestive system— from mouth to anus—is adapted

for eating, pro cessing, and using specifi c types of

food.

■ Insects as a whole are chemically quite similar to

each other in terms of the composition of their

tissues and bodily fl uids. Not surprisingly, then,

the nutrients needed to maintain homeostasis are

through metabolic reactions and energy use, or in the

synthesis of other molecules, membranes, or other cel-

lular components. This fi nal destination is assimilation,

which can be mea sured by the following equation:

Assimilation effi ciency =

Rate of digestion × Food retention time

Concentration of food × Midgut volume

The true or impor tant effi ciency of the digestive sys-

tem is mea sured by whether the food ingested is incor-

porated into cellular uses. After all, it really means little

overall that an insect eats the right foods if the nutrients

never reach the audience (the cells) in need. With that in

mind, just how good are insects in assimilating food? The

answer depends on what kinds of feeders they are— that

is, what types of foods are primarily consumed (e.g.,

phytophagous, carnivorous, etc.). Overall, most insects

demonstrate assimilation effi ciencies at or just below

50%, placing them much higher than most vertebrate ani-

mals but nowhere near the highest effi ciencies known

to be achieved. Well, just who in the animal world is most

effi cient? The answer may surprise you: insects. Now

things have really gotten confusing! Certain insect spe-

cies have adapted lifestyles that permit them to display

extraordinary food assimilation capacities. These insects

are parasitic on other invertebrates as well as vertebrate

animals. Assimilation is maximized because the parasites

obtain concentrated, predigested food materials from the

host, which requires very short pro cessing time in the

insect’s gut before it can be absorbed and assimilated.

Consequently, food assimilation effi ciencies can exceed

90% under these conditions.

Do parasitic insects have any rivals in the animal

world for the title of kings of assimilation? Actually, yes

they do. Other parasitic invertebrates display similar

prowess in food assimilation. The other insect contend-

ers are fl y maggots that feed necrophagously on carrion

or dead animals. Larvae from the families Calliphoridae

and Sarcophagidae form large feeding aggregations or

maggot masses as they consume an animal carcass.

Flies in the maggot masses generate heat that can elevate

the internal temperatures of the aggregations to more

than 30° to 50°C above ambient conditions. Remember

that temperature is an impor tant factor infl uencing the

rate of diff usion, and in this case, it can accelerate enzy-


214 Insects

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and lacked a true head but did have a simple acron

with an opening anteriorly located for food to

enter and possessed several ventral appendages

that aided in locomotion. The fi rst six trunk

segments are believed to have fused with the

acron to form a complex head. Im por tant in this

evolutionary transition is that the pair of ventral

appendages located on each trunk segment

became head structures, including mouthparts

and antennae, with each powered by the vestiges

of the ancestral ventral appendage’s muscles,

meaning leg muscles.

■ Adding to the versatility of the mandibular

mouthpart is the complex yet effi cient design of

the accompanying structures. A pair of large,

relatively fl at accessory jaws is pres ent that

allows the insect to manipulate food, along with

leg- like palps that occur in pairs on maxillae as

well as on the lower lip, the labrum. The pres-

ence of lips (labrum, or front lip, and labium, or

rear lip) is a remarkable evolutionary advance-

ment in itself, as these structures allow insects

to close their mouths once food is ingested.

By doing so, ingested food does not roll back

out of the mouth, and a unique internal environ-

ment can be maintained that facilitates the

initiation of chemical digestion. Keep in mind

that most animals do not have lips! Within this

internal space is a tongue or glossa (or hypo-

glossa) that does function somewhat similarly to

that of humans: it is covered with thousands of

taste receptors and can also be used to lap up

liquids.

■ Mandibles are well suited for solid foods, whether

derived from plants or animals. However, for

those species that feed exclusively on liquid or

semiliquid diets, the mandibular design needed to

be adapted into something more functional. The

evolved design for liquid feeding occurs in

modern insects in three basic forms: as siphon-

ing, piercing- sucking, and sponging mouthparts.

Why so many mouthpart adaptions?

Simple— liquid diets are not all the same. Gener-

ally some sort of muscular pump is also required

to promote intake. The take- home message is that

also very similar among diff er ent insect species.

Although some species do indeed display very

unique diets, as a whole, the basic nutritional

requirements of insects inhabit common ground.

As with most animals, certain nutrients must be

obtained in the diet of an insect, and those are

termed essential. Since the insect hunger drive

is activated in response to nutritional needs,

foraging and subsequent ingestion generally leads

them to foods that contain the essential nutrients.

Many of the needed nutrients can be synthesized

from precursors found in ingested foodstuff s or

derived from breakdown products. Such nutrients

are called nonessential.

■ The basic nutrients needed by insects are the

same as those needed by other animals, includ-

ing us, and fall into the categories of amino acids,

carbohydrates, lipids, vitamins, and inorganic

compounds.

d Tools of the trade: Structures used for food

collection

■ The primary tools for collecting and ingesting

food are the mouthparts. As we discussed in

chapter 5, insect mouthparts are quite varied. In

fact, the structure of the mouthparts directly

refl ects dietary tendencies, particularly in terms

of the physical form of the food. Mouthpart type,

and hence diet, is also correlated with insect

groups or specifi c orders. The majority of insect

species and groups use mandibles for food acquisi-

tion and manipulation. Is there any special

reason(s) for this trend? Well, actually, yes, there

is! For one, mandibular mouthparts are quite

adaptable, off ering utility in obtaining, pro-

cessing, and even tasting many types of food

diff ering in physical composition and structure.

More impor tant, though, mandibles are the

ancestral mouthpart type of the class Insecta,

the original mouthpart type that all others are

derived from.

■ The way mandibles evolved— really, the entire

head, for that matter— accounts for much of the

remarkable effi ciency that insects display in food

acquisition and manipulation. Insects evolved

from a wormlike ancestor that was segmented



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digestion or mastication, and on chemical diges-

tion, a pro cess that relies on enzymatic digestion

and the chemical environment (i.e., pH and ions)

in which the food is contained. The insect gut

tube is open at two ends (mouth and anus),

permitting one- way fl ow of food. It is also

designed for specialized functions to be carried

out in diff er ent locations, thereby permitting

multiple digestive events to occur si mul ta neously.

■ The tube is portioned into three sections, each

evolved for a specialized function: the foregut or

stomodeum, the midgut or mesenteron, and the

hindgut or proctodeum. Both the foregut and

hindgut are derived from ectoderm and also lined

with cuticle. Yes, the same cuticle discussed in

chapter 6 as a major component of the insect

exoskeleton. Cuticle, regardless of location, is

shed during molting. So the lining of two- thirds

of the gut tube is ripped out with each molt and

replaced with a new one. The midgut is produced

from endodermal tissue and does not possess a

cuticular lining. Instead, epithelial cells synthe-

size a protective membranous tube (peritrophic

membrane) that fi lls the inside of the midgut.

■ The type of digestive enzymes found in any given

insect refl ects the chemical composition of the

primary food source. Most insects must digest

the major nutrient macromolecules, meaning

protein, carbohydrates, and lipids. Therefore the

same basic types of digestive enzymes are

pres ent in all insects. What diff ers is the exact

form of the foodstuff s ingested. Insects with a

solid diet of either plant or animal material will

ingest predominantly proteins of diff er ent sizes,

polysaccharides, and lipids in the form of glycer-

ides, phospholipids, and glycolipids. By contrast,

fl uid- feeders benefi t from tapping into the

circulating fl uids of their hosts, meaning that the

dirty work of digestion has already occurred, so

much less investment in chemical digestion is

needed. When digestion does occur, it happens

primarily in the midgut, regardless of the feeding

habits of the insects in question. Epithelial cells

lining the midgut synthesize the bulk of the

digestive enzymes, although some are produced

all insects do not eat the same foods, and their

mouthparts refl ect this diversity in diet.

d Why insects don’t get fat but people do

■ Under natu ral conditions insects do not overeat.

How do they pull off what humans have never

mastered? Do they possess unique mechanisms

of self- control that other animals lack? Most

research examining insect nutrition, specifi cally

those aspects associated with hunger and satiety,

suggests that the mechanisms employed by insects

are quite similar to vertebrate animals. Broadly

speaking, diet is regulated throughout the insect

digestive system and can be observed from the

time of food ingestion, through food movement

from one location to another, to the release of

digestive enzymes, and to the pro cesses of

absorption and assimilation.

■ In the world of insects, their sleek, trim fi gures

are maintained through hunger and satiety

controls. Basically hunger and satiety are

regulated through a series of interconnected

mechanisms, which include feedback from taste

receptors, chemical modulation, and neural

mechanisms.

■ Just how eff ective are these dietary control

mea sures? Several studies have shown that if

insects are not off ered the right food, they will

not eat. Starvation and death will result, rather

than insects eating nonnutritious food. Some

rely on complex foraging be hav iors before even

making an attempt to taste a potential food

source. For any insect species, obeying the neural

and chemical signals from the hunger and satiety

neurons results in food intake that meets specifi c

nutritional needs. Under normal or natu ral

conditions, insects cannot ignore these signals

and, consequently, they do not overeat or get fat.

d Eating “crap” makes sense! Food pro cessing depends

on what is eaten

■ For an insect to take advantage of its ability to

eat the right foods, it must have the capacity to

release the valuable nutrients trapped inside

ingested foods, through the pro cess of digestion.

Digestion relies on physical manipulation or

breakdown of foodstuff s, termed mechanical


216 Insects

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exceptional effi ciency in digesting what has been

ingested. Of course, none of these accolades

matter if the highly sought after nutrients never

reach their targets. The nutrients liberated by

digestion cross the epithelial cells to enter the

hemolymph, so that they eventually reach target

tissues to become assimilated. The absorbed

nutrients are used (assimilated) in cellular

functions such as metabolism and the assembly of

molecules or structures, or as fuels. Effi ciency in

food assimilation has two components: nutrient

absorption by midgut cells and uptake and usage

(assimilation) by cells and tissues throughout the

body.

■ Two main mechanisms drive absorption in

insects: passive and active transport. Passive

transport is diff usion, and as we discussed earlier,

the mechanism for diff usion is concentration

diff erences. Generally, absorption by passive

transport relies on the solute in question moving

down a concentration gradient: that is, a from a

region of higher concentration (the midgut

lumen) to a region of lower concentration (intra-

cellular environment). When operating properly,

nutrients absorbed by passive transport move for

free, meaning cellular energy is not required.

■ Movement of solutes against a concentration or

electrochemical gradient requires energy or active

mechanisms for transport. It appears that ATP is

the energy currency used by most insects to drive

pumps (vacuolar type ATPase) located in apical

membranes ( those facing the lumen) of midgut

epithelial cells.

■ Most critical nutrients released in the gut are

absorbed via passive transport. For instance,

amino acids are usually in higher concentration

within the gut lumen than in the intracellular

environment of epithelial cells.

■ Absorption is not the fi nal step for macromol-

ecules, for to truly be a nutrient, the substance

must be used by a cell or tissue in need. Absorbed

molecules circulate via hemolymph and move

across target cells by active and passive mecha-

nisms. Utilization of the nutrients occurs through

metabolic reactions, as energy use, or in the

by the salivary glands and travel with the food

bolus to the midgut.

■ What types of digestive enzymes do insects

produce? For the most part, the same types that we

make. There are subtle diff erences, such as mole-

cular sizes, precise target sites on substrates, and of

course enzyme names, but the basic enzyme types

are very similar to those produced by humans.

■ The presence of the peritrophic membrane in

theory poses a potential prob lem to several

aspects of digestion and absorption. The prob-

lem can be summed up succinctly as diff usion.

Diff usion is the mechanism that accounts for

digestive enzymes making contact with solutes—

that is, the food particles to be digested. It is the

same mechanism used for liberated nutrients to

cross the peritrophic membrane into the ectoperi-

trophic space and then migrate to the epithelial

cell surfaces for absorption. What ever is an insect

to do, then, to help itself overcome the barrier of

the peritrophic membrane? Amazingly, the

solution is to stir the gut fl uids.

■ Insects sometimes need help with digesting

certain types of foods or in acquiring specifi c

nutrients. Aid commonly comes in the form of

microorganisms that take up residence within the

gut tube of a host insect and take on the role of

either digesting foodstuff that the insect lacks the

necessary enzymes to digest or producing a

nutrient needed by the host. In turn, the insect

provides habitation and nutrients needed by the

microorganism. The relationship is endosymbi-

otic, as both members of the association benefi t.

Generally, insects with simple straight gut tubes

contain very few microorganisms, largely

because their diet is not complex and therefore

fairly straightforward to digest and then absorb.

Insects with a complex, highly convoluted gut

tube are more apt to harbor a wide range of

microorganisms, and often rely on endosymbiotic

organisms to fulfi ll certain aspects of digestion

and nutrition.

d It is only effi cient if you can use it: Food assimilation

■ Insects demonstrate extraordinary regulation

over what foodstuff s are consumed and then



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4. Explain the advantages a gut tube provides an insect in

terms of digestion.

Level 3: Synthesis/Evaluation1. Speculate on how an insect can increase food assimila-

tion effi ciency under natu ral conditions.

2. Speculate on the state of the hunger and satiety centers

in a caterpillar that has initiated apolysis.

3. If insects only eat what is good for then, meaning they

obey the hunger and satiety centers, then explain why

certain species, like the Indianmeal moth Plodia inter-

punctella, can be found infesting candy bars.

references

Canavoso, L. E., Z. E. Jouni, K. J. Karnas, J. E. Pennington,

and M. A. Wells. 2001. Fat metabolism in insects. Annual

Review of Nutrition 21:23–46.

Carle, E. 1969. The Very Hungry Caterpillar. Scholastic, Inc.,

New York, NY.

Chapman, R. F. 1998. The Insects: Structure and Function.

4th ed. Cambridge University Press, Cambridge, UK.

Dethier, V. G. 1954. Evolution of feeding preferences in

phytophagous insects. Evolution: 8(1): 33–54.

Downer, K. E., A. T. Haselton, R. J. Nachman, and J. G.

Stoff olano Jr. 2007. Insect satiety: Sulfakinin localization

and the eff ect of drosulfakinin on protein and carbohy-

drate ingestion in the blow fl y, Phormia regina (Diptera:

Calliphoridae). Journal of Insect Physiology 53:106–112.

Fraenkel, G., and M. Blewett. 1943. The vitamin B- complex

requirements of several insects. Biochemical Journal

37:686–692.

Karasov, W. H., and C. Martínez del Rio. 2007. Physiological

Ecol ogy. Prince ton University Press, Prince ton, NJ.

Klowden, M. J. 2013. Physiological Systems in Insects. 3rd

ed. Academic Press, San Diego, CA.

Lehane, M., and P. Billingsley. 1996. Biology of the Insect

Midgut. Academic Press, San Diego, CA.

Nation, J. L. 2008. Insect Physiology and Biochemistry. 2nd

ed. CRC Press, Boca Raton, FL.

Rahmathulla, V. K., and H. M. Suresh. 2012. Seasonal

variation in food consumption, assimilation, and

conversion effi ciency of Indian bivoltine hybrid silk-

worm, Bombyx mori. Journal of Insect Science 12:82.

Available online at insectscience . org / 12 . 82.

Reynolds, S. E., and S. F. Nottingham. 1985. Eff ect of

temperature on growth and effi ciency of food utilization

synthesis of other molecules, membranes, or

other cellular components.

■ The true or impor tant effi ciency of the digestive

system is mea sured by whether the food ingested

is incorporated into cellular uses. After all, it

really means little overall that an insect eats the

right foods if the nutrients never reach the

audience (the cells) in need. With that in mind,

just how good are insects in assimilating food?

The answer depends on what kinds of feeders

they are— that is, what types of foods are primar-

ily consumed. Overall, most insects demonstrate

assimilation effi ciencies at or just below 50%,

placing them much higher than most vertebrate

animals, but nowhere near the highest effi cien-

cies known to be achieved.

mushroom farming (self- test)

Level 1: Knowledge/Comprehension1. Defi ne the following terms:

(a) assimilation (d) essential nutrient

(b) maxillae (e) hyperphagia

(c) countercurrent fl ow (f ) satiety

2. Describe the basic design of the insect digestive tract.

3. What is the function of the peritrophic membrane?

4. Explain how midgut fl uids fl ow during digestion and

absorption.

5. What structures are involved in tasting food?

6. Describe the mechanisms used for absorption of (a) amino

acids, (b) fatty acids, and (c) monosaccharides.

Level 2: Application/Analy sis1. Explain how insects regulate food intake to avoid

overeating.

2. Make a diagram of an insect head, say, that of a cock-

roach, including the basic mouthparts.

3. Diagram the path that a protein follows through the gut

tube, identifying the regions in which chemical digestion

occurs.


218 Insects

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Parasites & Vectors 4:105. http:// www . parasitesandvectors

. com / content / 4 / 1 / 105.

Kunieda, T., T. Fujiyuki, R. Kucharski, S. Foret, S. A. Ament,

A. L. Toth, K. Ohashi, et al. 2006. Carbohydrate meta-

bolism genes and pathways in insects: Insights from the

honey bee genome. Insect Molecular Biology 15:563–576.

Nässel, D. R., and M. J. Williams. 2014. Cholecystokinin-

like peptide (DSK) in Drosophila, not only for satiety

signaling. Frontiers in Endocrinology 5:219. doi:10.3389/

fendo.2014.00219.

Panizzi, A. R., and J. R. Parra. 2012. Insect Bioecol ogy and

Nutrition for Integrated Pest Management (Con-

temporary Topics in Entomology). CRC Press, Boca

Raton, FL.

Sapolsky, R. M. 1998. Why Zebras Don’t Get Ulcers.

W.H. Freeman Com pany, New York, NY.

Terra, W. R. 1990. Evolution of digestive systems of insects.

Annual Review of Entomology 35:181–200.

Waldbauer, G. 1996. Insects through the Seasons. Harvard

University Press, Cambridge, MA.

Wigglesworth, V. B. 1982. The Princi ples of Insect

Physiology. 7th ed. Springer, London, UK.

additional resources

Alien Empire

http:// www . pbs . org / wnet / nature / alien - empire - introduction

/ 3409/

Insect adaptation to plant defenses

http:// www . reeis . usda . gov / web / crisprojectpages / 0186708

- insect - adaptation - to - plant - defense - decryption - of - gut

- protease - deployment - and - development - of - synthetic

- resistance . html

Insect digestive and excretory systems

http:// www . cals . ncsu . edu / course / ent425 / tutorial / digest

. html

International Centre for Insect Physiology and Ecol ogy

http:// icipe . org/

Spider digestion and food storage

http:// www . atshq . org / articles / Digestion . pdf

Termite digestion and biofuels

http:// www . purdue . edu / newsroom / releases / 2013 / Q1

/ understanding - termite - digestion - could - help - biofuels,

- insect - control . html

in fi fth instar caterpillar of tobacco hornworm, Manduca

sexta. Journal of Insect Physiology 31:129–134.

Scriber, J. M., and F. Slansky. 1981. The nutritional ecol ogy

of immature insects. Annual Review of Entomology

26:183–211.

Simpson, S. J., F. J. Clissold, M. Lihoreau, F. Ponton, S. M.

Wilder, and D. Raubenheimer. 2015. Recent advances in

the integrative nutrition of arthropods. Annual Review

of Entomology 60:16.1–16.19.

Slansky, F., Jr. 1982. Insect nutrition: An adaptationist’s

perspective. Florida Entomologist 65:45–71.

Spit, J., L. Badisco, H. Verlinden, P. Van Wielendaele, S. Zels,

S. Dillen, and J. Vanden Broeck. 2012. Peptidergic control

of food intake and digestion in insects. Canadian Journal

of Zoology 90:489–506.

Stoff olano, J. G., Jr., M. A. Lim, and K. E. Downer. 2007.

Clonidine, octopaminergic receptor agonist, reduces

protein feeding in the blow fl y, Phormia regina (Meigen).

Journal of Insect Physiology 53:1293–1299.

Waldbauer, G. P., and S. Friedman. 1991. Self- se lection of

optimal diets in insects. Annual Review of Entomology

36:43–63.

Wei, Z., G. Baggerman, R. J. Nachman, G. Goldsworthy,

P. Verhaert, A. De Loof, and L. Schoofs. 2000. Sulfaki-

nins reduce food intake in the desert locust, Schistocerca

gregaria. Journal of Insect Physiology 46:1259–1265.

Widmaier, E. P. 1999. Why Geese Don’t Get Obese (and We

Do): How Evolution’s Strategies for Survival Aff ect Our

Everyday Lives. W. H. Freeman Com pany, New York,

NY.

Winston, M. L. 1987. The Biology of the Honeybee. Harvard

University Press, Cambridge, MA.

the entomologist bookshelf (supplemental readings)

Cohen, A. C. 2004. Insect Diets: Science and Technology.

CRC Press, Boca Raton, FL. Dethier, V. G. 1976. The

Hungry Fly: A Physiological Study of the Be hav ior

Associated with Feeding. Harvard University Press,

Cambridge, MA.

Gaio, A. de O., D. S. Gusmão, A. V. Santos, M. Berbert-

Molina, P. F. P. Pimenta, and F. J. A. Lemos. 2011.

Contribution of midgut bacteria to the blood digestion and

egg production in Aedes aegypti (Diptera: Culicidae) (L.).


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