The Insect World

Introduction to the insects

• Insects are the most diverse and numerous group of organisms that have ever lived on the earth

• There are estimated to be over 1.5 million species of named animals on earth, with insects making up 1 million of this number

• Some researchers estimate there are as many as 10 to 30 million species of all kinds of animals yet to be described, with a large percentage of these expected to be insects

How many insects are out there?

• The most numerous of the insects are the beetles (Coleoptera) with 375,000 species worldwide, followed by the moths (Lepidoptera) and the wasps .

• Insects live mostly on land with only about 5% of the species utilizing an aquatic habitat, either in their immature or adult stage

• Insects occupy most terrestrial habitats including the Arctic circle, hot springs, and the Namibia Desert while there are almost no insects that live in the marine environment

What about weight, not numbers?

• Of the total animal biomass on earth, ants and termites may comprise 29%, while other insects contribute an additional 16% for a total of almost half of the weight of animals on the planet

• Surprisingly, the contribution by the class of animals humans are in (Mammalia) is only about 14% - in a tie with worms such as the earthworm; also 14%

Why are insects so successful?• Their small size allows them to occupy many

different nitches• They have an extremely high reproductive

potential coupled with a short generation time– The classic example of this reproductive potential is

the fruit fly which has 3 or 4 generations in a single month with hundreds of eggs produced by each female

– On a theoretical basis, if all progeny survived, the earth would be covered by several feet of fruit flies in a very short time!

Why are insects so successful?

• Insects can readily adapt to changing conditions, and are highly resistant to environmental extremes by entering periods of dormancy until conditions improve.

• Perhaps the most important adaptation which allowed the insects to disperse and occupy so many different habitats was the ability of most insects to fly

How long have insects been around?

• Insects originated over 400 million years ago• While there is no agreement on who the

ancestors of the insects were, scientists agree they were aquatic and that the insects today developed as a group on land

The evolution of insects

• Insects were highly successful in diversifying in an evolutionary sense and were greatly assisted in this process in several ways– Mass extinction of all forms of life which occurred

millions of years ago, allowing insects to occupy habitat nitches that had been previously occupied by other forms of life

– Development of the ability to fly

The evolution of insects

• Development of metamorphosis• Co-evolution of insects with flowering plants,

which greatly enhanced the development of new species of insects

• Development of a rigid exoskeleton which helped control the loss of moisture from the insects body to the atmosphere

Movement of insects back into the aquatic world

• While insects are well equipped to live in a terrestrial world, they have had to undergo a number of adaptations to move back into the aquatic environment

• Some of the adaptations have to do with obtaining food through modification of the mouthparts (dragonfly nymphs) or developing a streamlined body for moving through the water as have the diving beetles

Movement of insects back into the aquatic world

• The major hurdle, however, to resuming an aquatic life was the problem of obtaining sufficient oxygen

• Insects rely on a series of trachea (hollow tubes) which allow oxygen to enter the insect body through small holes called spiracles

• The oxygen diffuses through the trachea and then passes into the insect tissues– Diffusing is much like the smell of burnt toast moving

across the kitchen to where you are sitting

How insects “breath”

• Several types of insects (dragonflies, mayflies, stoneflies, etc) have immature stages which are entirely aquatic

• Since spiracles could not bring in oxygen under water, the young have developed thin tissues or gills which have a network of trachea within

• Dissolved oxygen diffuses from the water, through the thin tissues of the gills, and into the tracheal system which then supplies oxygen to the insect tissues

How insects “breath”• other kinds of insects use the aquatic environment

as adults, including the beetles and true bugs (giant water beetles, water boatman and water striders) and need oxygen while in the water– Diving beetles hold oxygen among their long hairs;

oxygen is picked up at the surface and taken with the beetles when they dive

– The rat-tailed maggot and water scorpions use a breathing tube which extends to the surface

– Some mosquitoes and leaf beetle larvae obtain oxygen by tapping into plants and breathing atmospheric oxygen through the stem

Why so few marine insects?• One of the more interesting theory discusses

the link between how insects breath and fish predation

• Diffusion of oxygen into the tracheal systems of insects works best at standard atmospheric pressure

Why so few marine insects?

• At depths greater than 120 feet, total pressure is 10 times greater than at the surface

• At this pressure, oxygen would not be able to diffuse properly into the trachea and nourish the tissues

• This would limit the insects to living near the surface, where atmospheric pressure allows proper diffusion of oxygen into the trachea

• However, at the surface is where fish predation would be most intense

Gigantism

• People are duly impressed with the large size of several native moths

• The Cercropia Moth reaches almost 6 inches, while the Luna Moth is only slightly smaller

• Both of these are dwarfed by the Queen Alexandra Birdwing Butterfly from New Guinea which has a wingspan of 12 inches.

• Some species of modern dragronflies have a wingspan of 8 inches

You ain’t seen nuthin yet

• From the fossil records, the largest insect that we know of was a dragonfly which had a wingspan of almost 3 feet!

• Several other species of dragonflies and mayflies are in the same category

• Obviously, not survived to today. What not??

What happened to the big dragonflies?

• the same link occurs between tracheal system of those large insects and its dependence upon the diffusion of oxygen from the atmosphere

• With today’s concentration of oxygen (20%) in the atmosphere, oxygen would not have diffused a great distance to all tissues and survival would not be possible

What happened to the big dragonflies?

• Then, how did insects reach 30 inches if their size was limited by oxygen conditons

• Geologists have found that oxygen conditions have not always been constant

• During the Carboniferous period (300 million years ago) and a lesser peak during the early Tertiary period (50 million years ago), there was a spike in oxygen concentration in the air

Change in atmospheric oxygen

• Both of these periods were followed by lower oxygen conditions

• Fossil records indicate that with higher oxygen conditions (about 35% higher than today), the higher oxygen concentration may have allowed insects to evolve to sizes approaching 2 or 3 feet

• However, as oxygen levels decreased, oxygen concentrations were inadequate to maintain life and the giant insects disappeared

A new variation to this theory

• Maximum insect size does correlate with oxygen concentration surprisingly well for about 200 million years to about the end of the Jurassic Period

• At that point, oxygen levels lower to current world conditions

• However, about 150 million years ago, it was noted that oxygen levels were increasing again, but insect size was going down, not up

A new variation to this theory

• The reduction in insect size correlates well with the evolution of birds

• With predatory birds flying around, the need for maneuverability became a driving force in the evolution of flying insects, favoring smaller body size.

Ability to Fly

• Most insects have the ability to fly• Wings were developed very early in the

evolutionary life of insects, first appearing in the fossil record about 350 to 400 million years ago

• Development of wings was crucial to the evolutionary success of insects – the ability to escape predators and to occupy new habits

• This led to the great diversity in the insects we observe today

Ability to Fly

• The fossil record shows evidence of three pairs of wings; however, the wings of modern insects consist of only two pairs

• One of the hypotheses is that wings evolved from a lobe or outgrowth of the body wall of the thorax

• Some researchers believe that wings came from appendages similar to the legs on a crayfish

Ability to Fly

• One of the intriguing hypotheses (never proven) is based on the observation of tropical ants “gliding” down from the canopy of the rainforest.

• The ants ere able to control the direction of their descent by moving their left hind legs outward and rotating them.

• Further research showed that the ants were not gliding randomly, but actually select their landing targets

The development of metamorphosis• The earliest insect did not undergo

metamorphosis – the juvenile stages looked very similar to the adults except in size

• There are still groups of insects today which also do not change except in size from juvenile to adults (called direct metamorphosis)

• However, other insects have developed different shapes and sizes as they matured– Complete metamorphosis; egg, larva, pupae, and

adult– Incomplete: egg, larva, and adult

The development of metamorphosis

• Metamorphosis is thought to have evolved 200-300 million years ago

• The striking differences between the larval and adult forms allows each or the states to occupy different habitat nitches as well as utilize different foods

• By doing this, one stage (usually the adult) could be adapted for a different role such as dispersion

The development of metamorphosis

• This process of metamorphosis combined with rapid life cycles, led to an explosion of insect species

• As you might expect, in today’s insect world there is a greater number of species which have some kind of metamorphosis – approximately 600 thousand or 77% have all 4 stages; egg, larva, pupae, and adult

Co-evoluton

• Co-evolution describes the process by which two different species adapt to evolutionary changes which occur in each other

• That means that an evolutionary change in one species causes an evolutionary change in the other

• Each species is totally dependent upon the other for their survival

Co-evoluton

• This relationship is mutually beneficial• An example is the development of flowering

plants and the insects such as bees and butterflies that pollinate them

• Insect diversity increased dramatically following the origin of the following plantas

Co-evoluton• Plants have a number of ways to increase pollination

by insects – the color of the petals and petal shape, fragrance, the amount of pollen, etc.

• In response, insects have developed specialized ways and means of gathering food as pollen and nectar

• A flow that attracted specific pollinators on a regular basis had an advantage over flowers that attracted random pollinators

• It is also an advantage for the pollinator to have its own “private” food source because there is less competition

Co-evoluton

• Insects which are totally dependent upon flowers as a source of food include bees, wasps, butterflies, and moths

• Approximate 20% of the insects living today are dependent on flowers, nectar or pollen for their food source

• The importance of co-evolution of flowers and insects is shown by the fact that about 2/3 of flowering plants are insect pollinated

Examples of coevolution

• Yucca flowers are shaped in such a way that only the tiny yucca moth can pollinate them. The moths lay their eggs in the yucca flowers and the caterpillars live in the developing ovary and eat yucca seeds

Examples of coevolution

• Acacia trees have large, hollow thorns with acacia ants living within. On the tips of the leaflets, the plants makes a substance used by the ants as food.

• The ants defend the tree from herbivores by stinging any animal that brushes up against the tree

Examples of coevolution

• Moth-pollinated plants often have tubes the exact length of a certain moth’s “tongue”.

• Charles Darwin predicted the existence of a moth in Madagascar based on the size and shape of a flower he saw there.

• The moth was discovered 40 years later.

Examples of coevolution

• Butterflies are diurnal and have excellent vision but a poor sense of smell

• They must perch to feed. Flowers pollinated by butterflies are brightly colored but give off no odor

• Flowers chosen by the butterflies usually have a convenient place for the butterfly to land

• Each flower has a tube that is equivalent in length to that of the butterfly’s tongue