RESEARCH PAPER

Behavioral Plasticity and Variation in Pit Construction of AntlionLarvae in Substrates with Different Particle SizesVesna Klokocovnik, Dusan Devetak & Marina Orlacnik

Faculty of Natural Sciences and Mathematics, Maribor, Slovenia

Correspondence

Vesna Klokocovnik, Faculty of Natural

Sciences and Mathematics, Koroska cesta

160, 2000 Maribor, Slovenia

E-mail: vesna.klokocovnik@uni-mb.si

Received: April 24, 2012

Initial acceptance: June 7, 2012

Final acceptance: August 10, 2012

(T. Tregenza)

doi: 10.1111/eth.12012

Abstract

Behavioral plasticity allows animals to maximize their fitness in a variety

of environmental conditions. Trap-building predators represent case stud-

ies in such plasticity as the characteristics of their traps are dependent

upon the substrate available. We investigated the effect of sand particle

size on pit construction in antlions (Euroleon nostras), sand-dwelling insect

larvae that build pitfall traps to capture prey. The pit construction behav-

ior of the species comprises six stages. When antlions were exposed to dif-

ferent sand particle sizes, their behavior differed in terms of the

occurrence and duration of particular stages and in the frequency of jerks

produced during sand tossing. Jerk frequency was negatively correlated

with sand particle size and also changed during pit construction. Further-

more, at larger particle sizes, individuals occasionally constructed irregular

traps with a figure of eight shape, and they crossed the center of the trun-

cated cone during deepening. In the largest substrate, particle size of ant-

lions did not construct pits. Our results demonstrate that variation in traps

under differing environmental conditions stems directly from behavioral

plasticity in this species.

Introduction

Different environments offer a wide variety of habi-

tats for animals (Farji-Brener 2003). The evolutionary

importance of habitat selection is based on the

assumption that animals actively select habitats that

are more suitable relative to those not selected, sug-

gesting that fitness is higher in preferred microhabi-

tats (Pyke 1984; Martin 1998). Habitat selection has

been primarily studied on mobile animals; however,

many organisms are limited in their mobility and

dependence on their immediate surroundings is con-

siderable, therefore exogenous factors largely dictate

behaviors such as foraging and habitat selection (Ori-

ans 1991; Scharf & Ovadia 2006; Scharf et al. 2011).

The majority of antlion larvae (Myrmeleontidae)

are psammophilous predators; therefore, habitat

structure plays an important role in foraging behavior.

Most species do not construct pit-fall traps but gener-

ally wait for prey just below the substrate, attacking

with rapid head movement toward the prey, and

grasping the prey by closing the mandibles (New

1986; Cain 1987; Mansell 1996, 1999). Only 10% of

antlions have evolved highly specialized apomorphic

pit-fall traps to catch small arthropods, primarily ants,

and usually inhabit suitable, arid sand, sheltered from

rain (Gepp 2010).

The use of traps for predation has evolved indepen-

dently in a small number of animal groups (Alcock

1972; Foelix 1996; Ruxton & Hansell 2009). This

strategy is energy-and time-consuming, but reduces

the amount of energy expended in prey searching

given that ambush predators invest no energy or time

in searching for prey but need only a suitable place for

constructing a trap (Lucas 1985; Riechert 1992; Foelix

1996; Eltz 1997).

Pit-constructing activity in antlions is determined

by a number of different factors including predator

and prey size (Griffiths 1980a; Scharf et al. 2009),

availability of prey (Griffiths 1980b; Scharf & Ovadia

2006), temperature, food and population density of

the antlions (Arnett & Gotelli 2001), disturbance

Ethology 118 (2012) 1–9 © 2012 Blackwell Verlag GmbH 1

Ethology

regime (Gotelli 1993; Barkae et al. 2010), microcli-

mate factors such as photoperiod (Scharf et al.

2008b), substrate temperature (Marsh 1987), mois-

ture (Gotelli 1993; Morrison 2004), and shade (Scharf

et al. 2008a). Pit construction is also affected by the

physical properties of the substrate, such as particle

size (Griffiths 1980a; Lucas 1982, 1986; Lomascolo &

Farji-Brener 2001; Botz et al. 2003; Devetak et al.

2005) and density (Devetak et al. 2012).

Trap-building antlions can modify their traps in

response to smaller-scale variations in their environ-

ment (e.g., the proportion of the fine particles in the

sand) (Lucas 1982; Farji-Brener 2003). Some of these

responses are probably due to various environmental

constraints (e.g., small diameter or pit depth may

depend on the space available for construction), but

some of the responses, particularly responses to biotic

factors, are adaptive, owing to their positive effect on

capture success. Antlions exhibit impressive behav-

ioral plasticity, resulting in efficiency of capture suc-

cess (Scharf et al. 2011). Behavioral differences are

tightly linked to the optimization of life-history traits

(Arnett & Gotelli 2001).

Most trap-building antlions prefer fine substrates

with small particle size for pit construction if they

have a choice (Youthed & Moran 1969; Griffiths

1980a; Lucas 1982; Kitching 1984; Allen & Croft

1985; Loiterton & Magrath 1996; Lomascolo & Farji-

Brener 2001; Botz et al. 2003; Devetak et al. 2005).

Their behavior can be analyzed as a cost–benefit rela-tionship, where benefits are related to capture success

and cost to pit construction (Griffiths 1986). Con-

struction in sand with smaller particles results in

higher benefits and lower costs, as such construction

requires less time and energy expended. Large parti-

cles are probably more difficult to handle; thus, the

antlions require more time for construction (Lucas

1982). In finer substrates, pits have features that max-

imize capture efficiency, such as pit diameter, pit

depth and the slope of the pit walls (Wilson 1974;

Griffiths 1980a, 1986; Lucas 1982; Farji-Brener 2003;

Fertin & Casas 2006). A study of Farji-Brener (2003)

indicated that pits constructed in fine-grained sand

were bigger, deeper, and significantly more efficient

at capturing prey than those in coarser sand.

The choice of whether or not to build a pit is also an

important component of antlions’ foraging strategy

(Arnett & Gotelli 2001). Devetak et al. (2005) studied

pit-building decision and pit size in different sand

fractions in all three larval instars of the European

species, Euroleon nostras, where a preference for a sand

particle size of 230–540 lm was shown in the third

larval instar. No pits were constructed in coarser

sands, that is, in substrates with a particle size

>1540 lm.

The pit is constructed in such a way as to maximize

prey capture (Lucas 1986). Pit construction behavior

has already been described (Wheeler 1930; Haub

1942; Youthed & Moran 1969; Tuculescu et al. 1975;

Topoff 1977; Bongers & Koch 1981; Klein 1982; Lucas

1982; Matsura & Kitching 1993), but most authors did

not divide this behavior into its various stages, with

the exception of Youthed & Moran (1969), studying

an African species, Myrmeleon obscurus. They described

seven stages of pit-building behavior, where the first

stage represents the antlion’s emergence at the sur-

face. In the second stage, the antlion excavates a very

small pit. During the third stage, the antlion moves

around randomly just beneath the sand surface. Stage

four represents the beginning of actual pit construc-

tion (initial circle), this continues in stage five where

the antlion moves spirally inward to the center of the

pit. Stage six represents the end of pit construction;

the pit is completed. Finally, stage seven represents

the enlargement of the pit, after the pit has been fin-

ished.

Lucas (1982) detected four stages of pit construction

behavior in the species Myrmeleon crudelis, while

Tuculescu et al. (1975) and Topoff (1977) noted some

variation in the construction behavior of Myrmeleon

immaculatus larvae, which changed direction while

circling or moved through the center of the truncated

cone.

The digging activity of antlion larvae includes back-

ward movements just beneath the substrate surface.

These backward movements are accompanied by peri-

odic sand-tossing behavior, consisting of rapid jerks of

the head and mandibles, expelling the sand. The sedi-

ment is accumulated on the dorsal side of the head

and the mandibles with lateral movements of the

head and by vibrating one foreleg on the inner side of

the furrow (Youthed & Moran 1969; Tuculescu et al.

1975; Lucas 1982).

The aim of our study was to focus on the pit con-

struction behavior of Euroleon nostras (Geoffroy in

Fourcroy, 1785) and to answer the following ques-

tions: (i) does variation in particle size affect pit con-

struction behavior?, (ii) does variation in particle size

affect the frequency of jerks produced during sand

tossing?, and (iii) is jerk frequency consistent across

all stages of pit construction?

We observed the sequences of pit-building behavior

and analyzed the relationship between sand particle

size and the following parameters: occurrence and

duration of each stage; duration of pit construction

and number of circles while deepening; frequency of

Ethology 118 (2012) 1–9 © 2012 Blackwell Verlag GmbH2

Pit Construction of Antlion Larvae in Different Substrates V. Klokocovnik, D. Devetak & M. Orlacnik

jerks produced during sand tossing in pit construction.

We also recorded the occurrence of irregular pit con-

structions, where the regular pit construction was

considered as a moving spiral toward the center and

changing direction while deepening.

Methods

Study Species

Thirty-five third-instar larvae of the antlion Euroleon

nostras (Geoffroy in Fourcroy, 1785) were studied.

Antlions were collected from their pits on Boc and the

surroundings of Maribor in Slovenia in early summer

2009 and 2010. Larval stages were determined by

measuring head capsule width and body length

(Devetak et al. 2005). Each antlion larvae was used

only once in all three substrates. Before the experi-

ment, the antlions were fed once a day, to achieve the

same hunger level in all animals, because antlions dis-

play a number of responses owing to a low rate of prey

arrival, which can reduce the activities associated with

pit construction and pit maintenance (Eltz 1997).

During the experiment, the antlions were fed after the

pit had been constructed. The antlions were fed with

worker ants of the species Lasius fuliginosus (Latreille).

After the experiment, the antlion larvae were

returned to the locations where they were found.

Experimental Setup

The experiment was performed in the laboratory at

the Faculty of Natural Sciences and Mathematics,

University of Maribor. Before the start of the experi-

ment, the antlions were kept in plastic containers

filled with quartz sand (Kema Puconci d.o.o.) with a

particle size of <540 lm. The sand was purchased

commercially. The larvae were kept at room tempera-

ture (23–27°C) in a light/dark regime of LD 14:10 h.

During the experiment, antlions were placed individ-

ually in plastic containers (1.4 l, Ø20) filled with

700 ml of quartz sand with a certain particle size.

Large quantities of dry sand were sieved by sifting

through a series of sieves, to collect the following sand

fractions: 60–110 lm (G1), 230–540 lm (G2), 1000–1540 lm (G3), and 1540–1750 lm (G4). Video

recording took place after 17:00 hours in the after-

noon, when the activity of the antlion increases. We

observed all stages of pit-building behavior. The

behavior of each antlion was recorded in all four sand

fractions. Each day, only three antlions were recorded

in just one sand fraction, and on the following days in

others, from the finest (G1) to the coarsest fraction

(G4). For each individual, recording took place over

4 d to test it in all sand fractions. In G1, we recorded

34 pit constructions, in G2, 20 pit constructions, and

in G3, 31 constructions. In G4, none of the antlions

constructed a pit. Observations of the behavior lasted

from July till August in 2009 and 2010.

Video Analysis

Pit-building behavior was recorded with Panasonic

NV-DS29 (Osaka, Japan) and Sony DCR-HC32E video

cameras (Tokyo, Japan), using mini DV cassette and

Canon XLH1a (Tokyo, Japan), using the Firestore Pro-

HD 60 GB digital disk. The antlions were recorded from

a distance of 20–30 cm. Recordings of pit-building

behavior were analyzed with the PC-supported video

analysis software Pinnacle Studio 12.0 (Pinnacle Sys-

tems Inc., Mountain View, CA, USA). We calculated

the frequency of jerks produced during sand tossing,

expressed as a number of jerks per minute during one

pit construction. Unclear behaviors were not analyzed.

Statistics

A one-way analysis of variances F-test was used to

test the differences in duration of each stage of pit

construction in different substrates. Differences

between pairs of treatments were subsequently tested

with Tukey’s post hoc comparison, because the number

of values in treatments differed.

To test the differences in frequency of jerks pro-

duced during sand tossing between treatments, we

used the non-parametric Kruskal–Wallis test. We used

a two-tailed test with a critical p value of p < 0.05. All

analyses were performed using GraphPad Prism 4

(GraphPad Software Inc., San Diego, CA, USA) and

the statistical package SPSS 14.0 (SPSS Inc., Chicago,

IL, USA).

Results

Pit Construction Behavior

We observed pit construction behavior in substrates

with four different sand particle sizes to determine

whether any variations occur in different stages of pit

construction. In all stages, excluding the last one,

sand-tossing behavior was present. Pit construction

was divided into six stages:

1 Construction of a small pit—the antlion comes to

the surface of the substrate and excavates a small pit

(with a few movements of the mandibles to the left

and right, accumulating and expelling the sand),

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V. Klokocovnik, D. Devetak & M. Orlacnik Pit Construction of Antlion Larvae in Different Substrates

where it stays for an indefinite period of time; the

head with eyes, mandibles and the antennae are

visible;

2 Random movement—the antlion moves randomly

around (backwards) just beneath the sand surface,

creating a furrow on the surface; during the move-

ment the sand-tossing behavior is present;

3 Excavation of the initial furrow—actual pit con-

struction begins with the excavation of the initial

(first) circular furrow of the pit;

4 Deepening—the antlion continues a pit construc-

tion by spiral movements toward the center (deepen-

ing), in a clockwise or counter-clockwise direction,

sometimes changing direction or moving through the

center of the pit;

5 Finishing—after deepening, the antlion stops

circling and moves backward into the sand with left

and right movements of the head and the mandibles,

accumulating the sand on the surface of the mandi-

bles and expelling the sand out of the pit. Sometimes

the head roll movement occurs, where the head of

the antlion is raised and swept along the pit wall,

gathering sand to the center of the pit and expelling

the sand out of the pit;

6 Jaw set—the last stage is jaw set (full opening of the

mandibles); the antlion’s head, with the eyes, anten-

nae, and part of the mandibles usually remains visible.

Sometimes, after the pit has been constructed the

antlions enlarged their pits, but the increase was not

considered as a construction stage.

The Effect of Sand Particle Size on Pit Construction

Particle size affected the percentage occurrence of the

first two stages of pit construction behavior (Table 1).

The first stage (construction of a small pit) in G4 was

difficult to observe because of the large sand particles,

but it was most common in the finest sand fraction

(G1).

The second stage (random movement of the antlion

larvae) occurred more often in the finest (G1) and in

two coarser substrates (G3 and G4) than in the G2

fraction that seemed the most convenient for pit

building. In G4, where none of the antlions completed

pit construction, only the second stage occurred, and

its duration was longer than in the other sand frac-

tions.

Sand particle sizes also affected the duration of the

stages of pit construction (Fig. 1). The first stage was

not observed for duration. An F-test revealed that the

duration of the second stage differed between treat-

ments (F2,41 = 9.175, p = 0.0005). Tukey’s post hoc

comparison of the means between groups showed sig-

nificant differences between G1 and G2 (p < 0.01)

and between G2 and G3 (p < 0.001). There was no

significant difference in duration between G1 and G3.

In G2, the duration of the second stage was short.

Duration of the third stage depended on the size of

the pit diameter with differences between treatments

(F2,39 = 7.412, p = 0.0019). The largest pits were con-

structed in G2 (mean ± SD = 52 ± 8 mm, n = 20),

with a significant difference from G3 (mean ± SD =45.5 ± 10 mm, n = 31). In G1, the pit diameter was

small (mean ± SD = 31 ± 12 mm, n = 34); therefore,

the duration differed significantly compared with G2

(p = 0.0009) and G3 (p = 0.0191). Usually, the pit

diameter of pits constructed in G1 and G3 was smaller

Table 1: Percentage occurrence of pit construction stages in substrates with different particle sizes

Substrate

particle size (lm)

Number of

complete pit

construction

observed

I

(small pit)

II

(random

movements)

III

(initial

circular

furrow)

IV

(deepening)

V

(finishing)

VI

(jaw set)

G1 (60–110) 34 44 21 100 100 100 100

G2 (230–540) 20 30 10 100 100 100 100

G3 (1000–1540) 31 23 71 100 100 100 100

G4 (1540–1750) 0 – 100 0 0 0 0

Fig. 1: Duration (mean ± SD) of four stages in three different sub-

strates.

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Pit Construction of Antlion Larvae in Different Substrates V. Klokocovnik, D. Devetak & M. Orlacnik

than the initial circular furrow of the pits. In G4,

the antlions constructed an incomplete initial circular

furrow.

During the fourth stage, as the antlions were mov-

ing spirally toward the center, most of the variation

occurred. The duration of the fourth stage was the

longest in G2 with significant differences between

treatments (F2,41 = 22.28, p < 0.0001). A Tukey’s

post hoc comparison showed no significant difference

in duration between G2 and G3, but indicated signifi-

cant differences between G1 and G3 (p < 0.001) and

between G1 and G2 (p < 0.001). In the duration of

finishing the pit, an F-test showed no significant dif-

ference of variance between groups.

During the deepening activity, antlions showed

some variations in circling toward the center of the pit

by changing circling direction once or more, moving

through the center of the truncated cone (Fig. 2b) or

forming ‘a figure eight’ (Fig. 2a). Changing direction

was considered as regular construction, whereas

forming ‘a figure eight’ and moving through the cen-

ter counted as an irregular construction.

The percentage occurrence of regular and irregular

pit constructions in different sands is shown in

Table 2. The figure eight occurred in only a few con-

structions in G2 and G3, and was most common in

G3, whereas crossing the center, that is, moving

through the center of the pit while deepening,

occurred more often in both substrates.

In G1, only changing direction occurred. In G3, the

antlions often stopped circling and waited with open

mandibles for an indefinite period of time and then

continued circling. This type of the behavior also

occurred in G2 but less often.

The number of circles in pit construction differed

between G1 (mean ± SEM = 5.6 ± 0.6, n = 34) and

the other two sand fractions (G2: 9.6 ± 0.6, n = 20;

G3: 9.5 ± 0.5, n = 31) (Fig. 3).

The antlions invested maximum time in pit con-

struction in the most convenient sand fraction, G2.

The constructed pits usually had the same diameter as

the initial furrow of the pit. The slope of the pit wall

had a smooth surface, which was not usual for pits in

G1 and G3, where the surface of the wall had visible

furrows.

The effect of substrate particle size on the frequency of

jerks produced during sand tossing

Substrate particle size had an effect on the frequency

of jerks produced during sand tossing. The course of

pit construction was divided into ten equal sequences,

(a)

(b)

Fig. 2: An irregular pit construction forming ‘a figure eight’ (a: 1–3) and crossing the center of the truncated cone (b: 1–4).

Table 2: Percentage occurrence of regular and irregular constructions

during deepening

Substrate

particle

size (lm)

Number of

pit constructions

observed

Regular

construction

(circling inward

or changing

direction) %

Irregular

construction

(figure eight

or crossing

the center) %

G1 (60–110) 34 100 0

G2 (230–540) 20 45 55

G3 (1000–1540) 31 64 36

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V. Klokocovnik, D. Devetak & M. Orlacnik Pit Construction of Antlion Larvae in Different Substrates

and courses in different sand fractions were com-

pared. The frequency was lower in coarser sand and

increased with decreasing sand particle size (Fig. 4).

The slope of the frequency curves in different sand

fractions is similar (Fig. 4). We found a significant dif-

ference between treatments using the multiple com-

parison Kruskal–Wallis test: (H = 43.21, df = 2,

n = 410, p < 0.0001). Significant differences in the

frequency of jerks produced during sand tossing were

found between G1 and G3 (p < 0.0001) and G2 and

G3 (p < 0.0001), while no significant difference was

found between G1 and G2.

Discussion

Antlions prefer sands with fine particle size to build

pits, a fact which has already been demonstrated by

several authors (Youthed & Moran 1969; Lucas 1982;

Kitching 1984; Allen & Croft 1985; Loiterton & Mag-

rath 1996; Botz et al. 2003; Farji-Brener 2003; Deve-

tak et al. 2005). Lucas (1982) showed that antlions

are able to regulate the mechanisms of pit construc-

tion, which is carried out through the manipulation

of sand particle velocity and trajectory angle, in addi-

tion to an initial sorting through foreleg vibration.

In previous studies, no individual variation or only

minor variability in pit construction in antlions was

described. Youthed & Moran (1969) divided the pit

construction activity of M. obscurus into seven stages,

but described only one pattern of pit-building

behavior. The differentiation between the fourth to

sixth stages is not entirely clear. In the study of

M. crudelis, four stages of pit construction are shown

(Lucas 1982). The stages are similar to those described

for M. obscurus (Youthed & Moran 1969), but without

the first and second stage where the antlion comes to

the surface and constructs a small pit. Lucas (1982)

also observed pit construction activity in three other

species: Myrmeleon carolinus, M. mobilis, and M. immac-

ulatus. Pit building was virtually the same in all three

species, and no variations in pit construction were

detected.

Tuculescu et al. (1975) and Topoff (1977) described

some variations in pit construction in the antlion spe-

cies M. immaculatus, where the larvae sometimes

change direction while circling inwards or stop cir-

cling and cut straight back and forth through the cen-

ter of the truncated cone.

In the present study, we showed that sands differing

according to particle size do affect pit construction

behavior in Euroleon nostras larvae. Pit construction

behavior was observed in four sands with different

particle sizes. As far as we know, this is the first study

showing pit-building behavior in substrates differing

according to sand particle size. The study shows that

by manipulating different sand particle sizes, antlions

are able to regulate their mechanisms for pit construc-

tion (see also Lucas 1982). Pit construction behavior

was divided into six stages. It seems that sand particle

size affects all stages of the pit construction process.

Variations occurred during construction in the occur-

rence and duration of the stages, in deepening with

irregular construction, and in the frequency of jerks

produced during sand tossing. The frequency of the

first stage, that is, excavating a small pit, differs

between experimental groups. With increasing sand

particle size, the frequency of excavating the small pit

decreases. In coarser sand, the first stage is hard to

observe, owing to the large sand particle size. The sec-

ond stage occurred frequently in less suitable sub-

strates where the antlions were probably searching for

more appropriate sand. The duration and occurrence

of the second stage also depended on the size of the

substrate particles. In G2, the most appropriate

substrate for E. nostras (Devetak et al. 2005, 2007),

Fig. 3: Number of circles (mean ± SD) in pit construction.

Fig. 4: Frequency of jerks produced during sand tossing depends on

the size of sand particles. Mean ± SE is shown.

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Pit Construction of Antlion Larvae in Different Substrates V. Klokocovnik, D. Devetak & M. Orlacnik

the presence and duration of the second stage was the

lowest compared with other substrates and has

occurred in only 10%. As the antlions occupied the

most suitable sand (G2), searching for an appropriate

place was probably not necessary, so in 90% of obser-

vations the larvae constructed pits on the sites where

they had previously excavated the small pit. This type

of behavior was also observed in the study by Farji-

Brener (2003). This claim can also be confirmed by

comparing the antlion behavior in G4 (1540–1750 lm), where only random movement occurred.

This stage in G4 was usually extended, lasting till the

antlion ceased with random movement and

submerged below the sand, without constructing a

pit. Stages three to four, which comprise construction

of the initial circle and deepening, differ between

groups mostly in the duration of construction and the

number of circles made while deepening. The struc-

ture of the pit is influenced by the physical properties

of the sand, and trap efficiency is strongly affected by

trap design and by the material from which it is built

(Lucas 1982; Botz et al. 2003; Fertin & Casas 2006).

The antlions spent the most time on pit construction

in the most suitable substrate, in this case in G2,

where the duration of pit construction was the lon-

gest. Even though the number of circles while deep-

ening did not differ greatly between the G2 and G3

groups, there was a difference in the duration of deep-

ening.

The pits in G2 were most precisely built and had

large diameters. The constructed pits usually had the

same diameter as constructed first furrow of the pit.

The slopes of the pit wall were smooth with no visible

furrows, what was not usual for pits in G1 and G3. In

coarser sand, the antlions had to construct larger pits

because of the angle of the pit wall, to prevent scatter-

ing the sand into the center of the pit (Botz et al.

2003; Fertin & Casas 2006). According to Lucas

(1982), larger particles have a lower angle of repose

than small ones; thus, fine sand tends to stay on the

pit walls, while and bigger particles fall into the center

of the pit. In G1, the pits had the smallest diameter;

thus, the duration of construction was short.

While deepening, the antlions frequently stopped

making circular movements and crossed the center of

the truncated cone to resume circling (Tuculescu

et al. 1975) or changed direction while moving

toward the center (Topoff 1977). Our study shows

one additional variation in pit construction occurring

in G2 and G3, where the antlions excavated an initial

pit, but then moved in different directions and formed

‘a figure eight’. Sometimes the antlions excavated an

actual pit in the second circle or circled whole ‘figure

eight’ structure and started deepening. In the finest

sand (G1), only spiral movements toward the center

or changes in direction occurred. By manipulating

material, antlions can maximize the capture efficiency

of the pit. In G2, where sand particle size was small

and the constructed pit had small diameter, moving

through the center was not necessary. When the par-

ticles and the diameter of the pit became bigger, the

antlions avoided large quantities of sand in the middle

of the pit by moving through the center of the pit and

expelling the accumulated sand; this behavior has also

been shown in research by Tuculescu et al. (1975).

The present study also shows that substrate particle

sizes did have an effect on the frequency of jerks pro-

duced during sand tossing. Bongers & Koch (1981)

provided information about the frequency in E. no-

stras where the frequency was the same in all three

larval instars and ranged between 0.5 and 1.0 jerks

per second.

We showed that the frequency has a negative

correlation with substrate particle size. As particle

size increases, frequency decreases, and that may

be because movement through coarser sand

becomes difficult, and because during construction

in coarser substrate, antlions often stop construction

for indefinite period of time, and that was more

frequent in G3. Frequency also changed during pit

construction. Construction of the initial circle was

slow. Frequency increased after the first circle but

gradually decreased as the antlions were moved

inward, when the circles became smaller and

movement more difficult. This frequency pattern

was similar in all substrates.

In summary, the pit-building behavior of antlions

showed a distinct dependence on the sand particle

size of substrates. The most convenient substrate

for E. nostras is finer sand with particle sizes of 230

–540 lm.

Acknowledgements

We express our thanks to Prof. Dr. Franc Janzekovic

for advice on the statistical analysis. This research was

supported by the Slovene Ministry of Education, Sci-

ence, Culture, and Sport within the Biodiversity

Research Programme (Grant No. P1-0078).

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