Cleaning Up After a Meal: The Consequences of Prey Disposal for Pit‐Building Antlion Larvae

Predators use a variety of strategies for capturing prey. Trap‐building predators can save on searching and encountering costs by investing in the construction and maintenance of traps such as webs and pits. However, what to do with partially consumed prey poses a potential problem. Antlion larvae (Myrmeleon acer) catch ants in conical pits, and dispose of partially consumed carcasses by flicking them a short distance away. We tested whether this prey‐disposal behaviour affects the effectiveness of antlion pits. We observed ant behaviour around artificially constructed pits and compared falls into pits with clean margins to those with conspecific ant carcasses or control objects around the pit edge. The presence of objects near pits affected the behaviour of live ants, and reduced the effectiveness of pits. Live ants spent the most time examining fresh ant carcasses, but the presence of any object near pits deterred pitfalls. Ants fell into pits significantly more often when pit edges were clean, suggesting that antlions could incur a prey capture cost from flicking carcasses from pits as well as from the accumulation of other debris around pit margins.     

Behavioral Plasticity and Variation in Pit Construction of Antlion Larvae in Substrates with Different Particle Sizes

Behavioral plasticity allows animals to maximize their fitness in a variety of environmental conditions. Trap‐building predators represent case studies 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 behavior of the species comprises six stages. When antlions were exposed to different 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. Furthermore, at larger particle sizes, individuals occasionally constructed irregular traps with a figure of eight shape, and they crossed the center of the truncated cone during deepening. In the largest substrate, particle size of antlions did not construct pits. Our results demonstrate that variation in traps under differing environmental conditions stems directly from behavioral plasticity in this species.     

Foraging decisions and behavioural flexibility in trap‐building predators: a review

Foraging theory was first developed to predict the behaviour of widely‐foraging animals that actively search for prey. Although the behaviour of sit‐and‐wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap‐building predators (mainly pit‐building antlions and web‐building spiders), a specific group of sit‐and‐wait predators that construct traps as a foraging device, with those of widely‐foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap‐building and widely‐foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap‐building predators. In each chapter of this review, we summarize the response of trap‐building predators to a different factor, while contrasting it with the equivalent response characterizing widely‐foraging predators. We provide here evidence that the behaviour of trap‐building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.     

Effects of larval antlions Euroleon nostras (Neuroptera, Myrmeleontidae) and their pits on the escape-time of ants

Abstract. Pit-building antlions are predators with a unique predation strategy, namely using pitfall traps constructed in loose sand to catch prey. Here, prey escape-time in the field is measured by introducing ants into one of four different treatment arenas. The first treatment lacks pits and antlions, the second includes 10 antlions that did not build pits, the third comprises eight artificially constructed pits, and the fourth is a treatment of eight antlions in pits and two without pits. Their pits are of a similar size to those used in the third treatment. When antlions are present without constructing pits, they impede the dispersal of prey. The mean escape-time for one half of the released ants is twice as long with antlions present as without them. When pits are present, the time taken for one half of the released ants to escape the predator is more than 10 times as long as when pits are absent. Escape-time from artificial pits is three times that from nonpit building antlions. Pits hinder the escape of ants and therefore increase the amount of time that the prey is available for capture. In the area where the pits are occupied by antlions, escape-time is four times longer than in a treatment with similar sized artificial pits. Thus, it appears that not only a pit, but also the presence of antlions influences the capture success.     

Optimal range of prey size for antlions

1. Antlions are opportunistic trap building predators that cannot control prey encounter. Their trap should ideally retain a great diversity of prey. However, building a single trap that captures many prey with varying characteristics can be challenging. 2. A series of five different ant species ranging from thin to large, of sizes ranging from 2.75 to 6.5 mm, and a mean weight ranging from 0.54 to 6.00 mg were offered in a random succession to antlions. The state of satiation of the antlions was controlled, and their mass and the depth of their pit were recorded. The reaction of antlion to the prey, the probability of capture as well as the time to escape were recorded. 3. The probability of an antlion reaction is an increasing function of the pit depth and a decreasing function of antlion mass. The probability of capture is highest for intermediate prey mass and is an increasing function of pit depth. The time to escape is a declining function of prey mass and an increasing function of pit depth. 4. There is an upper limit to prey mass given that large prey escape out of the pit. There is a lower limit to prey mass given the difficulty to apprehend the smallest, thin species. Consequently, there is a range of prey mass, corresponding to a medium‐sized ant of 2 mg, for which the pit functions best. The physics of insect locomotion on sandy slopes was identified as the key to understanding the functioning of antlion pits.     

An experimental study on the foraging behavior of a pit-building antlion larva,Myrmeleon bore

Foraging behavior of a pit-building antlion larva, Myrmeleon boreTjeder was investigated experimentally to elucidate the relation between the feeding level and pit relocation.

1. In artificial sands constructed in the field the 3rd instar larvae of M. bore rarely changed the positions of their pits, though several antlions had moved actively until they constructed pits. The average feeding rate was 0.3 prey/day/pit, and about 60% of prey captured were ants.
2. To examine whether or not M. bore larvae concentrate into the area where they can capture more prey, 8 antlions were released into each of 6 boxes filled with sand. I divided the sand surface of each box into two half areas, then gave prey to the pits built in a half area and gave no prey to the pits built in the other half. During the 50-day observation period, nonfed antlions never moved into the area where prey were given.
3. The 3rd instar larvae were reared separately without food. Even under starved conditions they rarely relocated their pits until dealth. The average duration of survival period was 83.9 days.
4. The experimental results indicate that M. bore larvae adopt a tactic of sedentary ambushing. These larvae exhibit low movement rates which are independent of prey capture rates.

     

The biophysics of pit construction by antlion larvae (Myrmeleon, Neuroptera)

An antlion pit is lined with fine particles during construction. This feature appears to increase the effectiveness of the pit in prey capture. Pit structure is influenced by physical properties of sand and the building behaviour of the antlion. Two physical properties of sand govern pit structure: the angle of repose and Stoke's Law drag force. These two properties complement each other as follows: (a) Since larger particles have a lower angle of repose than smaller particles, fine sand grains tend to stay on the pit walls, whereas larger particles fall to the pit's centre. (b) Large particles have a lower drag to momentum ratio than do small particles. Thus, larger particles are more likely to be thrown out of the pit than are smaller particles. Several behavioural modifications were demonstrated that increase the number of fine particles on the pit walls while reducing construction costs for the antlion. (a) A trajectory angle of 45° is used when the antlion throws particles out of the pit. This angle will maximize the distance to which larger particles are thrown. A trajectory angle of 60° is used at the end of pit construction when the antlion is throwing fine particles on the sides of the pit. This angle reduces the number of these fine particles leaving the pit. (b) Antlions can alter the velocity with which they throw particles. When discarding prey carcasses and debris that have accumulated during prey capture, they use a velocity that is approximately 39% higher than the velocity used during pit construction. (c) By vibrating their forelegs, antlions appear to sift out the finer particles before each throw. This increases the percentage of larger particles discarded from the pit.     

The involvement of sand disturbance, cannibalism and intra-guild predation in competitive interactions among pit-building antlion larvae

Competition in trap-building predators such as antlion larvae is a complex biotic interaction, potentially involving exploitation competition, sand throwing (i.e., interference competition), cannibalism and intra-guild predation. We investigated the short-term behavioral and developmental responses of the strict sit-and-wait antlion predator Myrmeleon hyalinus to sand disturbance (i.e., quantification of the impact of severe sand throwing), and to con- and hetero-specific competition by a larger sit-and-pursue antlion species Lopezus fedtschenkoi. We found that antlions subjected to sand disturbances reduced their pit construction activity and relocated less often. Furthermore, the reduction in pit construction activity was stronger among antlions subjected to disturbances prior to feeding. Almost no death occurred during the sand disturbance experiment, but as expected, disturbances caused reductions in the relative growth rates of antlions. This negative effect was stronger in the group exposed to sand disturbances prior to feeding. The presence of the sit-and-pursue competitor led to reductions both in pit construction and in relocation activities of M. hyalinus. Although the per-capita food supply was identical in both experiments, only 48% of M. hyalinus larvae survived the competition experiment, and this pattern was consistent between the con- and hetero-specific treatments. However, in the presence of hetero-specific competitors, the relative growth rate of surviving larvae was significantly lower than that measured in the presence of con-specific competitors. Our study demonstrates that investigating the different components of complex biotic interactions can markedly improve our understanding of how these different factors interact to influence the behavior and life history of organisms.     

Variation in pit size of antlion (Myrmeleon carolinus) larvae: the importance of pit construction

Abstract.Larvae of several antlions build pits that vary in size across and within species. The influence of food limitation and pit building experience on variation in pit size of the larvae of Myrmeleon carolinus was investigated in the laboratory. Unfed larvae that were allowed to build pits had smaller pit diameters than fed larvae. However, fed antlions that had been previously prevented from pit building, initially did not build larger pits than unfed antlion larvae that, too, had been prevented from pit building. Therefore, physiological constraints associated with food limitation alone are not sufficient to explain the reduction in pit size of food limited antlions of this species.     

Substrate selection for pit making and oviposition in an antlion, Myrmeleon bore Tjeder, in terms of sand particle size

The larvae of the pit‐making antlion Myrmeleon bore Tjeder live in open sand in riverbeds with a substratum consisting of various particle sizes. We analyzed the spatial distribution of their pits in a sandy floodplain to determine their larval and adult responses to the heterogeneous substrate. The spatial distribution pattern of their pits had an aggregated distribution, and there was a significant positive correlation between pit density and the ratio of medium‐size sand particles to total weight of sand. We examined the size of sand particles selected in the larval pit‐building behavior and the oviposition behavior of the adult. Both larvae and adults selected medium‐size sand particles. The larvae of M. bore are relatively sedentary predators and rarely move great distances. Thus, the present results suggest that habitat selection by adult females is a major factor causing the aggregative distribution of the pits.     

Prey detection in antlions: propagation of vibrational signals deep into the sand

Trap‐building antlion larvae detect their prey according to the substrate vibrations produced during movement of the prey on the sand surface. Although most studies are devoted to surface vibrational waves, in the present study, we determine the role of vibrations travelling through deeper sand layers. A behavioural experiment confirms that vibrational stimuli from prey insects on the surface of the sand stimulate the antlions buried in deeper sand layers to move towards the surface. Sand depth and particle size both have a strong effect on signal transmission. The damping coefficient (α₁₀) varies from 0.49 dB to 3.30 dB cm^?1 and depends on frequency (in the range from 100 to 300 Hz), particle size (from finest to coarse sand) and distance from the source of the vibrations. The deeper the sand, the narrower the frequency range of the signal becomes. Sand is a filter for higher frequencies. The smaller the sand particles, the more intense the filtering becomes. Fine sand with a mean sand particle size of 360 μm is a more efficient filter than coarse sand; consequently, high frequencies (> 2.5 kHz) are eliminated at a depth of 3 cm. Mean frequency depends on both depth and particle size. However, low frequency signals still propagate at a certain distance, which is biologically important in prey detection. Although the most efficient signal propagation appears to occur in coarse sand, it contains overly large particles that are inconvenient for relatively small antlion larvae. Predators seek a compromise between fine and coarse sand choosing medium sand.     

Determining sensitive stages for learning to detect predators in larval bronzed frogs: Importance of alarm cues in learning

Successful survival and reproduction of prey organisms depend on their ability to detect their potential predators accurately and respond effectively with suitable defences. Predator detection can be innate or can be acquired through learning. We studied prey–predator interactions in the larval bronzed frogs (Sylvirana temporalis), which have the innate ability to detect certain predators. We conducted a series of experiments to determine if the larval S. temporalis rely solely on innate predator detection mechanisms or can also learn to use more specific cues such as conspecific alarm cues for the purpose. The results of our study clearly indicate that larval S. temporalis use both innate and learned mechanisms for predator detection. Predator-naïve tadpoles could detect kairomones alone as a potential threat and responded by reducing activity, suggesting an innate predator detection mechanism. Surprisingly, predator-naïve tadpoles failed to detect conspecific alarm cues as a potential threat, but learned to do so through experience. After acquiring the ability to detect conspecific alarm cues, they could associate novel predator cues with conspecific alarm cues. Further, post feeding stages of larval S. temporalis are sensitive for learning to detect conspecific alarm cues to label novel predators.     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Conspecific alarm cues, but not predator cues alone, determine antipredator behavior of larval southern marbled newts, Triturus pygmaeus

Predation imposes selection on the ability of prey to recognize and respond to potential threats. Many prey species detect predators via chemoreception, particularly in aquatic environments. Also, chemical cues from injured prey are often perceived as an indication of predation risk. However, because antipredatory behavior can be costly, prey responses should depend on the current level of risk that each predator poses, which may depend on the type of chemical cues detected. We exposed larval newts, Triturus pygmaeus, to chemical cues from predator larval beetles or to alarm cues from conspecific larval newts and examined the behavioral changes of larval newts. Results showed that larval newts reduced activity levels when conspecific alarm cues were present but not when the predator cues alone were present. These results might suggest that larval newts are unable to recognize predator chemicals. To avoid costs of unnecessary antipredatory behaviors, larval newts may benefit by avoiding only predators that represent a current high level of threat, showing only antipredatory responses when they detect conspecific alarm cues indicating that an actual predatory attack has occurred.     

Orientation of the pit-building antlion larva Euroleon (Neuroptera, Myrmeleontidae) to the direction of substrate vibrations caused by prey

Pit-building antlion Euroleon nostras constructs efficient traps in sand to catch its prey. The predator is known to react to substrate vibrations produced by movements of its prey outside the pit with sand-tossing behaviour but it has not yet been ascertained if this reaction is directed towards the prey. The accuracy of the sand-tossing response in the presence of four prey species was measured using a video recording method. The sand-tossing angle was highly positively correlated with the prey angle. Sand tossing was most frequently elicited when prey was on the posterior sand surface. Covering the larval photoreceptors did not influence the antlion's localizing behaviour.     

Diet of a polyphagous arthropod predator affects refuge seeking of its thrips prey

Antipredator behaviour of prey costs time and energy, at the expense of other activities. However, not all predators are equally dangerous to all prey; some may have switched to feeding on another prey species, making them effectively harmless. To minimize costs, prey should therefore invest in antipredator behaviour only when dangerous predators are around. To distinguish these from harmless predators, prey may use cues related to predation on conspecifics, such as odours released by a predator that has recently eaten conspecific prey or alarm pheromones released by attacked prey. We studied refuge use by a herbivorous/omnivorous thrips, Frankliniella occidentalis, in response to odours associated with a generalist predatory bug, Orius laevigatus, fed either with conspecific thrips or with other prey. The refuge used by thrips larvae is the web produced by its competitor, the two-spotted spider mite, Tetranychus urticae, where thrips larvae experience lower predation risk because the predatory bug is hindered by the web. Thrips larvae moved into this refuge when odours associated with predatory bugs that had previously fed on thrips were present, whereas odours from predatory bugs that had fed on other prey had less effect. We discuss the consequences of this antipredator behaviour for population dynamics. Copyright 2000 The Association for the Study of Animal Behaviour.     

Morphological and life‐history responses of anurans to predation by an invasive crayfish: an integrative approach

Predator‐induced phenotypic plasticity has been widely documented in response to native predators, but studies examining the extent to which prey can respond to exotic invasive predators are scarce. As native prey often do not share a long evolutionary history with invasive predators, they may lack defenses against them. This can lead to population declines and even extinctions, making exotic predators a serious threat to biodiversity. Here, in a community‐wide study, we examined the morphological and life‐history responses of anuran larvae reared with the invasive red swamp crayfish, Procambarus clarkii, feeding on conspecific tadpoles. We reared tadpoles of nine species until metamorphosis and examined responses in terms of larval morphology, growth, and development, as well as their degree of phenotypic integration. These responses were compared with the ones developed in the presence of a native predator, the larval dragonfly Aeshna sp., also feeding on tadpoles. Eight of the nine species altered their morphology or life history when reared with the fed dragonfly, but only four when reared with the fed crayfish, suggesting among‐species variation in the ability to respond to a novel predator. While morphological defenses were generally similar across species (deeper tails) and almost exclusively elicited in the presence of the fed dragonfly, life‐history responses were very variable and commonly elicited in the presence of the invasive crayfish. Phenotypes induced in the presence of dragonfly were more integrated than in crayfish presence. The lack of response to the presence of the fed crayfish in five of the study species suggests higher risk of local extinction and ultimately reduced diversity of the invaded amphibian communities. Understanding how native prey species vary in their responses to invasive predators is important in predicting the impacts caused by newly established predator–prey interactions following biological invasions.     

Multi-predator effects across life-history stages: non-additivity of egg- and larval-stage predation in an African treefrog

The effects of multiple predators on their prey are frequently non‐additive because of interactions among predators. When prey shift habitats through ontogeny, many of their predators cannot interact directly. However, predators that occur in different habitats or feed on different prey stages may still interact through indirect effects mediated by prey traits and density. We conducted an experiment to evaluate the combined effects of arboreal egg‐stage and aquatic larval‐stage predators of the African treefrog, Hyperolius spinigularis. Egg and larval predator effects were non‐additive – more Hyperolius survived both predators than predicted from their independent effects. Egg‐stage predator effects on aquatic larval density and size and age at hatching reduced the effectiveness of larval‐stage predators by 70%. Our results indicate that density‐ and trait‐mediated indirect interactions can act across life‐stages and habitats, resulting in non‐additive multi‐predator effects.     

Effect of density on spatial distribution,pit formation and pit diameter of Myrmeleon acer Walker, (Neuroptera: Myrmeleontidae): patterns and processes

The effect of increasing population density on the formation of pits, their size and spatial distribution, and on levels of mortality was examined in the antlion Myrmeleon acer Walker. Antlions were kept at densities ranging from 0.4 to 12.8 individuals per 100 cm². The distribution of pits was regular or uniform across all densities, but antlions constructed proportionally fewer and smaller pits as density increased. Mortality through cannibalism was very low and only occurred at densities greater than five individuals per 100 cm². Antlions in artificially crowded situations frequently relocated their pits and when more space became available, individuals became more dispersed with time. Redistribution of this species results from active avoidance of other antlions and sand throwing associated with pit construction and maintenance, rather than any attempt to optimise prey capture per se.     

Balancing predation risk,social interference,and foraging opportunities in backswimmers,<Emphasis Type="Italic"> Notonecta maculata</Emphasis>

Loss of foraging opportunities and intraspecific competition for prey may be important costs of using refuges, because a hiding animal is unable to use or defend its foraging area from conspecific intrusions. Thus, animals should balance antipredator demands with other requirements in deciding when to come out from a refuge after a predators unsuccessful attack. Observations on foraging and social interactions of backswimmers Notonecta maculata suggest that foraging may be costly in terms of intraspecific agonistic interactions. When prey density is low, increasing the probability of finding a prey may require active exploration of a larger area, but this also increases the probability of encountering a competitor. After simulated exposure to predators, unfed bugs resumed feeding positions after a significantly shorter hiding period than recently fed bugs. We hypothesized that hiding time may also be reduced by recent interactions with conspecific competitors, due to an increased perceived need to defend feeding opportunities. Thus, when a predator attack occurred immediately after an agonistic conspecific interaction, backswimmers resumed feeding positions more quickly, and closer to the original position from which they were disturbed, suggesting short-term defense of particular positions. We conclude that when foraging, backswimmers balance the benefits of finding prey with the costs of predation risk and social interference in deciding their foraging strategy.Communicated by P.K. McGregor