Adaptive short-term changes in pit design by antlion larvae (Myrmeleonsp.) in response to different prey conditions

Desert Ants Learn to Avoid Pitfall Traps While Foraging

Simple Summary

Animals living in nests leave their nests to search for food and often use constant routes. We tested how workers of ant colonies cope with pitfall traps placed on their way to food. Such pits can represent those dug by the ant-hunting pit-building antlions. The pitfall traps delayed the arrival at the food and increased the workers’ tracks, but the ants improved in searching after accumulating experience. Furthermore, workers learned to avoid falling into the pits with experience. Removing or adding pits led to a fast change in the worker behavior and they ignored the past conditions, except for tracks that were longer than expected, after pitfall traps were removed. The ants fell much more frequently into pits closer to the arena entry, suggesting that such positions are especially profitable for sit-and-wait predators, ambushing such ants.

Abstract

Central-place foragers, such as social insects or nesting birds, repeatedly use the same routes from and to their nests when foraging for food. Such species forage more efficiently after accumulating experience. We examined, here, a relatively neglected aspect of such an improvement with experience—the avoidance of pitfall traps. Similar pits are built by antlions, which co-occur with the ants, but they also resemble other natural obstacles. We used the desert ant Cataglyphis niger, common in sandy habitats, and allowed it to forage for three successive runs for a food reward. Ant workers discovered food more slowly and in smaller numbers when pits were in their path. Pit presence also led to longer tracks by ants and slower movement. However, with experience, the ants fell into such pits less often and reached the food more quickly. To understand how past conditions affect current behavior, we investigated whether removing or adding pits led to a different result to that with a constant number of pits. Workers adjusted their behavior immediately when conditions changed. The only carryover effect was the longer tracks crossed by workers after pit removal, possibly resulting from the mismatch between the past and current conditions. Finally, the workers were more likely to fall into pits that were closer to the nest than those that were further away. This is a good example of the advantage that ambush predators can derive from ambushing their prey in specific locations.

Behavioral differences between pit-building antlions and wormlions suggest limits to convergent evolution

Antlions and wormlions are distantly related insect taxa, both digging pits in loose soil and ambushing arthropod prey. Their hunting method, which is rare in the animal kingdom, is a clear example of convergent evolution. There is little research directly comparing the 2 pit-building taxa. Using the same experimental platform to investigate how they respond to biotic and abiotic environmental factors enables an examination of their convergence and its limits. We investigated the response of antlions and wormlions to 3 factors common in their daily life: disturbance to the pits, prey arrival, and conspecific competitors. Although both increased the pit size following disturbance, wormlions increased it faster than antlions. Antlions responded to prey faster than wormlions, but wormlions improved their response time over days. The most diverging response was toward conspecifics. Whereas antlions relocated their pits fast in response to increasing conspecific density, wormlions never relocated. We suggest explanations for the behavioral differences between the taxa. Our results imply that despite the similar hunting method of the 2 taxa they may differ greatly in their behavior, which in turn might have consequences for their habitat use and population dynamics.

Operant conditioning in antlion larvae and its impairment following exposure to elevated temperatures

Although ambush predators were previously considered limited in their cognitive abilities compared to their widely foraging relatives, there is accumulating evidence it does not hold true. Pit-building antlions are already known to associate vibrations in the sand with the arrival of prey. We used a T-maze and successfully trained antlions to turn right or left against their initial turning bias, leading to a suitable substrate for digging traps. We present here the first evidence for operant conditioning and T-maze solving in antlions. Furthermore, we show that exposure of second instar larvae to an elevated temperature led to impaired retention of what was learned in a T-maze when tested after moulting into the third instar, compared to larvae raised under a more benign temperature. We suggest that climate change, involving an increase in mean temperatures as well as rare events (e.g., heatwaves) might negatively affect the retention of operant conditioning in antlions, alongside known, more frequently studied effects, such as changes in body size and distribution.

Efficiency of antlion trap design and larval behavior in capture success

Traps constructed by an animal reduce the amount of energy required to seek prey. The main risk of trap-building predators is the greater uncertainty of encountering prey, owing to their immobility. Sometimes environmental characteristics do not allow them to build efficient traps, resulting in lower capture success. We observed prey escape, capture success, and behavior of two antlion species, Cueta lineosa, a habitat specialist, and Myrmeleon hyalinus, a generalist, building geometrically different traps. The traps of C. lineosa are elaborate and deep, consisting of two inverted cones, while M. hyalinus builds simple inverted cones. Prey escape was observed from traps with antlion larvae present and from artificially constructed traps without antlions. We used a 3D printer to create a replica model of both trap types, pressing the model onto the substrate surface to create a trap. The C. lineosa artificial trap slowed prey escape more effectively than the simpler artificial trap of M. hyalinus. Prey escape time was four times longer for two ant species and three times longer for woodlice from C. lineosa traps. Escape time also decreased with increasing prey length. We also found behavioral differences between these two antlion species. The behavior of M. hyalinus is much more efficient in catching prey than that of C. lineosa. The results indicate that both species are efficient trap-building predators; however, it appears that capture success depends not only on trap design but also on larval behavior.

Past thermal conditions affect hunting behaviour in larval antlions

Some sit-and-wait predators, such as antlion larvae, construct traps to capture passing prey. The location of these traps depends on many abiotic and biotic factors, including temperature and the presence of conspecifics, which probably stimulate behaviours that minimize the costs and maximize the benefits of trap building. Here, we exposed second instar antlion larvae to elevated temperatures of 25°C (mild treatment) or 31°C (harsh treatment) for one month and then transferred them to common conditions (20°C) to examine the effects of previous thermal treatment on aggregation tendency and trap size. We predicted that antlions that experienced harsh conditions would subsequently increase the neighbouring distance and trap diameter to reduce competition with conspecifics and improve prey capture success, compensating for past conditions. In contrast with these predictions, antlions exposed to harsh conditions displayed a trend in the opposite direction, towards the decreased neighbouring distance. Furthermore, some of these antlions also built smaller traps. We discuss possible reasons for our results. The effects of previous thermal exposure have rarely been considered in terms of trap construction in antlions. Described effects may possibly apply to other sit-and-wait predators and are significant considering that many of these predators are long-lived.

Wormlions prefer both fine and deep sand but only deep sand leads to better performance

Wormlions are small fly larvae that dig pits in loose soil to trap their prey. Similar to other trap-building predators, like spiders and antlions, they depend on the habitat structure for successful trap construction and prey catch. We examined whether sites at which wormlions are present differ in sand depth and particle size from nearby sites, at which wormlions are absent. Next, in the laboratory we manipulated both sand depth and type (fine vs. coarse) to determine their joint effect on microhabitat preference, the size of the constructed pit, wormlion movement, and their latency to respond to prey. We expected better performance by wormlions in fine and deep sand, and the sand in wormlions’ natural sites to be finer and deeper. However, in only partial agreement with our expectations, wormlion sites featured finer sand but not deeper sand. In the laboratory, wormlions preferred both fine and deep sand, and moved more in shallow and coarse sand, which we interpret as an attempt to relocate away from unfavorable conditions. However, only deep sand led to larger pits being constructed and to a faster response to prey. The preference for fine sand could, therefore, be related to other benefits that sand provides. Finally, body mass was a dominant factor, interacting with the preference for both deep and fine sand: deep over shallow sand was more favored by large wormlions and fine over coarse sand by smaller ones. Our results suggest that several factors should be incorporated when studying microhabitat selection.

The effect of previous experience on trap construction and movement distance in a pit-building predator

Wormlion larvae are sit-and-wait predators that construct cone-shaped pits in sandy patches to capture prey. Wormlions select microhabitats that feature favorable conditions for pit construction, in a similar way to other trap-building predators, like spiders and antlions. We investigated whether wormlions exhibit an experience-based behavioral plasticity in their pit construction behavior. In a laboratory experiment, pit sizes and relocation distances were compared between larvae that experienced either a period of unfavorable conditions, i.e., surface obstacles, shallow or coarse sand, or a period of favorable conditions, i.e., clear, deep, and fine sand and were able to construct pits undisturbed. We expected that wormlions experiencing improving conditions would build larger pits than those experiencing deteriorating conditions. In addition, we expected that larvae experiencing unfavorable conditions would be less choosy in their new microhabitat and move over shorter distances. We observed a certain effect of recent experience on the trap-building behavior; however, it was not consistent among treatments. Additionally, we detected a correlation between larval body mass, relocation distance, and pit area. These findings might suggest that past experience does not influence wormlion foraging behavior in a simple manner but that different types of experience induce different behavioral responses.

Effect of substrate temperature on behavioural plasticity in antlion larvae

Temperature is of crucial importance, affecting all aspects of insect life such as survival, development and daily activity patterns, and consequently behaviour. In the present study we evaluated the effect of temperature on the behavioural plasticity of antlion larvae, the sit-and-wait predators, which are considerably more dependent on local habitat conditions. We provided ethological descriptions of pit construction and feeding behaviour. An increase in temperature led to greater activity and consequently to greater frequency of sand tossing during pit construction. Larvae constructed bigger pits at higher temperatures, but required less time than at lower temperatures, when the resulting pits were the smallest. At low temperature, larvae required more time for feeding, and behaviour followed a core pattern with little variety, in comparison to behaviour at high temperatures. Two behavioural patterns occurred only at the highest temperature: ‘relocation’ and ‘submergence’, presumably in response to high temperatures.

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.