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.

Influence of rainfall regime in the Cerrado biome on the maintenance of traps built by Myrmeleon brasiliensis (Navás) (Neuroptera: Myrmeleontidae) larvae and the morphology of adults

ABSTRACT Seasonality in the Cerrado biome of Brazil is characterized by a dry season and a rainy season, affecting the availability of water and generating nutritional limitations. Thus, plants and animals have developed adaptive mechanisms in order to survive in this environment. Insects known as antlions (Neuroptera: Myrmeleontidae) occur in areas of the Cerrado and build traps in dry soil to capture prey items. In the rainy season, these insects are unable to forage due to the waterlogged soil. The aim of this study was to investigate the influence of the rainfall regime in the Cerrado on aspects of trap-building behavior, larval development and morphological characteristics of adult antlions. Larvae of the antlion Myrmeleon brasiliensis (Návas, 1914) were observed and collected in an area of the Cerrado biome in the municipality of Aquidauana (MS), Brazil. Observations were performed in the rainy and dry seasons to determine the abundance of traps built by M. brasiliensis larvae. In the laboratory, experiments were performed involving the manipulation of the frequency of simulated rain on the traps. The results revealed that variations in rainfall due to seasonality in the Cerrado affect M. brasiliensis larvae, with greater foraging observed in the dry season. The laboratory experiments demonstrated that differences in the frequency of rains affect the mortality of the larvae, larval development time and the size of the adults. Thus, variations in rainfall patterns can lead to variations in the characteristics of the population structure of M. brasiliensis in areas of the Cerrado biome in Brazil.

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.

Antlions are sensitive to subnanometer amplitude vibrations carried by sand substrates

The antlion larvae (Myrmeleontidae) are ambush predators. They detect substrate-borne vibrations induced by the movement of the prey. European pit-building antlions (Myrmeleon inconspicuus) are studied for their ability to perceive vibrations generated by the locomotion of an ant (Cataglyphis cursor) outside the pit. These strides have been recorded and copied in detail in their time sequences. The signal created was emitted by piezoelectric transducers placed several centimeters outside the peripheries of the pits: the ant movements create waves with particle accelerations that are three orders of magnitude less than g, alleviating any possibility of sand avalanche towards the bottom of the pit. Depending on the amplitude of the vibrations, the antlions answer back, generally by sand tossing. One remarkable feature is the time delay between the start of the cue and the predatory behaviour induced by this cue. This time delay is studied versus the cue amplitude. We found that antlions answer back within minutes to cues with amplitudes of nanometer range, and within seconds to these same cues if they are preceded by a sequence of signals at the Ångström amplitude. This difference in latency is used to evidence the sensitivity to vibrations at an extremely low level.

Digging the optimum pit: antlions, spirals and spontaneous stratification

Most animal traps are constructed from self-secreted silk, so antlions are rare among trap builders because they use only materials found in the environment. We show how antlions exploit the properties of the substrate to produce very effective structures in the minimum amount of time. Our modelling demonstrates how antlions: (i) exploit self-stratification in granular media differentially to expose deleterious large grains at the bottom of the construction trench where they can be ejected preferentially, and (ii) minimize completion time by spiral rather than central digging. Both phenomena are confirmed by our experiments. Spiral digging saves time because it enables the antlion to eject material initially from the periphery of the pit where it is less likely to topple back into the centre. As a result, antlions can produce their pits—lined almost exclusively with small slippery grains to maximize powerful avalanches and hence prey capture—much more quickly than if they simply dig at the pit's centre. Our demonstration, for the first time to our knowledge, of an animal using self-stratification in granular media exemplifies the sophistication of extended phenotypes even if they are only formed from material found in the animal's environment.

Increased Risk Proneness or Social Withdrawal? The Effects of Shortened Life Expectancy on the Expression of Rescue Behavior in Workers of the ant Formica cinerea (Hymenoptera: Formicidae)

In social insects behavioral consequences of shortened life expectancy include, among others, increased risk proneness and social withdrawal. We investigated the impact of experimental shortening of life expectancy of foragers of the ant Formica cinerea achieved by their exposure to carbon dioxide on the expression of rescue behavior, risky pro-social behavior, tested by means of two bioassays during which a single worker (rescuer) was confronted with a nestmate (victim) attacked by a predator (antlion larva capture bioassay) or immobilized by an artificial snare (entrapment bioassay). Efficacy of carbon dioxide poisoning in shortening life expectancy was confirmed by the analysis of ant mortality. Rescue behavior observed during behavioral tests involved digging around the victim, transport of the sand covering the victim, pulling the limbs/antennae/mandibles of the victim, direct attack on the antlion (in antlion larva capture tests), and snare biting (in entrapment tests). The rate of occurrence of rescue behavior was lower in ants with shortened life expectancy, but that effect was significant only in the case of the entrapment bioassay. Similarly, only in the case of the entrapment bioassay ants with shortened life expectancy displayed rescue behavior after a longer latency and devoted less time to that behavior than ants from the control groups. Our results demonstrated that in ant workers shortened life expectancy may lead to reduced propensity for rescue behavior, most probably as an element of the social withdrawal syndrome that had already been described in several studies on behavior of moribund ants and honeybees.

Natural aversive learning in Tetramorium ants reveals ability to form a generalizable memory of predators' pit traps

Many species of ants fall prey to pit-digging larval antlions (Myrmeleon spp.), extremely sedentary predators that wait, nearly motionless at the bottom of their pit traps, for prey to stumble inside. Previous research, both in the field and laboratory, has demonstrated a remarkable ability of these ants to rescue trapped nestmates, thus sabotaging antlions' attempts to capture them. Here we show that pavement ants, Tetramorium sp. E, an invasive species and a major threat to biodiversity, possess yet another, more effective, antipredator strategy, namely the ability to learn to avoid antlion traps following a single successful escape from a pit. More importantly, we show that this learned antipredator behavior, an example of natural aversive learning in insects, is more complicated than a single cue-to-consequence form of associative learning. That is, pavement ants were able to generalize, after one experience, from the learned characteristics of the pit and its specific location, to other pits and other contexts that differed in many features. Such generalization, often described as a lack of precise stimulus control, nonetheless would be especially adaptive in nature, enabling ants to negotiate antlions' pit fields, which contain a hundred or more pits within a few centimetres of one another. Indeed, the ability to generalize in exactly this way almost certainly is responsible for the sudden, and heretofore inexplicable, behavioural modifications of ants in response to an invasion of antlions in the vicinity of an ant colony.

Ants and antlions: The impact of ecology, coevolution and learning on an insect predator-prey relationship

A behavioural ecological approach to the relationship between pit-digging larval antlions and their common prey, ants, provides yet another example of how the specific ecological niche that species inhabit imposes selection pressures leading to unique behavioural adaptations. Antlions rely on multiple strategies to capture prey with a minimal expenditure of energy and extraordinary efficiency while ants employ several different strategies for avoiding capture, including rescue of trapped nestmates. Importantly, both ants and antlions rely heavily on their capacity for learning, a tool that sometimes is overlooked in predator-prey relationships, leading to the implicit assumption that behavioural adaptations are the result of fixed, hard-wired responses. Nonetheless, like hard-wired responses, learned behaviour, too, is uniquely adapted to the ecological niche, a reminder that the expression of associative learning is species-specific. Beyond the study of ants and antlions, per se, this particular predator-prey relationship reveals the important role that the capacity to learn plays in coevolutionary arms races.

Foraging strategy switching in an antlion larva

Antlion larvae are typically considered as trap-building predators, but some species of antlions always forage without using pits or only sometimes use pits to capture prey; they can ambush prey without pits. This study examined a species that switches its strategy between pit-trapping and ambushing and asked the mechanism behind the switching behaviour. A dynamic optimization model incorporating tradeoffs between the two strategies was built. The tradeoffs were prey capture success and predation risk (both are higher when pit-trapping). The model predicted that antlions should use the trap-building strategy when their energy status is low and should use the ambush strategy when their energy status is high. These predictions as well as an assumption (i.e., predation risk associated with pit-trapping is higher than that associated with ambushing) of the model were empirically confirmed. The results suggest that antlions flexibly switch between pit-trapping and ambushing to maximize their fitness by balancing the costs and benefits of the two strategies.

Specialized learning in antlions (Neuroptera: Myrmeleontidae), pit-digging predators, shortens vulnerable larval stage

Unique in the insect world for their extremely sedentary predatory behavior, pit-dwelling larval antlions dig pits, and then sit at the bottom and wait, sometimes for months, for prey to fall inside. This sedentary predation strategy, combined with their seemingly innate ability to detect approaching prey, make antlions unlikely candidates for learning. That is, although scientists have demonstrated that many species of insects possess the capacity to learn, each of these species, which together represent multiple families from every major insect order, utilizes this ability as a means of navigating the environment, using learned cues to guide an active search for food and hosts, or to avoid noxious events. Nonetheless, we demonstrate not only that sedentary antlions can learn, but also, more importantly, that learning provides an important fitness benefit, namely decreasing the time to pupate, a benefit not yet demonstrated in any other species. Compared to a control group in which an environmental cue was presented randomly vis-à-vis daily prey arrival, antlions given the opportunity to associate the cue with prey were able to make more efficient use of prey and pupate significantly sooner, thus shortening their long, highly vulnerable larval stage. Whereas "median survival time," the point at which half of the animals in each group had pupated, was 46 days for antlions receiving the Learning treatment, that point never was reached in antlions receiving the Random treatment, even by the end of the experiment on Day 70. In addition, we demonstrate a novel manifestation of antlions' learned response to cues predicting prey arrival, behavior that does not match the typical "learning curve" but which is well-adapted to their sedentary predation strategy. Finally, we suggest that what has long appeared to be instinctive predatory behavior is likely to be highly modified and shaped by learning.

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.

Learning in a sedentary insect predator: antlions (Neuroptera: Myrmeleontidae) anticipate a long wait

Pit-building antlions, the larvae of a winged adult insect, capture food by digging funnel-shaped pits in sand and then lying in wait, buried at the vertex, for prey to fall inside. The sedentary nature of this sit-and-wait predatory behaviour and, especially, antlions' innate ability to detect prey arrival, do not fit the typical profile of insects that possess learning capabilities. However, we show, for the first time, that learning can play an important role in this unique form of predation. In three separate experiments, individual antlions received, once per training day, either a vibrational cue presented immediately before the arrival of food or that same cue presented independently of food arrival. Signalling of food not only produced a learned anticipatory behavioural response (Experiment 1), but also conferred a fitness advantage: Associative learning enabled antlions to dig better pits (Experiments 2 and 3), extract food more efficiently (Experiments 2 and 3), and, in turn, moult sooner (Experiment 3) than antlions not receiving the associative learning treatment.

Sub-optimal pit construction in predatory ant lion larvae (Myrmeleon sp.)

The impacts on energy gains of two aspects of ant lion pit architecture were investigated in a natural population of pit-building ant lion larvae (Myrmeleon sp.) in Costa Rica. Field and laboratory settings were used to examine the impacts of circumference and depth of the pit on net energy gain rate. An optimization model predicted a point optimum circumference and angle of depression in an unconstrained system, and positive correlations between body mass, pit circumference, and pit angle of depression in the presence of physiological constraints on both measures. Such a physiological constraint is possible in this system due to a large one-time construction cost. All of these correlations were observed in a lab setting with filtered substrate and no competition; though none were significant in the field. Individuals additionally constructed wider, shallower pits in the field. These results are consistent with an angle of depression that is limited by the angle of repose of the substrate in the field, rather than physiology. These results provided suggestive evidence for sub-optimal pit dimensions in Myrmeleon sp., and for the importance of substrate type in understanding the architecture of natural ant lion pits. The model predicted that the frequency of relocation would not affect the optimal angle of depression, but it would affect the optimal pit circumference to a degree proportional to the square root of the change in the average time an ant lion occupies a single pit. These findings challenge the widely held assumption of adaptive optimality in animal foraging.

Intraspecific competition in ant-lion (Macroleon quinquemaculatus) larvae in the field

The number of larvae of the pit-digging ant-lion Macroleon quinquemaculatus at a food-poor site in Tanzania increased between May and October 1983 and then remained at high density until the following rainy season in March. Food availability was low and small larvae were most likely to suffer food shortage. Hunger level varied within and between instars: hunger was greatest after moults and only third-instar larvae weighing more than 100 mg were well fed. A simple model showed that exploitation competition was asymmetric and most likely to affect small larvae, by removing small prey. Exploitation was estimated to reduce food intake by 30-60%. Within-site differences in growth rate and size were associated with differences in larval density. Small larvae were much more likely to move than large larvae; the latter tended to occupy the periphery of the habitat. The reduction in food intake experienced by small larvae was a consequence of interference competition, which was the main factor operating at this site.