Pitfall vs fence traps in feeding efficiency of antlion larvae

Effects of high and low temperatures on the rhythmic patterns in pit-building behavior of antlion larvae

Temperature influences the survival, growth, and development of insects including the antlion. In the present study, we examined the 24-h rhythm in the pit-building behavior of the antlion larvae at three different temperature conditions, high (37°C), low (17°C), and room temperature (Control: 25.9 ± 0.2°C). We recorded pit-building activities by monitoring two variables: the Time Lag for the Initiation of Pit Reconstruction (TLIPR) after the demolition of the pit, and the Total Time for Pit Construction (TTPC). We monitored TLIPR and TTPC at four different time points with equidistant intervals (i.e. 08:00-10:00 h, 14:00-16:00 h, 20:00-22:00 h, and 02:00-04:00 h) each day over three consecutive days. We employed single Cosinor rhythmometry to evaluate the characteristics of 24-h rhythm in TLIPR and TTPC. We used one-way ANOVA to find out the effects of the temperature on TLIPR and TTPC. We detected a statistically significant 24-h rhythm in TLIPR at the group level, irrespective of the temperature conditions. 24-h rhythm in TTPC was abolished at high and low temperatures. Temperature significantly affected TLIPR and TTPC in general. It also significantly affected the mesor of the rhythms in both variables as well as the amplitude of TTPC. Finally, we noted that although high and low temperatures affected the pit-building behavior in general, the thermal conditions did not lead to a complete cessation of pit-building activities. The 24-h rhythmic pattern associated with TTPC is more susceptible to the effects of thermal regimes (17°C or 37°C) unlike that associated with TLIPR.

Heat stress during development makes antlion larvae more responsive to vibrational cues

We investigated the effects of heat stress on the responsiveness to vibrational cues, our measure of perceptual ability, in Myrmeleon bore antlion larvae (Neuroptera: Myrmeleontidae). We reared these trap-building predatory larvae under 2 heat stress regimes (mild, 30°C, and harsh, 36°C), and after they progressed from one instar stage to another, we tested their perceptual ability in common unchallenging conditions. We hypothesized that exposure to the harsh heat stress regime would impose costs resulting in handicapped vibration responsiveness. We found that the harsh heat stress regime generated more stressful conditions for the larvae, as evidenced by increased mortality and postponed molting, and the loss of body mass among larger larvae. Furthermore, among the individuals who remained alive, those originating from the harsh heat stress regime were characterized by higher vibration responsiveness. Our results suggest 2 not mutually exclusive scenarios. Costly heat stress conditions can sieve out individuals characterized by poor perceptual ability or surviving individuals can attempt to hunt more efficiently to compensate for the physiological imbalance caused by heat stress. Both of these mechanisms fit into the ongoing debate over how adaptation and plasticity contribute to shaping insect communities exposed to heat stress.

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.

Heat wave effects on the behavior and life-history traits of sedentary antlions

Research on the behavioral responses of animals to extreme weather events, such as heat wave, is lacking even though their frequency and intensity in nature are increasing. Here, we investigated the behavioral response to a simulated heat wave in two species of antlions (Neuroptera: Myrmeleontidae). These insects spend the majority of their lives as larvae and live in sandy areas suitable for a trap-building hunting strategy. We used larvae of Myrmeleon bore and Euroleon nostras, which are characterized by different microhabitat preferences-sunlit in the case of M. bore and shaded in the case of E. nostras. Larvae were exposed to fluctuating temperatures (40 °C for 10 h daily and 25 °C for the remaining time) or a constant temperature (25 °C) for an entire week. We found increased mortality of larvae under heat. We detected a reduction in the hunting activity of larvae under heat, which corresponded to changes in the body mass of individuals. Furthermore, we found long-term consequences of the simulated heat wave, as it prolonged the time larvae needed to molt. These effects were pronounced in the case of E. nostras but did not occur or were less pronounced in the case of M. bore, suggesting that microhabitat-specific selective pressures dictate how well antlions handle heat waves. We, thus, present results demonstrating the connection between behavior and the subsequent changes to fitness-relevant traits in the context of a simulated heat wave. These results illustrate how even closely related species may react differently to the same event.

Fine sand particles enable antlions to build pitfall traps with advanced three-dimensional geometry

Pit-building antlion larvae are predators that construct pitfall traps in fine sand. We used three-dimensional laser scanning and geometric morphometrics to reveal the shape of antlion pits of two antlion species, analysed the particle size composition of sands from the different natural habitats, and measured the slope angles of the pits of the two species. In most antlions, the pits are structured as a simple inverted cone, as in Myrmeleon hyalinus, studied here. The other antlion studied, Cueta lineosa, constructs a unique pit composed of two inverted truncated cones inserted into one another, which feature substantially steeper walls than the pits of any other antlion studied to date. Pit stability depends on the slope inclination, which oscillates between the maximum angle of stability and the angle of repose. The angles in C. linosa substrates were larger than those in M. hyalinus substrates. One reason for the steeper walls is the greater proportion of fine sand in the natural sand inhabited by C. lineosa However, video-recording revealed that both the natural sand of C. lineosa and the finest sand tested had a higher maximum angle of stability than any of the other substrates studied here. Furthermore, experiments with pits built in different substrates revealed that the shape of the pit is variable and depends on the structure of the sand. Myrmeleonhyalinus displayed a more flexible pit construction behaviour than C. lineosa The present demonstration of such differences in pit characteristics contributes to understanding how these two species co-exist in the same habitat.

Do pit-building predators prefer or avoid barriers? Wormlions' preference for walls depends on light conditions

Ambush site selection by sit-and-wait predators is a complex process, involving biotic and abiotic considerations, which greatly affect hunting success and costs. Wormlions are fly larvae that dig pit-traps in loose soil and hunt the arthropod prey falling into their pits. They are abundant in urban environments, found below buildings that provide cover, and many of their pits are dug adjacent to walls. We examined here under what conditions wormlions prefer to dig their pits next to walls. We analysed our dataset in two ways: frequency comparisons among the different treatment combinations and a simulation null model assuming random movement. While the frequency comparisons suggested that wormlions avoided the walls under some cases, the simulation null model suggested that a combination of shallow sand and strong light in the centre led to an attraction towards the walls, independent of the wormlions’ initial location. We suggest that wall attraction results from the certain amount of shade the walls provide. We also demonstrate that shallow sand and strong illumination are unfavourable microhabitats, either leading to more frequent movement or the digging of smaller pits. We locate our results within the broader context of sit-and-wait predators and of animals’ attraction to barriers.

The dynamics of prey selection by the trap-building predator Gasteracantha hasselti

Prey selection by generalist predators can be highly dynamic depending on the prey community structure. However, the dynamics of prey selection at the stage of prey entrapping are rarely investigated in trap-building predators, probably because their traps have been previously considered to intercept mobile prey proportionally to its availability in environment. Here we investigated the dynamics of prey selection by the orb-weaving spider Gasteracantha hasselti (Araneidae) depending on the composition of the available prey in tropical lowland forests located in north-eastern Thailand. We found that Gasteracantha captured a wide variety of prey but selected, on average, mostly Coleoptera and Diptera. The selectivity of Gasteracantha’s webs for Coleoptera was constant across the changes in overall prey availability and prey composition. The web selectivity for Hemiptera decreased rapidly with increasing relative densities of Hemiptera in the environment. The selectivity for Diptera and Hymenoptera increased and decreased, respectively, with their absolute densities in the environment. The relative selectivity of Gasteracantha’s traps for a particular prey type was driven by the presence and density of the highly selected prey rather than overall prey density. The results show that the selectivity of Gasteracantha’s traps for prey had both fixed and dynamic components and the dynamic component was determined by the relative as well as absolute densities of the particular prey types in the environment.

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.