Multi-axis niche examination of ecological specialization: responses to heat, desiccation and starvation stress in two species of pit-building antlions

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.

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.

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.

Thermal dependence of trap building in predatory antlion larvae (Neuroptera: Myrmeleontidae)

Trap-building predators remain under strong selection from thermal microenvironments. To address how soil temperature and body size affect trap building, we conducted a laboratory experiment using larvae of the antlion Myrmeleon bore at six ecologically relevant temperatures. Larger larvae built larger traps, and warmer soil led to more and larger traps. Body mass did not alter the dependence of trap building on temperature. Our results suggest that the physiological capacity of antlion larvae, which is affected by larval size and body temperature, is the major determinant of trap building. This effect should be considered when assessing interactions between antlions and prey.

Characterization of the Microbial Communities in the Ant Lion Euroleon coreanus (Okamoto) (Neuroptera: Myrmeleontidae)

Euroleon coreanus (Okamoto) is widely distributed in China, and the larval stage can be treated as traditional Chinese medicine. However, the host-bacterium relationship remains unexplored, as there is a lack of knowledge on the microbial community of ant lions. Hence, in the current study, we explored the microbial community of the larval ant lion E. coreanus using Illumina MiSeq sequencing. Results indicated that a total of 10 phyla, 126 genera, and 145 species were characterized from the second instars of E. coreanus, and most of the microbes were classified in the phylum Proteobacteria. Cronobacter muytjensii was the most abundant species characterized in the whole body and gut of E. coreanus, and the unclassified species in the genera Brevundimonas and Lactobacillus were relatively more abundant in the head and carcass. In addition, no Wolbachia-like bacteria were detected, whereas bacteria like Francisella tularensis subsp. Holarctica OSU18 and unclassified Rickettsiella were first identified in ant lion E. coreanus.

The effect of fasting and body reserves on cold tolerance in 2 pit-building insect predators

Pit-building antlions and wormlions are 2 distantly-related insect species, whose larvae construct pits in loose soil to trap small arthropod prey. This convergent evolution of natural histories has led to additional similarities in their natural history and ecology, and thus, these 2 species encounter similar abiotic stress (such as periodic starvation) in their natural habitat. Here, we measured the cold tolerance of the 2 species and examined whether recent feeding or food deprivation, as well as body composition (body mass and lipid content) and condition (quantified as mass-to-size residuals) affect their cold tolerance. In contrast to other insects, in which food deprivation either enhanced or impaired cold tolerance, prolonged fasting had no effect on the cold tolerance of either species, which had similar cold tolerance. The 2 species differed, however, in how cold tolerance related to body mass and lipid content: although body mass was positively correlated with the wormlion cold tolerance, lipid content was a more reliable predictor of cold tolerance in the antlions. Cold tolerance also underwent greater change with ontogeny in wormlions than in antlions. We discuss possible reasons for this lack of effect of food deprivation on both species' cold tolerance, such as their high starvation tolerance (being sit-and-wait predators).

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.