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

Abstract

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. In contrast to widely-foraging predators, sit-and-wait predators do not search for prey. Rather, they choose an ambush location and wait for the prey to enter their detection range before attacking it 1,2. This foraging mode allows sit-and-wait predators to save the energetic costs of searching and to survive long periods of shortage of prey by reducing their metabolic rate 3-5. Ambushing prey instead of searching can also lower the risk of predation on the sit-and-wait predator, due to the positive association between searching intensity and predation risk 6-8. This foraging mode's drawback, however, lies in the low encounter rate with prey, forcing such predators to be opportunistic foragers 1,9. Sit-and-wait predators must choose their ambush sites carefully. Because sit-and-wait predators count on the prey to reach them, they often select ambush sites rich in prey in order to maximize capture success 10-12. Sit-and-wait predators are therefore attracted to abiotic cues, which are either correlated with prey abundance or facilitate prey capture 13-16. However, in many other sit-and-wait predators, considerations related to minimizing ambush costs dominate over those related to high prey abundance. For example, sit-and-wait predators select sites that limit exposure to extreme thermal conditions 17-20. Trap-building predators (hereafter, TBPs) are a subgroup of sit-and-wait predators, which construct traps to facilitate the capture of prey 21,22. The most common trap-building predator groups are the web-building spiders and pit-building antlions or wormlions, presenting case studies of convergent evolution 21,23. The selection of suitable sites for trap construction and ambush for prey is even more important for this group of p...

Evidence for competition and cannibalism in wormlions

“…Regarding the pit area, the final location in deep or shallow sand had a stronger effect than the treatment per se, while the shade/light treatments were more important for the pit area than the final location. This fits previous studies indicating that while both deep sand and shade are preferred, sand depth affects the pit area more strongly 80 . The reason is probably that while deep sand is preferred in order to construct larger pits, shade is preferred in order to avoid exposure to high temperatures and desiccation 58 , 60 .…”

Urban Pit-Building Insects Are Attracted to Walls for Multiple Reasons

“…Shade could present a better explanation for the wormlion apparent preference for walls. First, a series of lab experiments has demonstrated that wormlions prefer walls when these provide shade [45]. Second, wall positions were more shaded, at least temporarily, than positions more distant from walls at four of the seven sites examined.…”

“…A recent study has suggested that wormlions prefer to dig pits next to walls or other barriers, but no quantitative support was provided for this suggestion regarding the wormlion's natural habitat [45]. In a series of lab experiments, wormlions chose to dig pits next to walls if these provided some shelter from direct light [45]. There are four possible reasons why pit positions next to walls should be preferred: (a) walls may provide shade for at least part of the day; (b) loose soil may accumulate next to walls driven by the wind and function as a "sand fence" [46,47], resulting in deeper soil in which to dig; (c) because wind carries fine particles more easily than coarse ones [48], sand next to walls may be finer than in other locations.…”

“…A recent study has suggested that wormlions prefer to dig pits next to walls or other barriers, but no quantitative support was provided for this suggestion regarding the wormlion's natural habitat [45]. In a series of lab experiments, wormlions chose to dig pits next to walls if these provided some shelter from direct light [45].…”

Wind impairs pit trap construction and hunting success in a pit‐building predator

“…It could also be that wind is not perceived as a ‘crucial’ problem, preventing hunting only temporally. In comparison, direct light, which correlates with high temperature leading to desiccation, as well as shallow sand, inhibiting pit construction, both trigger more relocation (Scharf et al., 2020). Relocation should also depend on the risk associated with movement, such as predation or cannibalism, and the pit construction costs.…”