Do pit-building predators prefer or avoid barriers? Wormlions

www.nature.com/scientificreports OPEN Do pit‑building predators prefer or avoid barriers? Wormlions’ preference for walls depends on light conditions Inon Scharf*, Akiva Silberklang, Bar Avidov & Aziz Subach 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 it1,2. 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 r ate3–5. 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 risk6–8. This foraging mode’s drawback, however, lies in the low encounter rate with prey, forcing such predators to be opportunistic foragers1,9. 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 success10–12. Sit-and-wait predators are therefore attracted to abiotic cues, which are either correlated with prey abundance or facilitate prey c apture13–16. 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 conditions17–20. Trap-building predators (hereafter, TBPs) are a sub-group of sit-and-wait predators, which construct traps to facilitate the capture of p rey21,22. The most common trap-building predator groups are the web-building spiders and pit-building antlions or wormlions, presenting case studies of convergent e volution21,23. The selection of suitable sites for trap construction and ambush for prey is even more important for this group of predators than for other sit-and-wait predators, owing to the investment in trap construction. This investment is expressed in time, higher metabolic rate during construction and in self-production of building materials in spiders22,24–27. Due to the high investment in trap construction and the risk in movement, most TBPs do not easily relocate their trap after building it28,29, which makes the choice of trap location even more important. When the trap-building predators are larvae (antlions and wormlions), the ovipositing female is the one choosing the initial location of the trap. While she can choose the most suitable s ubstrate30, this does not always hold true31. It is therefore School of Zoology, Faculty of Life Sciences, Tel Aviv University, 69978 Tel Aviv, Israel. *email: Scientific Reports | (2020) 10:10928 | https://doi.org/10.1038/s41598-020-67979-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  (a) Photos of wormlion pits in an urban habitat (Tel Aviv University, photographed by IS). Wormlion pits are often constructed adjacent to building walls. (b–d) Schemes of the experimental designs. We used arenas of 15 × 15 cm for all experiments. Wormlions were always placed individually in each arena. (b) Experiment 1: wormlions were placed either in shallow sand (0.5 cm depth; left) or deep sand (2 cm depth; right), either in the arena centre (dashed arrow) or next to the arena wall (continuous arrow). Experiment 3 was designed similarly to Experiment 1, but instead of a sand-depth treatment with two levels, we either exposed arenas to full light or covered them to induce full shade. (c) Experiment 2: The arena included a stripe of deep sand (rectangular grey area) surrounded by shallow sand. Wormlions were placed either in deep sand (left) or shallow sand (right). They were either placed next to the wall (continuous arrow) or in the arena centre (dashed arrow). Experiment 4 was designed similarly to Experiment 2, but instead of a sand-depth treatment, we used a rectangular-shaped cover providing full shade (the grey area) surrounded by full exposure to light in the rest of the arena. (d) The follow-up experiment comprised two treatments: the arena was fully exposed to light or one of its walls provided a 3-cm stripe of shade. Wormlions were individually placed 3 cm from the wall (arrows). important for the larvae to be able to later relocate, differentiate between possible options, and choose the most suitable microhabitat for trap construction. Similar to other sit-and-wait predators, TBPs balance their selection of ambush sites between the need to capture prey and the need to avoid unfavourable abiotic conditions, such as exposure to extreme temperatures or sites that restrict the ability to construct a functional trap32,33. For such reasons, TBPs often prefer specific natural or artificial structures. For example, web-building spider species prefer vegetation of a specific height, thorny plants or vegetation of a specific complexity l evel34–36. TBPs are sometimes attracted to natural barriers, such as rocks, cliffs or tree t runks37–40. Similarly, some spiders are more frequently found next to or prefer to construct their webs on man-made walls or f ences41–44. Such a preference for natural or artificial barriers has been explained by better thermal/humidity conditions next to the barrier, the reflection of light from a bright barrier that attracts potential prey, protection against rain, or help in the stabilization of the trap. Wormlions are common pit-building predators in the Mediterranean region and are highly abundant in cities23,45. Wormlion pits in urban environments are often attached to or constructed close to w (...truncated)