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

Simple Summary Wormlions are small fly larvae that dig pit-traps in loose soil to hunt ants and other prey. Their natural habitat is caves, but they are also abundant in Mediterranean cities below man-made shelters, even in thin layers of soil. They are especially common next to building walls. First, we show that wormlions are indeed closer to walls than expected by chance. Next, we tested several explanations for this observation: the possible effect of soil depth, soil particle size, shade, and prey abundance. We could not find a single explanation for the wormlion's proximity to walls, and in each site, a different set of explanations held true. The final step was to conduct an experiment. We placed wormlions on clear sand either in the center or next to the wall and observed whether they moved after a day. Those placed in the center moved over longer distances, and we interpret this result to indicate that those adjacent to the wall are more satisfied with their location. Our study provides an example for how animals take advantage of human-made changes in the habitat and prosper in urban habitats. Whereas most animals find urban habitats to be inferior to natural habitats, some urban specialist species thrive there. Wormlions present such an example. Common in Mediterranean cities, they cluster in thin layers of loose soil below man-made shelters. Wormlions are fly larvae that dig pit-traps in loose soil and hunt small arthropods. Our first aim was to determine whether wormlion pits accumulate next to walls. Wormlion pits were indeed closer to walls than expected by chance at most of the study sites. We examined possible factors behind this apparent preference, combining field observations and experiments, laboratory work, and theoretical analysis. We examined the effect of soil depth, particle size, shade, and prey abundance. Each factor provided a partial explanation for the wormlions' proximity to walls, but none provided an overall explanation. We developed a spatially explicit simulation model, demonstrating under which conditions wall-adjacent positions are favored. Finally, we created artificial microhabitats, and placed wormlions either in the center or next to the wall. The wormlions in the center moved over longer distances than those next to the wall and did so more in the wall's direction. The abundance of walls may help to explain the success of wormlions in urban habitats.

Automated summary

Wormlions are small fly larvae that dig pit-traps in loose soil to hunt ants and other prey. They are often found closer to walls, but the reasons vary by location; experimental results suggest that wormlions near walls prefer their location.