Comparison of wormlion behavior under man-made and natural shelters: urban wormlions more strongly prefer shaded, fine-sand microhabitats, construct larger pits and respond faster to prey

Behavioral differences between pit-building antlions and wormlions suggest limits to convergent evolution

Antlions and wormlions are distantly related insect taxa, both digging pits in loose soil and ambushing arthropod prey. Their hunting method, which is rare in the animal kingdom, is a clear example of convergent evolution. There is little research directly comparing the 2 pit-building taxa. Using the same experimental platform to investigate how they respond to biotic and abiotic environmental factors enables an examination of their convergence and its limits. We investigated the response of antlions and wormlions to 3 factors common in their daily life: disturbance to the pits, prey arrival, and conspecific competitors. Although both increased the pit size following disturbance, wormlions increased it faster than antlions. Antlions responded to prey faster than wormlions, but wormlions improved their response time over days. The most diverging response was toward conspecifics. Whereas antlions relocated their pits fast in response to increasing conspecific density, wormlions never relocated. We suggest explanations for the behavioral differences between the taxa. Our results imply that despite the similar hunting method of the 2 taxa they may differ greatly in their behavior, which in turn might have consequences for their habitat use and population dynamics.

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.

Abstract

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.

Adaptive Evolution in Cities: Progress and Misconceptions

Current narratives suggest that urban adaptation - the adaptive evolution of organisms to cities - is pervasive across taxa and cities. However, in reviewing hundreds of studies, we find only six comprehensive examples of species adaptively evolving to urbanization. We discuss the utility and shortcomings of methods for studying urban adaptation. We then review diverse systems offering preliminary evidence for urban adaptation and outline a research program for advancing its study. Urban environments constitute diverse, interacting selective agents that test the limits of adaptation. Understanding urban adaptation therefore offers unique opportunities for addressing fundamental questions in evolutionary biology and for better conserving biodiversity in cities. However, capitalizing on these opportunities requires appropriate research methods and dissemination of accurate narratives.

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.