Foraging behaviour of a neglected pit-building predator: the wormlion

Abstract

Wormlion larvae (Diptera: Vermileonidae) construct conical pits in fine loose soils and ambush arthropod prey. Their hunting strategy resembles that of pit-building antlions (Neuroptera: Myrmeleontidae), offering a classical example of convergent evolution, as they belong to different orders of insects. However, compared with other trap-building predators, spiders and antlions, the foraging behaviour of wormlions is almost unknown. In this study we used a combination of field observations and laboratory experiments to close this gap and investigate how hunger and ecological factors such as density and spatial pattern affect pit size, that is, how they shape investment in foraging and indicate competition between neighbouring larvae. We found slight, mainly decreasing, changes in pit size with hunger, with no change in response to prey. Surprisingly, body mass and length were not tightly correlated with pit size, unlike in antlions and spiders. Other factors, in addition to body mass and size, affect pit size, as the correlation between pits constructed in the field and in the laboratory was strong. The evidence for competition was mixed. On the one hand, we detected, in the laboratory, a change towards a regular spatial pattern with increasing pit densities, as expected, suggesting interference competition. On the other hand, we detected, in the field, a positive correlation between the sizes of neighbouring pits, and a negative correlation in the laboratory between pit size and distance to the nearest neighbour, both indicating clustering in favourable microhabitats, and not supporting strong competition. We discuss our findings in comparison with other trap-building predators and locate them within the general framework of foraging theory.