Fine sand particles enable antlions to build pitfall traps with advanced three-dimensional geometry

Abstract

Pit-building antlion larvae are predators that construct pitfall traps in fine sand. We used three-dimensional laser scanning and geometric morphometrics to reveal the shape of antlion pits of two antlion species, analysed the particle size composition of sands from the different natural habitats, and measured the slope angles of the pits of the two species. In most antlions, the pits are structured as a simple inverted cone, as in Myrmeleon hyalinus, studied here. The other antlion studied, Cueta lineosa, constructs a unique pit composed of two inverted truncated cones inserted into one another, which feature substantially steeper walls than the pits of any other antlion studied to date. Pit stability depends on the slope inclination, which oscillates between the maximum angle of stability and the angle of repose. The angles in C. linosa-substrates were larger than those in M. hyalinus. One reason for the steeper walls is the greater proportion of fine sand in the natural sand inhabited by C. lineosa. However, video-recording revealed that both the natural sand of C. lineosa and the finest sand tested had a higher maximum angle of stability than any of the other substrates studied here. Furthermore, experiments with pits built in different substrates revealed that the shape of the pit is variable and depends on the structure of the sand. M. hyalinus displayed a more flexible pit construction behaviour than C. lineosa. The present demonstration of such differences in pit characteristics contributes to understanding how these two species co-exist in the same habitat.