Soil particle size and the pit morphology of the Australian ant-lions Myrmeleon diminutus and M. pictifrons (Neuroptera: Myrmeleontidae)

A comparison of soil particle size and its influence on pit construction, pit morphology and capture success for Myrmeleon diminutus and M. pictifrons was undertaken. Both species constructed pits by the circuitous method, using the forelegs. The forelegs may aid in encouraging soil particles to fall from the forming pit wall and may help to direct soil onto the dorsal surface of the head for subsequent tossing. Both species showed a preference to build in fine soil (<0.3 mm) but the aversion to medium (0.3-0.6 mm) and coarse soil (0.6-1.2 mm) differed. In mixed particle size soil all ant-lion pits were lined with fine soil particles less than 0.3 mm diameter. This improved capture success by delaying the escape of prey and increasing the number of falls to the pit vertex during escape. Body weight was found to be the most important correlate of pit morphology. Pit diameter and pit depth increased linearly and pit angle increased curvilinearly with body weight. The smallest ant-lions optimize trap design by building pits close to the angle of repose of the soil, but larger ant-lions build relatively shallower, less optimal pits.