Microhabitat selection by antlion larvae, Myrmeleon crudelis: Effect of soil particle size on pit-trap design and prey capture

Selective pressure for choosing an adequate habitat should be strong in semisedentary animals because they have limited mobility once established. I examined microhabitat preferences and the adaptive value of these preferences in the antlion larva Myrmeloen crudelis, a semisedentary insect that digs pit traps in soils to capture small arthropods. I tested the habitat preferences of M. crudelis between two soil types in a tropical dry forest of Costa Rica. Specifically, I compared the soil particle composition size within and outside antlion aggregations and manipulated the availability of fine- and coarse-grained soil to assess how differences in soil grain size affect the design and performance of larval traps. Adjacent to antlion pits the soil was composed of a greater proportion of fine- grained particles (< 2 mm) than soil 1 m away from the pits. A set of experiments demonstrated that ( 1) in the presence of equal availability of fine- and coarse-grained soils, all larvae built their pits in fine- grained soil; ( 2) the larvae required less time to start and finish traps in fine- grained soil; ( 3) the larvae constructed larger and deeper pits in fine- grained soil; and ( 4) prey capture increased greatly in fine- grained traps compared with coarse-grained traps. Antlion larvae respond to variations in the proportion of fine particles in the soil, suggesting that antlion aggregations result from an active microhabitat selection. The preference for fine- grained soils is adaptive since pits constructed in such substrate are functional for longer periods and much more successful in trapping prey than pits in coarse-grained soil. Sit-and-wait predators that use sessile traps are spatially constrained to track prey abundance. Therefore, the ability to detect and select microhabitats with better conditions that enhance capture success may be under strong selection for this type of organism.