Disadvantages of living in a populous neighborhood for sit-and-wait predators: Competition for space reduces pit-trap size in antlion larvae

The study of how trap design responds to biotic and abiotic conditions can help to understand the selective forces affecting the foraging of trap-building organisms. We experimentally tested whether pit design can be modified by intraspecific competition for space in larvae of Myrmeleon crudelis, a common sit-and-wait predator that digs conical pit traps in the soil to capture walking arthropods. In a tropical forest in Costa Rica, we measured pit dimensions, larval body size, and the level of competition (i.e., density of neighboring traps) in 40 antlion larvae. These larvae were then taken to the laboratory and allowed to build new traps in individual containers. We measured within-individual changes in the size of traps in the field and in the laboratory, and related these to the level of competition experienced in the field. Larvae with relatively high levels of competition in the field showed a greater increase in the size of their pits in the laboratory. This change was independent of larval size. Larvae with none or few neighbors in the field showed little change in their pit sizes, whereas those with higher competition levels increased their diameter and depth up to 1,400% and 1,000%, respectively. Our results demonstrate that, at least in high-density aggregations, pit design is restricted by competition in addition to the constraints imposed by body size. This work suggests that biotic interactions can play a role in the design of extended phenotypes in sit-and-wait predators that live in dense aggregations.