Hunting positions in a sit-and-wait predator

Modifying decisions depends on the cost of change, especially in
sit-and-wait predators. Once situated, they should remain immobile, as
position changes can disturb their traps or alert prey and predators.
Antlion larvae (Myrmeleon spp.) dig conical pits in the soil to capture
walking insects. The larva then ejects the prey carcass using strong,
backward, pendular mandible movements. Therefore, their position on the
pit influences the direction to expel carcasses and debris. We tested the
hypothesis that antlion larvae orient at the bottom of their pits to avoid
future movements, and that they decide their position based on
environmental features such as the proximity of vertical obstacles and the
ground’s slope. In the field, we dropped debris (rice grains) into pits.
The expelled debris direction was consistent as larvae maintained their
position at the bottom of their pit throughout time, and their position
was independent of the pit's distance from a wall. In the lab, we
induced larvae to build pits in narrow and sloped terraria. Larvae
oriented themselves in positions that avoid expelling objects toward a
wall or uphill, preventing them from bouncing off or rolling back into the
pit. These results indicate that larvae use information about their
surroundings to determine how to position themselves at the pit to avoid
having to move if the pit gets compromised.