Abstract
Larvae of the antlion, Myrmeleon crudelis, build conical pitfall traps in sand that are asymmetric, with lower-sloped walls in the part of the pit where the antlion resides, and steeper walls in front of the antlion. The front walls also tend to be lined with a higher density of fine sand. Simulation of movement at the pit bottom showed that this asymmetry can result from movement by the antlion. If asymmetry reduces prey capture efficiency, antlions should shift their position in the pit, and thereby reduce the impact of their movement on any given wall. Alternatively, if asymmetry is a design feature, antlions can maintain this feature by remaining in the same position for extended periods of time. Observations support the later hypothesis. Sixty-one per cent of 41 antlions tested remained in the same orientation for the entire observation time (9–14 days), and 20% remained in the same orientation for at least 5 days, then maintained a different orientation for the remainder of the experiment. Ant escape behaviour was analysed in relation to the asymmetry of the pit walls. The initial escape direction was random, but if ants moved to another part of the pit after their initial escape attempt (66% of escapes), they tended to travel to a steeper wall. The opposite occurred when ants escaped from artificial pits containing no antlions. Thus by constructing an asymmetric pit, the antlion compensates for its inability to efficiently capture prey that escape to the front of the pit. The antlion can throw sand directly at prey that attempt to escape in the back of the pit; they can also reach over their backs to capture prey, and can walk after prey in this direction. None of these behaviours is available if prey try to escape forward in the pit. A change in pit design with antlion size is discussed in light of these results.
Published Version
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