Rapid swimming and escape movements in the aquatic larvae and pupae of the phantom midge Chaoborus (Diptera, Chaoboridae)

Abstract

Abstract Rapid locomotion in the aquatic larvae and pupae of the phantom midge Chaoborus was analysed. A 10 mm long larva moved sporadically by rapidly curling into a tight circle and then unfurling. The most common movement (70% of all movements) was a body rotation of 332 ± 22 degrees that lasted 63 ± 19 ms and reached a peak velocity of 0.07 ± 0.02 m s-1. If the head unfurled earlier in the cycle, the rotation was smaller and the larva dived downwards. A distinct category of single rotations of about 180 degrees (8%) resulted in a larva finishing with its head pointing in the opposite direction. A sequence of rotational movements (22%) resulted in more extensive displacements. The area of the tail fan was reduced by folding during part of a cycle. It was made of a row of 26 radiating filaments with interlacing hairs between adjacent filaments and resilin at their ventral midline articulations with the body. The fan sprang back passively to its splayed position after being forcibly folded. Reducing the area of the fan by 80% decreased angular rotation and impaired stability so that 33% of movements ended with the body upside down. A 6 mm long pupa also moved by curling and unfurling motions of the head and tail that lasted 215 ± 19 ms and generated slower velocities of 0.03 ± 0.01 m s-1. The pupal tail fan was membranous, oriented differently, had resilin at its articulations and its area could be changed.