Regulation of swimming in the Culex pipiens (Diptera, Culicidae) pupa: kinematics and locomotory trajectories.

High-speed videography was used to investigate swimming kinematics and locomotory trajectories during escape responses in the pupa of Culex pipiens (Diptera, Culicidae). The pupa can perform straight-line motion despite undergoing backward and forward somersaults through 400 degrees at a rate of 20 s(-)(1). For linear motion to occur, the effective stroke of the abdomen must be delivered during the part of each cycle when the body is facing forwards, allowing passive rotation to carry the body round to the correct position for the start of the next half-stroke. On-off motion of the abdomen is regulated by a click mechanism based on stressed integumentary plates which buckle at strategic points in the cycle. The importance of self-inertia as a component in positional regulation was demonstrated by increasing the kinematic viscosity of the medium: this retarded rotation and resulted in somersaulting on the spot. Whereas normally the pupa invariably directs its swimming downwards, reversal of the usual light gradient produced upward paths as well. It is concluded that straight-line motion is generated automatically by the locomotory machine without any need for sensory feedback about body orientation during each cycle, but that swimming direction can be influenced by sensory cues such as light.