Positional head reflexes and the role of the prosternal organ in the walking fly,Calliphora erythrocephala

Summary

• 1. In walking flies,Calliphora erythrocephala, gravity specific stimulation of proprioceptors of the legs causes positional reflexes of the head. For rotations around the longitudinal axis this response is maximal if the transverse axis of the fly is tilted 90° relative to the horizontal. For rotations around the transverse axis the head of the fly approaches (withdraws from) the prothoracic sternum in the head-down (head-up) position when the longitudinal axis is tilted 90° relative to the horizontal. The strength of this reflex depends on the strength of the mechanical stimuli applied to the pedal proprioceptors.

• 2. In the absence of gravity dependent proprioceptive afferences of the legs these positional head reflexes are not observed. Antennal afferences enhance only the reaction induced by the pedal proprioceptors. The heads are kept constant relative to the thorax when the insects are resting.

• 3. The prosternal organ (PSO) is part of a control system that stabilizes the position of the head relative to the thorax against mechanical disturbances. It is sensitive to passive movements around the longitudinal axis, but it is insensitive to passive movements of the head around the transverse axis of the fly. In contrast to intact flies, the heads of PSO-ectomized flies loaded with a three-fold head weight can no longer be held by the neck muscles in the normal position. A comparison of the angular positions of the weighted heads of non-anaesthetized PSO-ectomized and anaesthetized flies shows that there are other sense organs in the neck besides the PSO capable of controlling the position of the head.