Structure and kinematics of the prosternal organs and their influence on head position in the blowfly Calliphora erythrocephala Meig.

The blowfly Calliphora has a mobile head and various, presumably proprioceptive, sense organs in the neck region. The "prosternal organs" are a pair of mechanosensory hair fields, each comprising ca. 110 sensilla. We studied their structure (Figs. 2-4), kinematics (Figs.5, 6) and, after surgery, their influence on head posture (Figs. 7-11) in order to reveal their specific function. The hair sensilla are stucturally polarized, all in roughly the same direction, and are stimulated by dorsoventral bending of the hairs (Figs. 3, 4). This occurs indirectly by flap- movements of two contact sclerites (Figs. 3, 6); they move in the same direction during pitch turns of the head, in opposite directions during roll turns, and barely at all during yaw turns of the head (Fig. 5). Bending and arresting all hairs of one field elicits a head roll bias to the non-operated side (Fig. 7) during tethered flight in visually featureless surroundings. In contrast, shaving all hairs of one field elicits a head roll to the operated side (Figs. 8-10). The surgically induced bias of head posture is not compensated within three days (Fig. 10). Our results show that the prosternal organs of Calliphora sense pitch and roll turns of the fly's head, and control at least its roll position