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Abstract

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.