J Comp Physiol A (1992) 171:483-493

The blowfly Calliphora has a preferred flight attitude from which it may transiently deviate either actively by voluntary aerobatics or, passively, by air turbulence. In order to regain its normal flight attitude the fly performs simultaneously a corrective flight manoeuvre and a compensatory head turn. This is made possible because Calliphora can turn its head about all 3 principal axes of its body: yaw turns about the vertical axis and pitch turns about the transverse axis range between ± 20°, and roll turns about the long body axis may be up to ± 90°. Compensatory head movements stabilize the fly's eyes relative to the surroundings and thus provide for optimal vision by prevention of motion blurring, by the correct placement of the visual scene an the retina, and by the proper alignment of visual objects and visual motions with the retinal coordinate system (Hengstenberg et al. 1986; Hengstenberg 1991). The fly's head is in an ambiguous situation : on the one side it is coupled by visual links to the surroundings, and an the other side it is contiguous with the trunk. Conflicting orientations or error accumulation may lead to a large misalignment between head and trunk, especially in the roll direction, and may require correction. Various types of mechanical sense organs have been described in the neck region of different insect orders. They are present in varying numbers and arrangements, and their function is, in most cases, only partially known (reviews: Hoffmann 1963, 1964; Thurm 1965a, b; Hengstenberg 1992). Calliphora has two mechanoreceptive hair fields, the "prosternal organs (PO)" an the ventral side of the neck (Lowne 1895), a pair of prothoracic chordotonal organs (Vater 1961), several singly placed bristles, a small hair field an the prothorax (Milde et al. 1987) and a few large bristles on the vertex of the head which extend backwards and touch the thorax when the head is held in normal position (Theiss 1979). The prosternal organs are located in a ventromedial neck fold (Fig. 2); each hair field consists of about 110 Summary. 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 an head posture (Figs. 7-11) in order to reveal their specific function. The hair sensilla are structurally polarized, all in roughly the same direction, and are stimulated by dorsoventral bending of the hairs (Figs. 3, 4). This occurs indirectly by flapmovements 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.