Visual fields and optical properties of the ocelli of the blowfly Calliphora

Apart from the compound eyes Calliphora vicina (= C. erythrocephala) has three ocelli, each with a cuticular lens. The retina of the lateral ocellus contains 200 300 receptor cells (Rosser, unpublished). In Musca domestica the photoreceptors of the three ocelli are coupled to the dendritic tufts of 12 large L-neurons, 8 of which make functional contact with a pair of DNOVS-neurons situated in the ventrolateral protocerebrum (Bassemir 1986). These neurons could contribute to the control of head attitude (Strausfeld, Bassemir 1985). In C. vicina L-neurons have terminals also in the ventrolateral protocerebrum (Nassel, Hagberg 1985); the role of ocellar input in this area remains to be elucidated: Hengstenberg (1985) points out that during flight the influence of the ocelli on head movements around the roll axis is negligibly small, and there is no further behavioural evidence for ocellar control of head posture in C. vicina. In optical studies Cornwell (1955) showed that the visual fields of ocelli in C. vicina are not directed towards the horizon as in locusts (cf. Wilson 1978) but are pointing upwards. We illuminated the ocelli from different directions and could confirm Cornwell's results by analysis of the simultaneously recorded electroocellogram: 1. The angular widths of the visual fields of both the median ocellus and the lateral ocelli are about 180°. 2.In the walking animal an image of the frontal horizon is formed within the visual field of the median ocellus. The horizon is imaged about 45° above its lower border. The lateral horizon is seen by the median ocellus and by the lateral ocellus on the respective side. The horizon is imaged about 15° above the lower border of their visual fields. 3. During flight the head is tilted backward and the borders of the visual fields are shifted relative to the horizon. The head of animals flying in a cage of 50 x 50 x 50 cm is tilted by 29° (±15° s.d.). At a pitch angle of 29° the angles between the axes of the ocelli and the vertical are about 30° 45°.During flight the combined visual fields of the 3 ocelli cover the whole celestial hemisphere. This leads us to assume that ocelli of Calliphora are functionally different from those of locusts and dragonflies which are mainly used as horizon detectors mediating compensatory head and body movements (Taylor 1981; Stange, Howard 1979).