The fine structure of the dorsal ocelli in the male bibionid fly.

The dorsal ocelli of bibionid flies, details of which have not previously been described, were examined in males of Dilophus febrilis. The three ocelli are combined within an elevated chitin capsule, in a medial position between the enlarged dorsal compound eyes. The biconvex lenses show a multiple layering of up to 150 regularly spaced, clear and dense cuticle zones (100 nm spacing) which probably provide some spectral filtering, suggested by in vivo observations with an epifluorescence microscope. The corneagenous cells and the retina with 100-200 photoreceptor cells are adjoined proximally. A distal retina zone comprises the rhabdoms, which are laterally connected in an hexagonal network. The rhabdoms are between 4 and 15 mum in length; they decrease gradually from the dorsal to the ventral retina region. A middle retina zone comprises the receptor somata, a proximal zone, their axons. Synaptic contacts between axons and interneuron dendrites, feedback synapses to axons, and axo-axonic synapses are found, showing varying pre-synaptic structures. A possible functional role of the ocelli is discussed.