Projection and connectivity of sex-specific photoreceptors in the compound eye of the male housefly (Musca domestica).

The structure and synaptology of the axon of a novel photoreceptor class in the compound eye of the fly (Musca domestica) were studied with the electron microscope by means of intracellular injection of horseradish peroxidase, a rapid Golgi method, and conventional electron microscopy. The photoreceptor - a sex-specific class of R7 found only in the frontal-dorsal region of the male eye - projects directly to the lamina cartridge underlying its ommatidium, thus conforming to the neural superposition scheme. At the top of the lamina cartridge the axon makes an abrupt 90 degrees bend sending out at least two arms that surround the bundle of monopolar cell fibres and extend to the limits of the cartridge defined by the incoming crown of R1--6 photoreceptors. This "foot terminal" ends in the most superficial layer of the lamina cartridge. The axon forms abundant synapses with so-called L2 and L3 monopolar cells, but very few, if any, with either L1 or T1. It also forms gap junctions with most or all of the other receptor axons in the cartridge including R8, which otherwise has the typical morphology of a long visual fibre.