The optomotor response and spatial resolution of the visual system in male Xenos vesparum (Strepsiptera)

The Strepsiptera are an enigmatic group of parasitic insects whose phylogenetic relationships are hotly debated. Male Strepsiptera have very unusual compound eyes, in which each of a small number of ommatidia possesses a retina of at least 60 retinula cells. We analysed the optomotor response of Xenos vesparum males to determine whether spatial resolution in these eyes is limited by the interommatidial angle or by the higher resolution potentially provided by the extended array of retinula cells within each ommatidium. We find that the optomotor response in Strepsiptera has a typical bandpass characteristic in the temporal domain, with a temporal frequency optimum at 1-3 Hz. As a function of spatial wavelength, the optomotor response is zero at grating periods below 12° and reaches its maximum strength at grating periods between 60° and 70°. To identify the combination of interommatidial angles and angular sensitivity functions that would generate such a spatial characteristic, we used motion detection theory to model the spatial tuning function of the strepsipteran optomotor response. We found the best correspondence between the measured response profile and theoretical prediction for an irregular array of sampling distances spaced around 9° (half the estimated interommatidial angle) and an angular sensitivity function of approximately 50°, which corresponds to the angular extent of the retina we estimated at the centre of curvature of the lens. Our behavioural data strongly suggest that, at least for the optomotor response, the resolution of the strepsipteran compound eye is limited by the ommatidial sampling array and not by the array of retinula cells within each ommatidium. We discuss the significance of these results in relation to the functional organisation of strepsipteran compound eyes, their evolution and the role of vision in these insects.