Deerflies (Chrysops sp). strongly rely on vision. Females use it primarily to search
for prey and water bodies they need for reproduction. The species of the Chrysopsinae
subfamily have not yet been studied in detail, so we analysed it with microscopic and
electrophysiological techniques. The retina of deerflies is sensitive mainly to blue light and
not to green, as is often the case with younger taxonomic groups from the horsefly family
(fam. Tabanidae). Polarization sensitivity was rare in peripheral photoreceptors, but was
observed in the central photoreceptor, so we conclude that the role of the former is the
perception of motion and the latter the polarization of light. Fluorescence of photoreceptors
indicates that two types of ommatidia are found in the retina – pale and yellow. These
two types coincide with the orientation of rhabdomere R7, which is horizontal in the pale
ommatidial type and vertical (parallel to the dorsoventral eye axis)in yellow. Both types of
ommatidia are arranged stochastically in retina. The cornea of deerflies is of characteristic
metallic green colour with dark spots. We assumed that its coloration affects the detection
of light, but we refuted this hypothesis, as we were unable to detect such an effect with
electrophysiological measurements. Also, corneal staining is not related to the cellular
composition of the retina, as both types of ommatidia are distributed irrespective of the
corneal colour. The stained cornea therefore has no optical role, is not related to the structure
of the retina, and probably has a signalling role in visual communication.
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