Fig. 5.

Retinal substrate for visual guidance of polarotactic horseflies. (A) Compound eye of a T. bromius female is composed of a stochastic mosaic of ommatidia with horizontal (H) or vertical (V) distal microvilli of R7, analogous to the fly pale and yellow ommatidia. (B) Proposed scheme of horsefly visual guidance. Polarotactic attack of a horsefly, seeking a blood meal, is triggered by multistaged intraommatidial and interommatidial opponent processing of visual signals. The attack is facilitated by the short-wavelength polarized light, detected by the orthogonal analyzer pair of UV-sensitive R7 and UV- and blue-sensitive R8 in H or pale ommatidia, inhibited by the long-wavelength light, detected by the opponent pair of UV-sensitive R7 and UV- and green-sensitive R8 in V or yellow ommatidia. The attack is guided toward a dark object, which is detected by the outer photoreceptors R1–6 that sample the same direction in the visual space as the inner photoreceptors. ∣Δ∣, rectified difference; Δ, difference; ×, multiplication.