Electrophysiological analysis of fly retina. I: Comparative properties of R1-6 and R 7 and 8

Summary1.Intracellular recordings have been made from the photoreceptor classes R1–6, R 7, and R 8 in the flies,Calliphora (wild) andMusca (white).2.The half width of the angular sensitivity function (Δρ) inCalliphora R1–6 varies between 1.5° in the fovea and 3° in the lateral eye regions (Fig. 1). Light adaptation narrows these values by 20%. Both R 7 and R 8 haveΔρ values narrower than in R1–6, averaging 1.3° in foveal regions. InMusca (white)Δρ in R1–6=2.3° and in R7/8=1.5° (both values for foveal regions).3.The presence of an adapting light of sufficient intensity to activate the pupil mechanism shifts the 490 nm peak of spectral sensitivity in R1–6 towards shorter wavelengths by 40–50 nm inCalliphora but not in the pupilless mutant, white, ofMusca (Fig. 2). The shift is independent of photopigment equilibria and has a time course similar to that of the pupil closure mechanism (Fig. 3).4.There are two spectral classes of R7. Both have a single peak of sensitivity in the range 340–360 nm, but one is purely ultra-violet (UV) sensitive, having less than 10% sensitivity beyond 400 nm, whilst the other has a long tail of sensitivity extending to 500 nm. The majority of R 8 cells have a major peak of sensitivity at around 540 nm (Fig. 4).5.Polarisation sensitivity (PS) inCalliphora R1–6 averages 2.0 when measured in the green, and is not significantly affected by light-adaptation. InMusca R1–6, PS averages 1.9. In R7, PS is similar inCalliphora (average 2.2), but reaches values of up to 6 inMusca. PS could not always be detected in R8 but values of up to 3.5 have been recorded inCalliphora.6.Absolute sensitivities measured from the quantal flux required to generate a 50% maximum response using axial light of peak wavelength (APS50) are higher in R 7 and R 8 due to the higher voltage gain per quantum (Table 2).7.All receptor classes light adapt in a similar manner and continue responding to increments of intensity under the brightest adapting regimes used.8.These new results from identified receptors lead to a reappraisal of the possible roles of R1–6 and R 7 and 8 in optomotor responses elicited under different regimes of test pattern wavelength and intensity.