Tyler Cutforth

DEEP PSEUDOPUPIL IN WHITE- AND RED-EYED FLIES

The deep pseudopupil; is an optical phenomenon arising from the regular arrangement of ommatidia in the compound eye and the nature of the rhabdomeres in Diptera, which are open and can transmit light ( N. Franceschini in Information processing in the visual systems of arthropods [ed R. Wehner], pp. 75-82, 1972). By focussing in a plane below the outer surface of the eye, virtual images of the rhabdomere patterns from several ommatidia are superimposed and the stereotypic trapezoid arrangement of photoreceptor cells R1-R7 is apparent. We use the deep pseudopupil; to score living flies for defects in photoreceptor; arrangement which may be masked by a normal external morphology.

Observation of the deep pseudopupil of red-eyed flies requires illumination from below (antidromic), using a narrow, bright light source (such as the Magnalite T-150) with a dissecting microscope and a transparent, elevated stage for the flies. Positioning the light source is a matter of trial and error, however a general guideline is to place the tips of the light pipes just inside the field of view under low power and angled upwards to illuminate the centre of the field. The pseudopupil itself is best viewed under high power (5X), with the anesthetized flies lying on their side, backs toward the observer. This position permits scoring of the posterodorsal quadrant of the eye, and the light pipes may need adjustment at this point to maximize the contrast between the pseudopupil and the surrounding eye surface. Other regions of the eye may be scored by tilting the head, but this is more problematic since slight motions of the hand blur the image. The deep pseudopupil of white-eyed flies is viewed with illumination from above (orthodromic), but in all other respects is similar to that of red-eyed flies.