Rhabdomeric twist in bees — Artefact or in vivo structure?

Summary

It has recently been claimed (Ribi 1979, 1980) that “bee ommatidia show no twist” and that our earlier finding of rhabdom twist in bees (Wehner et al. 1975; forMyrmecia ants see Menzel and Blakers 1975) had been due toartefactual twisting of the rhabdoms. In this paper we present new histological data substantiating our previous hypothesis that bee rhabdoms twist in vivo. In addition, we demonstrate that Ribi's longitudinal section (Ribi 1979; Fig. 1) through part of a bee's rhabdom does not provide compelling evidence against the hypothesis of rhabdomeric twist.

• 1. Rhabdomeric twist is found in preparations in which special care was taken not to cut the eye (glutar-aldehyde fixation) or even the whole head (acrolein fixation) during the processes of preparation and fixation. Thus, changes in intraocular pressure that might be one cause of artefactual twist have been reduced to a minimum in both the whole-eye and the whole-head preparations. On the other hand, even when the retinal tissue is mechanically manipulated, e.g. by compressing the compound eye or by excising the retina, the rhabdoms twist as in the usual preparations. The direction of twist (clockwise or counter-clockwise) is always correlated with the geometry of the rhabdom (X-type rhabdoms or Y-type rhabdoms). The twist rate is about the same in the whole-eye preparations, the manipulated-eye preparations, and the isolated-retina preparations. Besides the absence of structural pecularities like disrupted membranes or enlarged intercellular clefts, there is some further support for the hypothesis that artefactual twist does not occur: Slender processes of the secondary pigment cells invade the intercellular clefts between adjacent photoreceptor cells but never show any signs of being sheared off or distorted. Furthermore, membrane junctions can regularly be found between secondary pigment cells belonging to different retinulae.

• 2. Ribi's (1979) electron micrograph presenting a relatively short longitudinal section through a bee's rhabdom does not allow for the conclusion that the rhabdoms of bees are straight. This is mainly a consequence of the fact that the section of the rhabdom shown in his figure was cut at about right-angles to its microvilli instead of parallel to them. It follows from simple geometrical considerations that the cut microvilli can appear rather circular even in the presence of substantial amounts of twist.