Interreceptor coupling in ommatidia of drone honeybee and locust compound eyes

Abstract

Microelectrode measurements from pairs of receptor cells in individual drone bee ommatidia show that the cells are electrically coupled, in contrast to cells in neighbouring ommatidia. By fitting the measurements to a model, it is found that coupling between drone retinulas is stronger than in the locust but weaker than in Limulus. The degree of coupling accounts satisfactorily for the difference in input resistance between Apis and Locusta cells. Spikes in the cell pairs always occur together but with occasional and reciprocal mismatch in spike size. Secondary spikes are sometimes found in one cell of a pair, and it seems likely that each cell can independently support a spike. Spikes appear to invade the soma in some cells but not others. The receptors in each fused-rhabdom ommatidium share identical receptive fields. The polarized-light (PL) response is always poor, but differences do sometimes occur between pairs of cells, as expected from the rhabdom tubule anatomy. Coupling can only partly account for the poor PL response in drone and locust cells, if there is a basic PL absorption ratio of 9:1 as in Crustacea, although strong basal coupling might explain this discrepancy. Receptors may uncouple at higher light intensities, since PL sensitivity increases slightly. Virtually all the cells are u.v.-sensitive in the central eye region examined, so that vision there is monochromatic. Lateral inhibition is absent at receptor level. Other implications of the results are discussed.