Retrograde labelling of photoreceptors in different regions of the compound eyes of bees and ants

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Abstract
The geometry of retinal receptor arrays and the projection patterns of photoreceptor axons are unravelled in the compound eyes of bees and ants by backfilling large populations of photoreceptors with horseradish peroxidase or Lucifer yellow. Such retrograde labelling techniques are applied in the insect retina for the first time. They brilliantly label populations of specific receptor types within more than 100 ommatidia. Such preparations are obtained in three distinctly different parts of the eye: (1) the part of the retina that is positioned at the uppermost dorsal margin of the eye and specialized for the detection of polarized skylight; (2) the remainder of the dorsal retina; and (3) the ventral retina. As shown by the large populations of labelled photoreceptors, the retinal receptor arrays differ strikingly between different parts of the eye. Furthermore, there exist similarities as well as marked differences between bees and ants. Former hypotheses concerning the location of photoreceptor terminals in the first and second visual neuropil have all been based on a few individual Golgi-labelled photoreceptors. The results presented in this paper, based on retrograde mass impregnations of selected types of photoreceptors, confirm some of the former hypotheses and reject others. The new findings are important for understanding how insects analyse polarized skylight.