Visual adaptations in the night-active Wasp Apoica pallens

The apposition compound eye of the nocturnal polistine wasp Apoica pallens shows, in comparison to the closely related diurnal wasp Polistes occidentalis, specific adaptations to vision at low light intensities. When considering recent work on nocturnal and diurnal bees, general principles for dim-light vision in hymenopterans become evident: The rhabdom diameters in nocturnal bees and wasps are 4 times wider compared to their diurnal relatives, leading to wide receptive fields, which in turn account for a 25-fold higher optical sensitivity. Interestingly, the rhabdom diameters in both nocturnal bees and wasps measure 8 mu m, which may represent the maximum width for nocturnal hymenopteran apposition eyes. A ratio of 1.8 times larger eyes is present in the nocturnal bees and wasps, which in A. pallens is achieved by increasing the facet number, instead of enlarging the facets, as in nocturnal bees. Although this initially indicates spatial resolution to be important for the nocturnal wasp, the wide receptive fields of the rhabdoms will reduce its potentially high acuity. As the optical sensitivity alone cannot account for the 8 log units intensity difference between day and night, a possible role of neural summation within the first optic ganglion (lamina) of nocturnal hymenopterans is discussed.