Species and Sex Differences in Eye Morphology and Visual Sensitivity of Two Crepuscular Sweat Bee Species (Megalopta Spp., Hymenoptera: Halictidae)
32 Pages Posted: 23 Oct 2019 Last revised: 15 Apr 2020
Date Written: October 14, 2019
Visually dependent dim-light foraging has evolved repeatedly across taxa, broadening species ecological niches. As most dim-light foraging species evolved from diurnal ancestors, visual sensitivity must increase immensely to compensate for light levels a billion times dimmer than daylight. Some taxa, e.g. bees, are anatomically constrained by their apposition compound eyes, which function well in daylight but not starlight. However, the sweat bee genus Megalopta has incredibly sensitive eyes, foraging in light levels up to 9 orders of magnitude dimmer than diurnal relatives. Despite years of behavioral study, variation in visual sensitivity and eye morphology has not been investigated within and across different Megalopta species. We describe eye morphology for two sympatric species of Megalopta, M. genalis and M. amoena, which both forage during twilight under little light. We use electroretinograms to find that males, which are smaller than females, have increased retinal sensitivity compared to females. Although males have relatively larger eyes compared with females, morphological features of the eye were not correlated with retinal sensitivity, suggesting males have additional adaptations to improve retinal sensitivity. These findings are foundational for future work into neural and physiological mechanisms that interface with morphology to increase visual sensitivity.
Keywords: allometry, compound eye, electroretinograms, eye morphology, facets, nocturnal foraging, visual sensitivity
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