Sex, head size, and male mate location behavior affect allometric scaling of the eyes in Centris pallida (Hymenoptera: Apidae) bees

Abstract

Variation in eye structure and function can allow organisms to exploit novel environments or perform unique behaviors. However smaller body sizes may impose morphological constraints on eye investment, limiting adaptive behavioral performance. Males of the solitary desert bee Centris pallida (Hymenoptera: Apidae) are dimorphic in both body size and sensory mate location behaviors: large-morph males are fixed on a scent-based strategy while small-morph males generally use a sight-based strategy. Females are not known to vary categorically in mating behavior but span nearly the full range of male body sizes, allowing for sex comparisons in allometric scaling. We hypothesized that sex, body size, and male mate location behavior would impact eye structure and estimated global and localized visual acuity in C. pallida bees. We predicted that male and female body size would correlate with ommatidia number and size, and visual acuity, due to allometric constraints. We further predicted that small-morph males would have increased relative investment in the eyes and improved visual acuity compared to large-morph males. We found that males had increased ommatidia numbers and larger eyes than similarly-sized females, as well as greater average and maximum ommatidia diameters. Females had improved localized visual acuity in the dorsofrontal hotspot than small-morph males, but similar acuity to large-morph males. Body size thus impacted localized acuity in the dorsofrontal region in males but not in females. Small-morph males had increased relative eye size and ommatidia density but reduced total ommatidia numbers, eye surface areas, and estimated visual acuities compared to large-morph males.