Externally-Expressed Fluorescence across Sexes, Life Stages, and Species of Spiders

摘要

Although all spiders possess fluorophores in their hemolymph, the expression of external fluorescence is much more restricted. The purpose of this study was to evaluate differences in externally-expressed fluorescence between sexes, life stages, and species of spiders. To approach this question, we developed novel instrumentation to capture fluorescence with photographs of our specimens. We paired these fluorescence measurements with spectrometer measurements to attempt to determine the role that fluorescence plays in the overall coloration in spiders. The study was divided into four sections. First, we examined how fluorescence varies in sexes and life stages in Misumena vatia, an ambush predator that typically preys on insects when they are on flowers. We found that adult females possess brighter external fluorescence than males in all body areas that we measured. We also found that external fluorescence remains relatively similar through life stages in females, but darkens over the course of a maleu27s life. It is likely that the differences between males and females relate to differences in feeding ecology. External fluorescence may contribute to a visual signal allowing females to visually blend in with flowers. The second study involved a series of experiments to determine whether freezing spiders at a temperature of -80°C affects their fluorescence intensity. In spiders considered “white thomisids”, fluorescence intensity increased after freezing, whereas fluorescence brightness in darker-pigmented spiders did not change to any similar extent. It seems likely that tissue trauma due to freezing is the cause of increased fluorescence intensity after freezing. The third study examined fluorescence brightness across ages and life stages of Araneus diadematus, a spider which is exposed to the sun, but builds large webs in which to snare prey. We found that, unlike M. vatia, adult males are the more brightly fluorescent sex, with adult females and all immature life stages possessing significantly less bright external fluorescence. It is unclear why these differences exist, but differences in ecology between adult males and all other life stages could play a role. Additionally, dim fluorescence may contribute to subtle patterning and/or convey photoprotection benefits to immatures and adult females. In the final study, we examined external fluorescence across the Thomisidae family. Because of a relatively large number of species with a small sample size, we divided them into “white” and “dark” thomisids based on taxonomy and what is known about ecology. The white thomisids tend to be prey on insects on the exposed surfaces of flowers, whereas dark thomisids more often reside in leaf litter and crevices. We found that white thomisids fluoresce more brightly than dark thomisids. There were no differences between the sexes in either group, however. The differences between white and dark thomisids may be related to differences in feeding ecology, whereas males and females of the same group tend to have similar ecological characteristics, and also possess similar levels of fluorescence brightness.