Photoreceptor physiology of two species of crab spiders (Araneae: Thomisidae)

Spiders are a diverse order of predatory arachnids with more than 53.000 described species, most of which have eight eyes. Many webless hunting spiders, most noticeably the jumping spiders (Salticidae) have been shown to have excellent eyes with high spatial resolution and colour vision. The family of crab spiders (Thomisidae) is also hypothesized to be visual hunters, employing a “sit and wait” or ambush hunting technique; however, little is currently known about their visual capacity. Here we use extracellular electrophysiology to examine the photoreceptor physiology of two crab spiders living in two different ecological niches. Ozyptila praticola (C.L. Koch, 1837) hunts on the ground in dim habitats whereas Xysticus cristatus (Clerck, 1757) hunts in the typical bright open grasslands. We test the hypotheses that (1) each species has special-purpose eyes, (2) that males and females have different photoreceptor physiology based on observed differences in behavior, and (3) the niche difference of the two species is reflected in different visual properties. We found support for the first hypothesis and also some support for differences between the two species, mostly in the temporal resolution of the eyes. We found no evidence of differences between males and females, indicating that the two sexes exhibit more shared visually guided behaviors than previously thought.