Do crab spiders perceive Batesian mimicry in hoverflies?

Many putative Batesian mimics only approximately resemble their supposed models, and such “imperfect” mimics are readily distinguished from defended species by humans and other vertebrates. One explanation for the existence of imperfect mimics is that the most important predators of many mimics have very different sensory and cognitive abilities from those of a typical vertebrate. In such circumstances, selection for more accurate mimicry, as perceived by humans, may be reduced. Little is known, however, about how invertebrate predators perceive and respond to mimicry in insect prey. Here, we investigate the foraging behaviour of the crab spider Synema globosum, an important predator of flower-visiting insects at our field site which frequently encounters both Batesian mimics (hoverflies - Diptera: Syrphidae) and their models (bees and wasps - Hymenoptera). In the field, we found that spiders can distinguish among dipteran and hymenopteran prey taxa, frequently attacking some models and mimics, but avoiding others. Laboratory experiments suggest that some apparently accurate mimic taxa are more likely to be avoided when spiders have prior experience of an aversive wasp model. Avoidance by spiders of black and yellow striped artificial prey suggests visual cues play a role in prey selection, but there was no evidence that olfactory cues are used to identify dangerous or noxious species. Overall, our results provide some support for the hypothesis that invertebrate predator behaviour can generate selection on visual signals in putative Batesian mimics

Evidence for Batesian mimicry in a polymorphic hoverfly

Palatable Batesian mimics are avoided by predators because they resemble noxious or defended species. The striking resemblance of many hoverflies to noxious Hymenoptera is a “textbook” example of Batesian mimicry, but evidence that selection by predators has shaped the evolution of hoverfly patterns is weak. We looked for geographical and temporal trends in frequencies of morphs of the polymorphic hoverfly Volucella bombylans that would support the hypothesis that these morphs are Batesian mimics of different bumblebee species. The frequency of the black and yellow hoverfly morph was significantly positively related to the frequency of black and yellow bumblebees across 52 sites. Similarly, the frequency of the red-tailed hoverfly morph was positively related to the frequency of red-tailed bumblebees. However, the frequencies of hoverfly morphs were positively spatially autocorrelated, and after controlling for this, only one of the two common hoverfly morphs showed a significant positive relationship with its putative model. We conclude that the distribution of V. bombylans morphs probably reflects geographical variation in selection by predators resulting from differences in the frequencies of noxious bumblebee species