Thomisus

Thomisus is a genus of crab spiders (family Thomisidae) with around 140 described species. The genus includes species that vary widely in their ecology, with some that are ambush predators that feed on insects visiting flowers. Like several other genera in the family Thomisidae, they are sometimes referred to as flower crab spiders, from their crab-like motion and their way of holding their front legs, reminiscent of a crab spreading its claws as a threat.

Description and behavior

As with most Thomisidae species, Thomisus exhibit sexual size dimorphism: females are 4 to 10 mm (0.16 to 0.39 in) in length, whereas males are only 2 to 7 mm (0.079 to 0.276 in). Many species are brightly colored, usually matching the color of the flower in which they are waiting in ambush. Not all species are flower-dwelling, but among those that are, at least some species can change their colour over a period of some days to match the flower colour. Studies suggest that bees are inclined to avoid a flower that contains a spider-sized object of a non-matching colour; whether this is specifically a mechanism for avoiding crab spiders, or simply that they are not attracted to flowers whose nectar guides are obscured however, is a more difficult question. The colour changes that such species can achieve are typically in ranges of white, pink, and yellow.

For example, in Thomisus spectabilis, the method of camouflage is similar to the Misumena vatia, though T. spectabilis are visible to their prey, but not their predators. This species of crab spiders is UV reflective while the flower is UV absorbing creating a contrast between the spider and flower through the eyes of the pollinator.