Crab Spiders

Lore & Background

Members of this family of spiders are ambush predators and do not spin webs. The two front legs are usually longer and more robust than the other legs. The back two legs are smaller, and are usually covered in a series of strong spines. Their coloration may be dull with colors such as brown or grey, or very bright green, pink, white or yellow to match their flower. These spiders are quite easy to identify, though can occasionally be confused with the usually larger huntsman spiders. The spiders of Thomisidae are not known to be harmful to humans. However, spiders of a distantly related genus, Sicarius, which are sometimes referred to as "crab spiders", or "six-eyed crab spiders", are close cousins to the recluse spiders, and are highly venomous, though human bites are rare.

In Their Own Story

Several different types of sexual dimorphism have been recorded in crab spiders. Some species exhibit color dimorphisms; however, the most apparent dimorphism is the difference in size between males and females. In some species, this is relatively small; females of Misumena vatia are roughly twice the size of their male counterparts. In other cases, the difference is extreme; on average, female Thomisus onustus are more than 60 times as massive as the males. Several hypothesized explanations are given for the evolution of sexual size dimorphisms in the Thomisidae and other sister taxa. The most widely acknowledged hypothesis for female growth is the fecundity hypothesis: selection favors larger females so they can produce more eggs and healthier offspring. Because males do not carry and lay eggs, a growth in size does not confer a fitness advantage. However, sexual size dimorphism may be a result of male dwarfism. The gravity hypothesis suggests that the smaller size allows the male to travel with greater ease, providing him with an increased opportunity to find mates. Females are comparatively stationary, and their larger size allows them to capture larger prey, such as butterflies and bees, granting females the additional nutrients necessary for egg production. Other hypotheses propose that sexual size dimorphism evolved by chance, and no selective advantage exists to larger females or smaller males.

Reader's Guide

Thomisidae do not build webs to trap prey, though all of them produce silk for drop lines and sundry reproductive purposes; some are wandering hunters and the most widely known are ambush predators. Some species sit on or beside flowers or fruit, where they grab and kill visiting insects. Individuals of some species, such as Misumena vatia and Thomisus spectabilis, are able to change color over a period of some days, to match the flower on which they are sitting. Some species await prey among leaves or bark, and some species are excellent mimics of bird droppings and sit out in the open; these species convergently evolved their mimicry with the so-called bird-dropping spiders of Araneidae. Other species of crab spiders with flattened bodies either hunt in the crevices of tree trunks or under loose bark, or shelter under such crevices by day, and come out at night to hunt. Members of the genus Xysticus hunt in the leaf litter on the ground. In each case, crab spiders use their powerful front legs to grab and hold on to prey while paralysing it with a venomous bite. The spider family Aphantochilidae was incorporated into the Thomisidae in the late 1980s; Aphantochilus species mimic and prey upon Cephalotes ants.

Did You Know?

• The common name crab spider is often linked to species in the Thomisidae, but may also be used for spiders from other families, including Sparassidae (giant crab spiders), Selenopidae (wall crab spiders), and Sicariidae (six-eyed crab spiders).
• Some distantly related orb-weaver spider species such as Gasteracantha cancriformis are also sometimes called crab spiders.
• Females of Thomisus onustus are on average more than 60 times as massive as the males.
• Individuals of some species, such as Misumena vatia and Thomisus spectabilis, are able to change color over a period of some days to match the flower on which they are sitting.
• The spider family Aphantochilidae was incorporated into the Thomisidae in the late 1980s; Aphantochilus species mimic and prey upon Cephalotes ants.