Crab Spider, any member of several families of free-living spiders that resemble small crabs. Crab spiders have short, wide, flattened bodies. The first two or three pairs of legs are longer than the rest and are normally held out from the sides of the body as a crab would hold its claws. Some crab spiders can walk forward, backward, or sideways like fiddler crabs. Worldwide there are about 2000 known species of crab spiders, 200 of which occur in North America.
Most crab spiders are less than 1 cm (0.4 in) in length, although the giant crab spider may reach 2.5 cm (1.0 in). Crab spiders do not spin webs to trap prey, but hunt on the open ground or on vegetation or flowers. In this, they resemble other free-living spiders such as jumping spiders and wolf spiders. Unlike other free-living spiders, however, all of a crab spider's eyes are small and serve primarily as motion detectors. Typical crab spiders are predators that lie in wait to ambush their prey. Though their chelicerae, or jaws, are rather small and slender, many crab spiders possess potent venoms that quickly immobilize their prey. Flower spiders, a particular type of crab spider, rest on flowers and remain motionless for long periods of time with their front two pairs of legs extended in readiness. They ambush butterflies, bees, flies, and other flower visitors; their venoms enable them to successfully attack insects much larger than themselves. They do not wrap their prey in silk after biting, but instead remain with the immobilized prey until they have sucked it dry.
In keeping with their ambush style of attack, many crab spiders are well camouflaged, blending in with their backgrounds. Some resemble tree bark, leaves, or fruits; others appear to mimic bird droppings. Some of the flower spiders are able to change their color over several days, typically between white and yellow, depending on the color of the flower on which they are resting. A common North American species is the goldenrod spider. The giant cockroach hunter is a warm-climate species which often moves northward on shipments of bananas.