Crab spiders are ambush predators. They sit in strategic locations, waiting for prey to arrive. Some species of crab spiders have evolved to blend in with bark and leaf litter. Others are more brightly colored and "hide" in flowers.
Typically a female crab spider lays her eggs in the fall and they hatch the following spring. The spiderlings spend the summer eating and growing. They will eat just about any insect that happens to come within the grasp of their powerful forelegs (including other spiderlings). By late summer and fall, the mature crab spiders become more conspicuous. Most of the crab spiders you see are females. Males are much smaller and marked differently.
Researchers say the venom of some crab spiders is more potent than most other spiders. This allows the relatively small spiders to quickly immobilize much larger prey. However, crab spider venom is not know to be especially harmful to humans, and unless you trap a crab spider in your hands and began squashing, you're highly unlikely to be bitten in the first place.
Most sources say crab spiders can slowly change their color to match their surroundings, although one claims that color is determined by where the females lays her eggs. I don't know, but I find it much easier to believe a crab spider can slowly adapt its color to its surroundings.
One thing I've noticed, but didn't find mentioned is that crab spiders often curl over a petal or two on a composite flower to help conceal themselves.
Three genera of flower crab spiders are common in North America. They can be a little tricky to identify because all three look similar and there can be considerable variation in color and markings within each genus. The ultimate determining factor in identifying these crab spiders is the position of their eight eyes. BugGuide has and excellent comparison article for helping with identification.
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Whitebanded Crab Spider (Misumenoides formosipes)
