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.
Both of the spiders on this page are Whitebanded Crab Spiders (Misumenoides formosipes). The identifying characteristics are: The white ridge across the spider's face just below its eyes. (Another common name for this species is Ridge-faced Flower Spider.) The forward-pointing, V-shaped marking on the spiders abdomen. The eyes: When viewed from the front and slightly above only six eyes are visible. Four are more or less in a row right above the white ridge. Two are above the four. The spiders other two eyes are at the ends of the top ridge and are actually on the sides of the spider's "head". (BugGuide has a photo showing eye placement more clearly than mine.)
Sources and Additional Information:
University of Kentucky Entomology
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