Crab spider on chokecherry blossom
The flowers that bloom in the spring (tra la!) breathe promise of . . . sudden death! We found this crab spider lurking around in the fresh blossoms of what I think is a chokecherry bush (Prunus virginiana) on May 20, 2012. To any bees or flies coming to the blossoms looking for nectar or pollen, this would be the biggest (and probably last) surprise of their lives.
She’s another example of our old friend Misumena vatia, the “goldenrod crab spider“. They are very common, and are probably the most common large crab spider that one is likely to find hanging out in blossoms in North America (and maybe in most of Europe and Asia, too, since they are found all around the Northern Hemisphere).
Their base color is white, but they can introduce a bright yellow pigment into their skins to slowly turn canary-yellow over a period of a few days. This means that they can camouflage themselves either on white blossoms, or on yellow blossoms. This ability seems to be under their intentional control; experiments have shown that they match themselves to the shade of yellow that they see, because if their eyes are covered they don’t change color. And they generally end up with a very close hue match to anything on the white/yellow spectrum.
The normal assumption is that they are camouflaged like this so that their prey won’t see them. But, there is at least one study suggesting that crab spiders have about equal hunting success whether they are matching the flower they are on or not. They have other features, like the ability to hold perfectly still, that help to keep prey from being alarmed by them. However, they are heavily preyed on by things like birds, so the camouflage is probably more important for helping the spider keep from getting eaten, than to help her get something to eat.
One last thing: if you look at the last segment of her front two pairs of legs, you can see some stiff bristles. I think these are so that she can keep a grip on her prey while she bites it in the face or neck.
 Everybody agrees that they do this, but I have yet to see a photo sequence or time-lapse actually showing them changing color. I really need to remember to do this next time I catch one. They supposedly change color more freely when they are young, so it will probably be best to find some immature specimens to try this with.