Crab spiders can scuttle, but apparently they can’t hide.
sciencenewsLong touted as an example of cryptic coloring, the female Misumena vatiaspider switches her body color over the course of days depending on the flower where she lurks. Contrary to the textbook scenario, though, a white spider on a white flower doesn’t catch more prey than a white spider moved to a yellow flower, researchers report online November 3 in Proceedings of the Royal Society B
Nor does a yellow spider on a yellow flower get a color-coordination bonus, says study coauthor Rolf Brechbühl of the University of Fribourg in Switzerland. He and his colleagues reached this conclusion after videotaping some 2,000 occasions when an insect buzzed over to a flower that held a spider. Sitting on a bloom ready to pounce on pollinators, the spider supposedly shifts to match her background by switching between white and yellow. To human eyes, she looks as if she’s becoming harder for her prey to see.
The study “finally shatters the myth of crypsis by color matching in crab spiders,” comments behavioral ecologist Marie Herberstein of Macquarie University in Sydney, who was not part of the study. “I suspect that textbooks may now need to be rewritten.”
crabspider2Color changing probably has some adaptive benefit for the spiders, according to ecologist Thomas C. Ings of Queen Mary University of London. What those benefits might be still isn’t clear, he says, “but this paper is exciting, as it shows that we may be focusing our attention in the wrong direction.”
Another possible direction — protection from the spider’s own predators — also doesn’t look encouraging in the new study. Brechbühl says that his research focused on spider prey, but he points out that all this videotaping took place in a field with plenty of birds and other possible menaces around. Even though he frequently moved spiders to flowers of the wrong color, he recorded only one predator (a bird) nabbing a spider.
Ideas about crab spider coloration have been unraveling since 2001 when Lars Chittka, also of Queen Mary, pointed out that bees see ultraviolet wavelengths but that non–UV-reflecting spiders often sit on UV-reflecting flowers.
To test for an effect of color on M. vatia crab spiders’ hunting, Brechbühl and his colleagues set up clusters of yellow, white and violet wildflowers in a field. The researchers filmed each spider for three days, tallying all potential prey. Spiders caught only 3.5 percent of insect visitors, and in terms of volume of insect meat, color-coordination didn’t make a difference to the catch.
Musing about other possible benefits of color changing, Ings notes that only adult females change color. “So is there a specific advantage to crypsis in mature females about to lay eggs?” he says. Or perhaps the color change worked against other predators or prey in the past and has not been lost.