The researchers also modeled what the insects would see in their visual systems, and found that the luckless prey may not be able to spot the difference between a hungry spider and actual bird droppings.
Not that we humans may do much better.
“Many people would not be able to even distinguish a spider from a bird dropping,” said Stano Pekar, a zoologist at Masaryk University in the Czech Republic who was not involved in the study and said its results were impressive. “I mean, they really have a very good masquerade.”
The findings have opened new questions on how the dung deception evolved. Other species of crab spiders bear different patterning and proportions of white and black on their bodies, which may affect how convincing their disguise is to insects, Dr. Li said. (The more “typical” species of crab spider are green and white, allowing it to blend into leaves; they also don’t smell like bird droppings and attract far fewer flies.)
Other animals have also evolved to masquerade themselves as inedible or inanimate objects for predator protection — larvae of early thorn moths appear like twigs and dead-leaf butterflies look like, well, dead leaves. But researchers rarely investigate whether coloring tricks can serve multiple functions in the same species. That could change, Dr. Pekar said.
“I think in the future,” he said, “we will see many more cases where both the coloration or the pattern will be both defensive and offensive.”