If you're going to move into someone's house and eat their children, it pays to be discrete. Predators that live in ant colonies, called myrmecophiles, get away with this because they smell, look, and behave just like ants. A new study shows how an Australian spider has reached new levels in this con game. Cosmophasis bitaeniata doesn't just smell like ant--it smells like home.
Cosmophasis prey only on weaver ants (Oecophylla smaragdina). These tropical ants build nests by stitching leaves together with silk produced by their larvae. It is these tender larvae that the spider is after, explains evolutionary biologist Mark Elgar of the University of Melbourne in Australia. He and his colleague Rachel Allan observed how the spiders would enter weaver ant nests and somehow persuade workers to hand over the larvae. In previous studies they had discovered that the gullible ants are tricked into thinking the spider is an ant because it carries the same chemical cloak of "cuticular hydrocarbons," organic compounds that make up arthropods' characteristic body odor.
As the researchers report in this month's Naturwissenschaften, there is a further twist to the story. When they collected ants from several colonies in and around Townsville, Australia, and used a gas chromatograph to separate their various cuticular hydrocarbons, they discovered that each colony has its own signature scent. Next, they divided 45 newly hatched spiderlings into three groups and fed each group ant larvae from a different nest. After 3 months, they squashed the spiders and ran them through their gas chromatograph. Each group of young spiders, they discovered, had picked up the characteristic scent of the nest whose larvae they had fed on. In the wild, these spiders probably spend their entire life in a single colony, says Elgar. Adopting the exact colony smell is a must in their game of deceit, because anything that smells unusual will be killed by the workers.
Tropical entomologist Brigitte Fiala of the University of Würzburg in Germany is intrigued with the study. But she cautions that more work is needed to reveal whether this is a real adaptation or simply a consequence of "smelling like what you eat."