Smelly spray signals free lunch for flies.

Stomp on a stinkbug, and you'll get a whiff of chemical weaponry. Stinkbugs -- and their cousins, the squash bugs -- unleash their aromatic ammunition to ward off predators such as spiders. But ecologists have now discovered that the smelly secretions hold an allure for tiny flies known as milichiids, which often swarm in to suck a meal from bugs held captive by spiders.

The chemicals that serve as dinner bells for these freeloading flies are the same odoriferous substances that repel people and most insects, the researchers report in the Sept. 15 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. Thomas Eisner of Cornell University and his co-workers determined that these chemicals -- hexanal and trans-2-hexenal -- trigger the flies' prey-poaching behavior, known as kleptoparasitism.

The flies are too small to kill stinkbugs or squash bugs on their own. But once the bugs have been immobilized and partially digested by a spider's venomous bite, they make a perfect meal for the milichiids, which slurp their dinners through long, needle-like feeding tubes.

"All the flies have to do is follow the scent of the spider's prey, eat their fill and fly away," Eisner explains.

Spiders usually ignore the flies, making no effort to chase them away, the researchers observed. And because spiders usually dine on their prey at the central hub of the web--where the silk is not sticky -- the flies themselves rarely become entangled.

To confirm that hexanal and trans-2-hexenal served as the chemical attractants, Eisner's group constructed gluey cardboard traps, some of which held tiny tubes of the chemicals. Each baited trap caught several flies, while the unbaited traps caught none.