NYBORG, DENMARK--The next time you step on a cockroach, you may be committing an act of mercy. At least its end will come quicker than it would have if the insect fell prey to the parasitic wasp Ampulex compressa, which delivers a paralyzing sting to the brain so that its hungry brood can devour the living roach from the inside out. Now researchers have found receptors on the wasp's stinger that may guide the neurotoxic strike.
The sting of A. compressa paralyzes its prey, the cockroach Periplaneta Americana, for 4 or 5 weeks--enough time for the wasp's eggs to hatch, feed, and pupate inside their helpless host. For this strategy to work, the wasp must deliver its venom--a cocktail of neurotoxins--directly to the roach's brain.
To investigate what guides the sting, Ram Gal and Frederic Libersat of Ben-Gurion University in Beer-Sheva, Israel, first introduced the wasp to roaches whose brains had been removed. Normally, it takes about a minute for the wasp to find its target, sting, and fly off. But in the brainless roaches, the wasps searched the empty head cavity for an average of 10 minutes. A radioactive tracer injected into the wasps revealed that when they finally did sting, they used about 1/6 the usual amount of venom. The wasps knew something was amiss, says Gal, who presented the findings here on 10 August at a meeting of the International Society for Neuroethology.
When the team took a closer look at the wasp's stinger with an electron microscope, they found tiny structures near the tip that resemble sensory receptors found in other insects. Using a dye that labels neurons, Gal identified stringlike axons stretching between the structures on the stinger and the wasp's brain, further evidence that these axons provide the wasp with information about what it is about to sting.
"It's a fascinating story," says neuroscientist Paul Katz of Georgia State University in Atlanta. The finding illustrates how specialized the wasp has become, he says. "Once you're invested in mind control, you have to find the mind." In future experiments, Gal plans to give the wasp a taste of its own medicine: By sticking electrodes in the wasp's brain and recording electrical activity there while he manipulates the stinger, he hopes to find out what chemical or mechanical signal the receptors detect.