Touching legs turns shy locusts gregarious.

Researchers wielding artists' paintbrushes have tickled some insects and come up with a new insight into how a plague of locusts gets started.

Left to itself, the desert locust of biblical fame stays camouflage green and shuns company, explains Stephen Simpson of the University of Oxford in England. However, when their population spikes because of such factors as abundant food, shy Schistocerca gregaria locusts become yellow-and-black partygoers.

Researchers have known that touch plays a central role in converting reclusive locusts into crop-destroying mobs. Now, Simpson and his colleagues say that touching one part in particular--the femur of the hind leg--triggers the shift. The other 10 parts the researchers stroked didn't evoke gregarious behavior, they report in the March 27 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Elizabeth A. Bernays of the University of Arizona in Tucson welcomes the new finding as "a fascinating detail." As she puts it, "Imagine if sitting on a crowded train with thighs touching made people gregarious or made their skin change color."

The paper "opens up the whole field of neurophysiology," Bernays adds. Narrowing the sensitive area should help find the specific nerves and chemical signals driving the dramatic shift.

That shift separates a true locust from the rest of the grasshoppers, Simpson explains. About a dozen species around the world switch from recluses to swarmers. "You hear millions of beating wings," he says. "It's an assault on all the senses."

His lab tested possible cues for the switch. Solitary locusts didn't respond much to air blown from a swarm or to the sight of a swarm they couldn't touch. However, a locust in a cage with a ball rolling back and forth--to simulate bumping from other locusts--changed the insect's behavior. After 4 hours, such a locust given a choice either to move toward a crowd or to retreat to a hideaway opted to congregate. Changes in color and subtle shifts in body shape followed more slowly.

Touch-sensitive hairs cover the desert locust's body, so Simpson's lab tried to localize the effect, an effort he derides as "mind-bogglingly tedious." Researchers poked fine-tipped paintbrushes through cage mesh to stroke a body part every 60 seconds for 4 hours on a total of 170 insects.

The hind legs "are a very good place to have [a crowd sensor]" remarks entomologist David Hunter at the Australian Plague Locust Commission in Canberra. He points out that a single locust going about its daily business of eating and hopping doesn't bump the outside of its rear legs as much as when it jostles around in a crowd. The real benefit of the new research will be in refining scientists' models for predicting locust behavior, Hunter says.

Grasshopper ecologist Gregory Sword of the U.S. Department of Agriculture in Sidney, Mont., adds that the shift from solitary to gregarious behavior offers insights into warning colors.

He and Simpson have shown that desert locusts don't seem affected by their buddies' colors. The researchers propose that the bright hues warn predators away. Locusts often have gutfuls of plants that other animals reject. To a predator, therefore, locusts "are like Twinkies, except with a filling of noxious plants," Sword says. The loud color makes a fine warning label.