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Sassy frass flingers. Skipper caterpillars like this one can shoot their poop great distances.

As the Frass Flies

It's a horrifying situation: Your house is full of stinky waste that everyone--even enemies--can smell for miles. Certain caterpillars have solved the problem, according to new research, by hurling their excrement far from their homes.

Evolutionary faecologist Martha Weiss of Georgetown University in Washington, D.C., began collecting skipper caterpillars to observe how they build leaf houses. That is, until she heard a "plink, plink, plink" coming from the plastic shoeboxes in which she kept them. After observing that the caterpillars "ballistically eject" Grape Nut-sized pellets of excrement, also known as frass, Weiss decided to investigate.

Old caterpillars shoot farthest, she found. One made a phenomenal 153 centimeter expulsion, she reports in the April issue of Ecology Letters. Based on the caterpillar's size, "that's the equivalent of a 76-yard field goal in football," Weiss said (beating the National Football League record by 13 yards). The caterpillars launch their loads through a hard, hatchlike plate on their backsides with a quick surge in blood pressure. Next, Weiss considered why the caterpillars might have evolved this peculiar talent.

Sanitation, perhaps? Weiss put the leaf houses in small boxes to see what would happen when frass accumulated. The caterpillars didn't seem to get sick or die young. Another possibility is that the caterpillars fling frass to avoid being crowded out of their homes and being forced to waste valuable energy building new homes. But when Weiss forced caterpillars to build 32 houses a month they fared just as well as those who constructed the typical nine per month. Finally, Weiss introduced a foraging wasp that preys on skippers. She found that 14 of 17 caterpillars whose shelters were decorated with frass were eaten within 5 minutes. But another group of caterpillars whose houses were decorated with similar-sized glass beads all survived, indicating that skippers shoot frass to avoid attracting predators.

"It's not surprising that chemicals in frass would be something that caterpillars have to worry about," says chemical ecologist Jack Scultz at Pennsylvania State University, University Park. After all, he says, feces are full of volatile chemicals that predators could home in on. "What's really cool is the adaptation that the animal has of dealing with it. That's impressive."

Related sites
Weiss's site