Caterpillars Build Silk "Alarm Systems"

Matt Kaplan
for National Geographic News

October 14, 2008

Metalmark moth caterpillars can build their own versions of home security systems out of silk, according to new research.

Until now, spiders were thought to be the only animals to detect tremors in their webs caused by foreign objects.

(Read "Deadly Silk" in National Geographic Magazine.)

The new study suggests that some caterpillars in the genus Brenthia can also sense disturbances and are using threads in their silk shelters like tripwires.

"I noticed that the caterpillars kept a couple of really long hairs in contact with the silk shelter, and was curious whether they were using the silk lines of the shelter as a predator detecting system," said lead study author Jadranka Rota, a biologist at the Smithsonian Institution in Washington, D.C.

The study could shed light on the evolution of animals that use silk to detect intruders.

"The metalmark [moth] is probably not alone," noted Lee Dyer, an ecologist at the University of Nevada in Reno who was not involved in the study.

"There are a lot of silk-weaving caterpillars. I think we are going to find this behavior in a number of other species."

Down the Hatch

Metalmark moth caterpillars are known to build protective shelters out of silk on the leaves where they dwell.

(Related: "Moths Elude Spiders by Mimicking Them, Study Says" [February 14, 2007].)

The larvae chew small holes beneath the shelters to use as escape hatches when danger nears.

Rota and co-author David Wagner, of the University of Connecticut in Storrs, set up a filmed experiment to study the caterpillars' behavior when they were disturbed in different ways.

Eighteen Brenthia monolychna caterpillars were brought into the lab and allowed to spin their shelters on leaves. Half of the shelters were subsequently taken away.

Rota and Wagner then used pencils to gently tap either the silk strands of the remaining shelters or the regions on the leaves where the removed shelters had been connected.

Later the team tapped the edge of the leaves and the caterpillars themselves to see if the animals' responses differed.

As soon as either the caterpillars or the silk strands were touched, the caterpillars bolted through their escape hatches and onto the other side of the leaves, with the fastest doing so in just a hundred thousandth of a second.

But when the scientists tapped the edges of the leaves or the regions where shelters had been removed, the caterpillars did not respond.

"This tells us that caterpillars and spiders have separately evolved the ability to use their silk threads in very similar ways," Rota said.

The research will appear next month in the journal Animal Behaviour.

Sound the Alarm

Michael Singer at Wesleyan University in Middletown, Connecticut, noted that the caterpillars could be detecting more than motion.

"Can the caterpillar use the web to detect the sound of incoming predators?" Singer asked.

It is known from a few cases, for example, that caterpillars exhibit evasive behavior when exposed to sounds that resemble the wingbeat frequencies of predatory wasps and flies.

"I wonder if the long [hairs] and web can amplify such sounds and provide an especially early warning system for the caterpillar," Singer said.

And because the metalmarks are such a large group, Smithsonian's Rota is wondering how many of them use their silk as an alarm trigger and how closely those that do are related.

"Ultimately," Rota said, "simultaneously examining the genetics of these caterpillars and their silk security systems could tell us a lot about how such a complex trait evolved."