A hairy back protects my innards, says the caterpillar.

The right amount of body hair in the right places could turn us or our partners into sex symbols, but that’s about it. Fortunately, body hair does not decide our fate. Yet Japanese scientists showed for the first time that hair length determines life or death, for caterpillars.

*Another version of this story first appeared on Earth Touch News*

 

Death rushes in to most caterpillars locked down by a carnivorous beetle. Some caterpillars though, stop death at a hair’s length. A recent study reported that caterpillars with long hairs survive beetle attacks because the hairs keep caterpillars beyond the beetle’s reach.

Although many caterpillars have skins as smooth as a baby’s, many grow hairs long and thick. The wooly bears for example, look like furry sausages. Scientists believed that the hairs protect the caterpillars from predators, but few have verified it.

In Osaka, Japan, Shinji Sugiura and Kazuo Yamazaki observed insects scurrying in the forest. Scores of caterpillars have hatched and were gorging on the sprouts and blossoms. Ground beetles, Calosoma maximowiczi, too were hunting for prey; caterpillar juices glistened on the jagged mandibles—protruding jaws—of some beetles.

Sugiura and Yamazaki saw Calosoma beetles attacked both hairy and smooth caterpillars—so could hairs save the caterpillars from predators, and how? The two Japanese behavioural ecologists examined further.

Calosoma beetle kills a Libythea celtis caterpillar. The beetle cuts the caterpillar's soft skin and sucks up the liquid.

Calosoma beetle kills a Libythea celtis caterpillar. The beetle cuts the caterpillar’s soft skin and sucks up the liquid.

Sugiura and Yamazaki pitted caterpillars of five species of butterflies and moths against Calosoma beetles from the same forests.  Of the caterpillars, three species had smooth skin whereas the other two, Lymantria and Lemyra, sported dense hair on their backs and sides. Every day, the scientists released a beetle and a caterpillar into a plastic arena. The confrontation lasted ten minutes, during which the scientists recorded how often the beetle attacked the caterpillar, and if it succeeded.

The Calosoma beetles attacked every caterpillar.  Each beetle killed the smooth caterpillars in its first attack, gashing the caterpillar’s naked soft flesh like cutting sausages with wire-cutters. The beetles drank the thick juices that oozed from the wounds.

The hairy Lemyra caterpillars though, held their ground. The beetles grappled with Lemyra, which often coiled up to shield its smooth underside, and struggled to tear the skin—some failed even after thirty attacks. In the end, only half of the beetles killed Lemyra caterpillars.

Hair helps, a lot. The plight of the other hairy caterpillar, Lymantria, shows that longer hair helps even more.

Against the hairy Lymantria, the beetles wrestled a little more than they did smooth caterpillars; yet almost every beetle feasted on a Lymantria within minutes. Although both Lymantria and Lemyra had dense hair, Lymantria’s hair raised little resistance against a hungry Calosoma beetle.

The difference between a live Lemyra and a dead Lymantria? A mere 2 millimeters. Lemyra caterpillars grow hairs 4.9 millimeters long, whereas Lymantria hairs measure 2.2 millimeters. Their foes, the beetles, brandish mandibles 2.2-2.8 millimeters long. Lemyra caterpillars’ hairs extend 2 millimeters longer than the beetle’s mandibles, just enough to keep the beetle’s weapons a safe distance away.

Calosoma beetle trying to penetrate the long, barbed hairs of a Lemyra imparilis caterpillar. Only half of the beetles managed to kill a Lemyra caterpillars within ten minutes. [Credit: Shinji Sugiura]

Calosoma beetle trying to penetrate the long, barbed hairs of a Lemyra imparilis caterpillar. Only half of the beetles managed to kill a Lemyra caterpillars within ten minutes. [Credit: Shinji Sugiura]

To test if hair length matters, Sugiura turned Sweeney Todd and trimmed the Lemyra caterpillars’ hairs to less than 1.5 millimeter. Within minutes, the beetles were lapping up the juices of these shaved, and now dead, Lemyra caterpillars. With hairs shorter than the beetle’s mandible, shaved Lemyra caterpillars succumbed just like Lymantria and other smooth caterpillars.

Lemyra caterpillars treasure their hairy coat, investing more in their hairs than the other caterpillars do. For every increment in body weight, the hairs of Lemyra grow three times longer than those of Lymantria. In contrast, Calosoma beetle’s mandibles measure the same length regardless of beetle sizes. As such, Lemyra caterpillar develops long hairs much faster than other caterpillars.

Why don’t Lymantria or the smooth caterpillars grow long hairs to save their skins too? Well, hairs make expensive armours. Caterpillars change into new skins as they grow, and new skins require new hairs. The resources invested into the hairs would be channeled from other physiological needs. If so, hairy caterpillars might develop slower than smooth caterpillars—a hypothesis that Sugiura and his colleagues are investigating.

Furthermore, caterpillars brave the world with more than just hairs. Caterpillars camouflage and mask their smells; hide in shelters; escape down silk threads; emit foul chemicals; hairs protect caterpillars, but many caterpillars without hairs still live to chew another leaf.

That said, a Lemyra caterpillar should not shave (certainly not waxing!). A shaved Lemyra flaunts the wrong fashion statement—a mistake it might not live to correct.

 

Source

Sugiura, S., and Yamazaki, K. (2014). Caterpillar hair as a physical barrier against invertebrate predators. Behavioral Ecology. doi:10.1093/beheco/aru080

 

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