(*We felt a little note from the editor was in order here. So, a word of warning: if you consider yourself compassionately pro-caterpillar, you might not want to proceed beyond this point. We're pretty sure caterpillars were harmed in this story). 

For most caterpillars, getting locked down by a carnivorous beetle usually means certain (and rapid) death. But some lucky survivors escape death by, quite literally, a hair's breadth. A recent study reports that caterpillars with long hairs survive beetle attacks because their hairs keep their soft flesh beyond the reach of the beetles' jaws.

Although many caterpillars have skin as smooth as a baby's, numerous others grow hairs that are long and thick – the wooly bears, for example, look like little furry sausages. Scientists have long believed that these hairs protect the caterpillars from predators, but few have actually gone out to verify it. That's why Japanese behavioural ecologists Shinji Sugiura and Kazuo Yamazaki decided to have a go.

“For most caterpillars, getting locked down by a carnivorous beetle usually means certain death. But some lucky survivors escape death by, quite literally, a hair's breadth.”

While observing critters scurrying in the foliage in a forest in Osaka, Japan, the duo watched scores of caterpillars hatching and gorging on leaves and blossoms. With all that tasty prey around, ground beetles (Calosoma maximowiczi) were in their element, caterpillar juices glistening on their jagged mandibles. Sugiura and Yamazaki noticed that the Calosoma beetles attacked caterpillars both hairy and smooth, and they wondered: did the hairs protect the caterpillars, and if so, how?

To find out, the researchers pitted the caterpillars of five species of butterflies and moths against Calosoma beetles from the same forests. Of the caterpillars, three species had smooth skin, while 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 Gladiator-style plastic arena. The confrontations lasted ten minutes, during which time the scientists recorded how often the beetle attacked the caterpillar and if its strikes were successful (to watch these gory gladiatorial games, you can check out this video).

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Hairless caterpillars didn't fare well against the beetles' wire-cutter jaws. Image: Shinji Sugiura

While the beetles took on every caterpillar, their smooth-skinned opponents were killed at first attack, their unprotected skin proving easy pickings for the beetles' wire-cutter jaws. The hairy Lemyra caterpillars, on the other hand, held their ground. Often coiled up to shield their smooth undersides, the caterpillars were a big challenge, and the beetles really struggled to tear the skin – some failed even after 30 attacks! In the end, only half of the beetles succeeded in killing Lemyra caterpillars.

So hair helps – a lot. And the fate of the other hairy caterpillar, Lymantria, shows that longer hair helps even more. Against the hairy Lymantria, the beetles had to wrestle just a little more than they did with smooth caterpillars – but almost every beetle feasted on a Lymantria within minutes. Although both Lymantria and Lemyra had dense hair, Lymantria's hair offered far less protection against a hungry Calosoma beetle. The difference between a live Lemyra and a dead Lymantria? A mere two millimetres.

Lemyra caterpillar hairs are 4.9mm long, whereas Lymantria hairs measure just 2.2mm. Their foes, the beetles, brandish mandibles that are 2.2-2.8mm long. Do the math and you'll see that the hairs of Lemyra caterpillars happen to be just two millimetres longer than the beetles' fearsome mouthparts – just long enough to keep the deadly weapons at a (survivable) distance.

To test just how much hair length matters, Sugiura turned 'Sweeney Todd' and trimmed the Lemyra caterpillars' hairs to less than 1.5mm. The results? Within minutes, the beetles were lapping up the juices of shaved Lemyra caterpillars. Left with hairs shorter than the beetles' mandibles, shaved Lemyra caterpillars succumbed just like their hairless (and short-haired) counterparts. 

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When it comes to warding off hungry beetles, the longer the caterpillar's hair, the better. Image: Shinji Sugiura

Unsurprisingly, Lemyra caterpillars treasure their hairy coat, investing more in their hairs than the other caterpillars do (for every increment in body weight, their hairs grow three times longer than those of Lymantria). So why don't the other caterpillars copy this successful strategy and grow long hairs to save their skins too?

Hair, it turns out, makes for expensive armour. Caterpillars change into new skins as they grow ... and new skins require new hairs. Investing a bigger chunk of your resources into all that hair-growing comes with tradeoffs (more hair-boosting resources equals fewer resources for other bodily needs). That's why Sugiura and his colleagues are investigating whether hairy caterpillars develop at a slower pace than their smoother cousins.

But it's not as if hairiness is the caterpillar world's only defensive measure. Caterpillars use camouflage, they lurk in hideouts, they mask their smells, escape down silk threads, emit foul chemicals – the list goes on. In short, while hairs do protect some caterpillars, many others live to chew another leaf without them.

That said, Lemyra caterpillars should never shave. And definitely no waxing. 

Top header image: Johnson Cameraface, Flickr