Shinji Sugiura via Science Illuminates

For you or me, a hairy back might be a disadvantage, but for a caterpillar, it can be a lifesaver.

Some caterpillars are smooth as a baby’s bottom, but others are covered in hair and look like a shambling shag carpet. The reason for the coat is usually defensive. Some species have bristly hairs that are attached to venom glands, while others sport barbed hairs that can stick to an attacker and irritate their skin. Even if the fuzz isn’t weaponized, though, it can still protect a caterpillar by acting like armor and keeping predators at arm’s length, or at least hair’s length.   

A pair of researchers in Japan figured this out by watching beetles hunt different moth and butterfly caterpillars of varying fuzziness in their lab. Every one of the beetles was able to catch and kill the three smooth-skinned types of caterpillar, often in one attack. They had similar success when the prey was a short-haired caterpillar, but it took them more tries to do it. When a long-haired caterpillar was presented as prey, though, beetles really started to struggle. More than half of them failed to capture and kill the fuzzy larvae of the moth Lemyra imparilis, despite making several attempts. And even the ones that were successful needed to make more attacks, sometimes as many as two dozen, to get the job done.

When attacked, the researchers say, the Lemyra caterpillars “frequently coiled up to defend their ventral sides from Calosoma mandibles in a manner similar to that of hedgehogs.” They didn’t find any evidence that the caterpillars were using venom or other chemicals to defend themselves. Instead, it looked like it was simply the length of their hair that kept them from being easy prey. At 4.9 millimeters long, the hairs might not seem long and flowing to us, but they’re about twice the length of the other caterpillars' hair and, more importantly, the beetles’ mandibles. The dense, long fuzz provides a protective barrier that keeps the beetle from reaching and biting the caterpillar’s flesh. Sure enough, when the researchers trimmed the caterpillars’ hair down to 1.5 mm with a small razor, more of the beetles were able to kill them, and with fewer attacks. 

Unfortunately for the fuzzy caterpillars, the hair that keeps them safe from some enemies also makes them attractive to others. The researchers say that some parasitoids will choose long-haired caterpillars over smooth ones as hosts because the fuzzy ones are less susceptible to predators and make for a safer place for their young to develop.