The monkey slug is not some experimental genetic monstrosity. But even if you see one in the flesh, you may still be unclear what this odd, brown hairy thing is, with its array of fuzzy tentacles. It moves over the surface of leaves with what can only be described as a lurching glide.
What is a monkey slug?
It’s neither a monkey nor a slug, of course. This is Phobetron pithecium and it is the caterpillar of the hag moth, native to North America. The species belongs to the Limacodidae, a global family of moths that are collectively called cup moths or slug moths, depending on which part of the world you are in. The former refers to their cocoons, the latter to their unusual gait.
What do monkey slugs look like?
Think of a caterpillar and you’ll probably picture a larva with six ‘true’ legs up front, coupled with varying numbers of simple organs called prolegs at the back. Prolegs are fleshy, gripping organs, each crowned with a fringe of hooked hairs called crochets.
However, the slug moths, including the monkey slug, are very different. If you were to watch one climbing up the side of a clear container, you would see a distinct lack of legs. The front ones are much reduced, and there are no prolegs as such either. Instead, the insect’s underside is modified into a series of suckers, and it shuffles along on these in peristaltic (wave-like) movements. These body parts are hidden from view, making the caterpillar appear to glide along, even laying down a thin carpet of silk as it goes. There also appears to be no head (it’s hidden beneath a cowl of fleshy folds).
While this explains one aspect of the monkey slug’s ‘uncaterpillariness’, for most people seeing one for the first time, there are other things to get to grips with. What are those ‘tentacles’ all about? The caterpillar almost seems to consist of nothing else, with nine pairs protruding out of either side of its body. Each is thick at the base, tapering to a curling point.
While the monkey slug’s scientific name translates as ‘a frightening monkey thing’, we might assume that it shares some of the family’s traits. I certainly have never trusted a monkey slug enough to give it a cuddle – or even a poke with a bare finger. Look at one closely and those curling appendages are each covered in dense fur that comes in varying shades of brown. There are also lots more rigid black spines protruding from this forest of fluff. Then look below this wondrous wig and fringing the entire insect’s body are even more tubercles, each armed with a barricade of even more stout hollow spines. So it looks like this is something to wonder at, but not touch.
What do other slug moths look like?
Among the other members of this family are numerous odd-looking, strange beasts. Some are lozenge-shaped, others are discs; some have fringes and frills; many are decked out in gaudy stripes, spots and blotches. They are a bright bunch that together resemble an alien pick and mix.
In nature, of course, this combination is a warning of something nasty unseen. Some of their common names bear this out, too – needle cats, nettle grubs, spitfires, warships and junks. Sure enough, many are armed with toxic spines, bristles and hairs.
What are slug moths also known as?
Slug moths are also known as cup moths due to their distinctive cocoons. (A weird caterpillar has to have a weird cocoon, right?) The cocoons are almost spherical and made from hardened silk, incorporating the caterpillars’ larval hairs for continued protection through to adult life. Some even have a round ‘hatch’ that pops open, allowing the adult moth to emerge.
Are monkey slugs venomous to humans?
While it isn't venomous to humans, there's debate over whether it stings humans. According to some sources, the monkey slug doesn’t sting or deliver any urticating symptoms; according to others, it does – possibly a result of the varying degrees of sensitivity expressed by humans. It certainly isn’t as bad as other insects, where an accidental brush can leave you feeling like you’ve been licked by flames.
So, back to the question: why? One line of thought is camouflage: it doesn’t look much like a caterpillar. This means it might get overlooked and, even if something does give it a peck, those ‘tentacles’ can break off, leaving its attacker with a mouth full of fuzz and not a lot else.
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Top image: larvae of the hag moth, also known as the monkey slug. Credit: Alison Hunter, CC BY-SA 2.5, via Wikimedia Commons
