The most beautiful thing we can experience is the mysterious.  It is the source of all true art and science.

~Albert Einstein

There are some creatures that are so bizarre that they leave me baffled as to why they are the way they are. This is one of them.

Monkey Slug, caterpillar of the Hag Moth (click photos to enlarge)

Even the name is a mystery – Monkey Slug. I suppose if you squint and look at it, you could imagine a very hairy cartoon-like monkey with a few extra arms. One reference said it resembled a terrestrial octopus. That I can see. And the Hag Moth part probably comes from the likeness of the hairy arms to an old witch’s disheveled hair. But, no matter how you look at it, you probably would not guess this strange-looking bug is a caterpillar.

Back lit Monkey Slug (early instar)

It belongs to the slug caterpillar family, Limacodidae, many of whom are so-called stinging caterpillars, due to the presence of urticating spines. There is disagreement in the references on this species’ stinging ability, with the guru of caterpillars, David Wagner (author of Caterpillars of Eastern North America), stating that he has been unable to get this species to “sting” even by rubbing it on his arm. Others say that people vary in their susceptibility, so I will let it go untested. They certainly look like they could hurt you, with a dense coating of short hairs and long spines, especially in the earlier stages (instars).

Later instar (perhaps the last) of a Monkey Slug

As they grow and molt, they take on a more carpeted appearance, with short, stiff, brush-like hairs, densely covering their dorsal surface. Monkey Slugs are characterized by six pairs of arms (some short, some long) that are deciduous and can break off without any apparent harm to the critter.

Monkey Slug showing curled tips to the arms

To my eye, a Monkey Slug (especially the last instar) resembles a dead leaf with curled edges. Some researchers have speculated that it looks like a tarantula shed, which also has urticating hairs. Many of our insect eating birds migrate to the tropics and thus might encounter these distasteful objects in the forest, which would then give a tarantula shed mimic some advantage, but who knows. I have never found an adult, but they, too, are mimics – the male looks like a wasp, and the female has the likeness of a bee, complete with faux pollen sacs on her legs.

From the side it looks like a crown

I have only seen a few of these caterpillars over the years, but two of my former colleagues at the Museum each found one and brought them to BugFest last week, where they drew lots of curious looks and questions from the hundreds of visitors wandering by our booth. Monkey Slugs can be found on a wide range of shrubs and trees but may be relatively scarce, or just darned hard to spot in vegetation.

Monkey Slug gliding along a twig

I’m just glad they found these two and I had a chance to reacquaint myself with one of the more outlandish denizens of our forests. May there always be mysteries…