The Monkey Slug

I received a call on the radio for an ID request at the Admissions Desk here at the Museum. The caller didn’t say what kind of an identification but just that “someone was in need of an identification.” Someone had brought in something that they couldn’t identify.

When this type of call occurs, as it sometimes does here at the Museum, it’s usually a rock, fossil, dead snake, or an insect that someone found in their yard. The object was unfamiliar to them and they wanted to know what it was.

I hopped in my golf cart and headed for the front desk.

When I arrived I saw a woman sitting on a bench with a mason jar in her hand. Inside the jar was a twig with two leaves attached, “ah, must be some kind of insect.” Looking closer, I saw a strange looking hairy, brown creature on one of the leaves. I knew right away it was a slug caterpillar.

Monkey slug.
Monkey slug.

Slug caterpillars are a group of often brightly colored and oddly shaped moth caterpillars. They’re unique in that they lack prolegs, those temporary legs on the abdomen worn by moth and butterfly larvae to help them get around on the leaves, twigs, or grass stems where they feed. Instead, slug caterpillars have suckers in place of the prolegs which give them a gliding, slug-like locomotion. Their true legs, the three pair that all caterpillars possess are on the thorax just aft of the head.

Caterpillar glides across leaf.
Caterpillar glides across leaf.

I’d seen pictures of this species of moth caterpillar before but never in person. I couldn’t remember its name but a quick search of “slug caterpillars” and there it was, hag moth caterpillar (Phobetron pithecium). The caterpillar is also called the monkey slug.

The woman told me she found it in her yard and simply wanted to know what it was and that I could have it if I so desired. I took it.

The name monkey slug may come from the fact that the caterpillar’s upper surface is covered with dense brown setae which resemble hair or fur and may appear monkey-like to some. I don’t get that impression, but a name is a name and I’ll call it a monkey slug if that’s what its official common name is. By the way, the day-flying female adult is said to mimic a bee and the male a wasp, so the monkey name doesn’t come from any particular characteristic of either of the adult moths.

Slug caterpillar species are typically covered with stinging spines which tend to cause pain in naturalists, or gardeners, that get too familiar with one of these larvae. I’ve read though, that the hairs on this caterpillar may not sting. Even so, I will not be poking this caterpillar with my finger.

The head end of the caterpillar.
The head end of the caterpillar.
From below the leaf.
From below the leaf.

The sides of the caterpillar, under all of that hair, are loaded with spines. They’re laid out in circular patches along the sides just below the hairy appendages. These appear menacing to me and should probably be avoided even more so than the brown hairs.

Spines are clearly visible beneath the "hairy overcoat."
Spines are clearly visible beneath the “hairy overcoat.”
Arrows point to spines and true legs (head on left).
Arrows point to spines and true legs (head on left).

Not all hairy or spiny caterpillars sting or cause itching when touched, but if you’re unfamiliar with a particular caterpillar, I’d avoid contact.

After taking the photos above, I released the caterpillar.

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