Slug Caterpillar Moths

Photo of a monkey slug caterpillar on an oak leaf
Scientific Name
More than 20 species in Missouri
Limacodidae (slug caterpillar moths)

Missouri has more than 20 species in the slug caterpillar moth family. The adult moths’ heavy bodies and wide, rounded wings are both thickly covered with scales, creating a furry appearance. Many are bright yellow-brown with variously colored lines and patches on the forewings. Some have large bold green or brown patches. Males have comblike (bipectinate) antennae (at least in the basal half).

The caterpillars are remarkable. Instead of the usual peglike prolegs that most moth caterpillars have in midbody, slug caterpillars have suckers, which gives them a gliding, sluglike movement, inspiring the common name. Although some have smooth green bodies, some are bumpy, ridged, spiny, or densely hairy. Many species are brightly colored or marked, with odd-shaped tubercles, horns, and numerous stiff spines or hairs capable of delivering a painful sting.

Some typical species in this family are the saddleback caterpillar, spiny oak-slug, hag, and skiff moths. The saddleback caterpillar’s sting feels much like a bee sting. The monkey slug caterpillar (larva of the hag moth) looks something like a fuzzy, brown, slightly elongated starfish.

Other Common Names
Wingspan: ½–1¾ inches.
Where To Find
Statewide. Certain species are more likely to be found in different regions. The saddleback caterpillar moth is most common in eastern Missouri, for example. Some species are widespread but have a very localized occurrence, being abundant where colonies occur, but rare elsewhere.
Adults of many species are attracted to lights at night, which is where many people see them. They are more common in or near wooded areas, since the larvae are often associated with oaks and other trees. Many species are locally abundant only, so seeing the weird-looking caterpillars can be hit-or-miss.
Favorite food plants vary with species. Some, for example, are associated with oak trees. But the larvae of many species may feed on a great variety of trees, shrubs, and nonwoody plants.
Life Cycle
Caterpillars may have as many as nine instars (molts) before reaching maturity. In late fall, caterpillars make an oval or spherical cocoon of brown silk, with a hinged cap on one end, through which the moth later emerges. Slug caterpillar moths overwinter as mature larvae in the cocoon. In spring, they pupate. Sightings of adult moths of most species usually peak in midsummer.

Look (and photograph), but don’t touch! The caterpillars in this group are famous for their stinging spines and hairs. Their purpose is to dissuade would-be predators. The caterpillars don’t “go after” people. But if you touch them or accidentally brush against them, you could get “zapped.” Many people are stung while gardening, so this is just one more good reason to use gloves, wear long sleeves, a floppy hat, and so on. Symptoms can include pain, rash, burning, itching, swelling, and blistering.

Usually the symptoms go away or decrease within about a day, but if they are severe or persist, seek medical help. Some experts suggest gently applying tape sticky-side-down to the affected area, then lifting it off to draw out the tiny hairs from your skin. A paste of baking soda and water can help relieve symptoms, as can hydrocortisone cream.

Although not all the caterpillars in this group have stinging spines or hairs, some people might be extra sensitive and have an allergic reaction to contact. This is generally true of larvae in all moth families, so we advise caution with any “hairy” or spiny caterpillars.

Apparently, the “fur” of the winged, adult moths is non-irritating for most people.

Moth caterpillars, which are soft and nutritious for potential predators, don’t move very fast and cannot fly, so generally speaking, they are prime dinner fare for many species of birds and other predators. Therefore, caterpillars of many species have developed ways to avoid being eaten. Camouflage is one typical defense strategy. But in this group, the bright color patterns of many species warn potential predators of stinging spines or hairs. This saves the caterpillar from expending time and energy building a leaf tent or other shelter, or from having to feed only at night.

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Similar Species
About Butterflies and Moths in Missouri
Butterflies, skippers, and moths belong to an insect order called the Lepidoptera — the "scale-winged" insects. These living jewels have tiny, overlapping scales that cover their wings like shingles. The scales, whether muted or colorful, seem dusty if they rub off on your fingers. Many butterflies and moths are associated with particular types of food plants, which their caterpillars must eat in order to survive.