Tom Turpin
Professor of
Purdue University


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Some bees sting; so do some caterpillars

When most of us think of insects that sting, bees, wasps and ants come to mind. And for good reason. At one time or another, one or more of these Hymenoptera has stung most people. Such an event produces a very memorable moment. And that is good for the insect! After such an encounter most animals, including humans, try to avoid stinging insects.

Bees, ants and wasps sting by injecting poison through a stinger. The insect stinging structure is a modified ovipositor, the egg-laying apparatus of female insects. So only female insects can sting. However another group of insects - caterpillars - also sting.

The sting of caterpillars is entirely different, although sometimes no less painful, than the sting of bees. A caterpillar's sting is delivered by a hair or spine on its body, not through a modified egg-laying structure. The stinging devices of caterpillars are called urticating hairs.

Urticating hairs are hollow spine-like structures that connect to poison sacs under the caterpillar's skin. These hairs can puncture and embed in a soft material such as human skin. When that happens the hair breaks off from the caterpillar, and a dose of poison is transferred through the hollow hair.

New world tarantulas also sport such defensive hairs. Scientists recognize six types of urticating hairs on tarantulas. Each type has a different structure, and not all types are found on all species of tarantulas. Like such hairs in caterpillars, most of the tarantula urticating hairs need direct contact to function as defense mechanisms. However, some tarantulas actually kick the hairs off their abdomens. In effect, the tarantula is throwing the hairs toward the target. Bald spots on the abdomen of a tarantula are an indication of such behavior. Such guided-missile hairs can get into the mucus membranes of mammals and cause both mechanical and chemical harm.

Stinging hairs are not just for arthropods. Some plants also possess these defensive hairs. The most widely recognized plants so adorned are appropriately called stinging nettles. Most people are familiar with the burning sensation that occurs when one touches the leaves of a stinging nettle plant and consequently understand how the system works for protection.

Tussock caterpillar
Tussock caterpillar

Caterpillars with stinging hairs occur in at least 11 insect families within the order Lepidoptera - the butterflies and moths. Most caterpillars with stinging hairs turn into moths, including those called tiger moths, tussock moths and the giant silkworm moths.

In general, human and caterpillar encounters of the stinging sort increase in the late summer and early fall. That is a time when caterpillars of many types have completed feeding and begin leaving food plants to crawl around in search of a place to pupate. But the insects are not itching for trouble. That is why most stinging caterpillars are strikingly marked in bright colors and patterns. Such markings are known as "warning colors" by scientists and are intended to send the message that the insect is dangerous to touch.

Saddleback caterpillar
Saddleback caterpillar

One of the more common of the stinging caterpillars is known as the saddleback caterpillar. This caterpillar is about an inch long with prominent horns and numerous bristles. It is brown at the ends with a green middle marked with white so that the insect appears to have a saddle and blanket on it. Hence the name saddleback. This bright-colored caterpillar turns into a rather dull-colored brown moth.

Io caterpillar
Io caterpillar

Another stinging caterpillar belongs to the group of moths that are called giant silkworm moths. The io moth, one of the smaller moths in this group, has a caterpillar with stinging hairs. As one widely used entomology textbook states, "This larva should be handled with care: for the spines sting." Larvae of some the tussock moths are also adorned with urticating hairs.

A caterpillar sting might not be as painful as a bee or wasp sting, but it does create a burning and itching feeling followed by swelling, redness and a rash. So what should you do if you become the recipient of a caterpillar sting? First, remove as many stinging hairs from your skin as possible by sticking and removing with tape or washing the affected area thoroughly. Apply a cold pack to reduce swelling.

In rare cases exposure to insect stinging hairs might result in an allergic reaction, with extensive swelling and rash. In such cases a physician should be consulted. In most cases though, encounters with stinging caterpillars result in a short period of discomfort and a lesson learned. The lesson is that bright-colored hairy caterpillars are sending a message. The message? Handle at your own risk!


Writer: Tom Turpin
Editor: Olivia Maddox