The Bizarre Saddleback Caterpillar Found in U.S. Packs a Nasty Sting

The bizarre-looking caterpillar of the moth species Acharia stimulea is found across large swaths of North America, but if you see one don't touch it because it can deliver a powerful sting.

The saddleback caterpillar can be identified by its characteristic markings—the body is brown on the front and rear ends, but the middle is green with a brown oval in the middle surrounded by a white outline. These markings resemble a lime-green saddle blanket on a brown saddle, hence the name.

These caterpillars, which can reach just under an inch in length, are relatively common across their extensive native range, which stretches across the eastern half of North America as well as into Mexico.

Saddlebacks can be seen feeding on a variety of plant species, although they often prefer to eat a shrub known as spicebush.

While it may come as a surprise to some, the caterpillars of several moth species are capable of stinging. Some research has indicated that about 150 of the approximately 160,000 documented moth species are known to cause adverse reactions, including saddlebacks.

These adverse reactions can occur after people come into direct contact with the venomous hairs or spines of the caterpillars, which act as a natural defense mechanism against predators.

The spines of the saddleback, for example, can break off easily and become embedded in the skin of a person or animal while delivering a potent, toxic venom that can result in tissue damage. When the spines become embedded in the skin, the victim may experience an intense burning sensation and inflammation, among other symptoms.

"The sting of the saddleback caterpillar can vary in pain level and seriousness based on the number of spines that made contact, contact time, caterpillar size and sensitivity of the individual person being stung," Margo Roseum, an education technician for Virginia's Shenandoah National Park, one of the locations where the caterpillars are found, told Newsweek.

"Removing the spines from the skin can reduce the continued pain. The sting can be serious enough to require medical attention," she said. "Symptoms can vary in severity and pain level per person but may include a pain, rash, headaches, hives, bumps, discoloration and discomfort in the affected area. Other symptoms can occur based on an individual's sensitivity."

In rare cases, victims may even experience anaphylactic shock, hemorrhaging or other severe reactions.

The stinging spines protrude from two horns found on each end of the caterpillar as well along its sides.

"Saddleback caterpillars make their unusual traits and behaviors well known by showing off bright colors, large horns and long, stinging spines," Roseum said. "The bright, green blanket-like shape on their backs warns predators of danger—in this case, stinging venom."

For victims who have been stung, treatment of the sting may include washing the site to remove toxic substances, stripping of the affected region with adhesive duct tape to pull out the spines, application of ice packs and administration of oral antihistamines or intramuscular corticosteroids, according to a 2016 report in Clinical Toxinology.

Aside from stinging hairs or spines, moths have evolved a variety of tactics to avoid predators in the larval stage. For example, caterpillars may blend into their surroundings using camouflage or hide under leaves. They may also have distinct markings that mimic larger animals and are designed to scare off predators.

Saddlebacks, like all moths and butterflies, go through several distinct life stages. They start off as an egg, from which the larva, or caterpillar, hatches.

In the larval stage, the caterpillar grows and eventually molts. The well-known coloration and large horns are not present in early larval stages but appear clearly in the late "instar" stage, according to Roseum. When the caterpillars are large enough, they pupate in a cocoon until emerging as an adult moth with a brown coloration and a furry appearance.