Caterpillars exhibit a wide array of forms and colors. While many are harmless, some have evolved striking appearances as a defense mechanism. These visual cues signal to potential predators that the caterpillar possesses irritating or toxic properties. Understanding these warning signs is important for avoiding uncomfortable encounters.
Warning Signs in Caterpillar Colors
Bright and contrasting colors serve as a warning for many irritating caterpillars, a strategy known as aposematism. Vibrant reds, oranges, yellows, blacks, and whites often indicate chemical defenses. Patterns such as stripes, spots, or bands further enhance this visual signal, making the caterpillar more conspicuous. Beyond color, some caterpillars also feature spiny or bristly textures, another visual and tactile warning. These physical characteristics, combined with bold coloration, suggest that the caterpillar should not be handled.
How Caterpillars Deliver Toxins
Caterpillars deliver defensive chemicals through specialized hairs or spines, known as urticating hairs. These structures are hollow and connected to venom sacs or glands containing toxins. When brushed against, these hairs can break off and embed in the skin, releasing their irritating contents. The reaction can range from mild itching and redness to more severe pain, swelling, welts, and blistering. The main concern for human interaction is contact dermatitis from these stinging hairs; some caterpillars might also contain toxins if ingested.
Common Toxic Caterpillars by Appearance
Several North American caterpillar species are known for their stinging properties and distinctive appearances.
The saddleback caterpillar (Acharia stimulea) is bright green with a prominent brownish-purple spot in the middle of its back, resembling a saddle. It has fleshy horns at both ends, covered in venomous urticating hairs. Contact with these hairs can cause sharp, burning pain, a swollen rash, and sometimes nausea.
The puss caterpillar (Megalopyge opercularis), also called the asp, is considered the most venomous caterpillar in North America. It has a dense, woolly coat that makes it resemble a tuft of cotton or fur, appearing in shades of brown or gray. Despite its soft appearance, its long, bristly setae conceal venom-filled spines that can cause excruciating burning pain, severe rashes, swelling, and even systemic symptoms like nausea or difficulty breathing.
The Io moth caterpillar (Automeris io) is light green and covered with clusters of branching, venomous spines. It displays a distinct red and white stripe running along its side. Contact with its spines commonly results in immediate painful stinging, itching, and the formation of localized welts and redness. While a mild to moderate irritation, allergic reactions can occur.
The hag moth caterpillar (Phobetron pithecium), also known as the “monkey slug,” has a peculiar, brown coloration and multiple pairs of fleshy, twisted appendages that resemble arms or legs. This unique appearance makes it look like a tarantula. Hidden beneath its short, brown hairs are stout, hollow spines connected to toxin glands, which, upon contact, can produce a burning, itching sensation, redness, and inflammation.
What to Do After Contact
Should contact occur with a stinging caterpillar, immediate action minimizes discomfort. First, gently remove the caterpillar if it is still on the skin, without using bare hands, perhaps with tweezers or a stick. Next, apply adhesive tape to the affected area, sticky side down. Press it firmly and then quickly pull it off to remove any embedded hairs or spines; this process may need to be repeated with fresh tape.
After removing the hairs, wash the affected skin thoroughly with soap and water. Applying a cold compress or an ice pack wrapped in a cloth to the area can help reduce swelling and pain. For symptomatic relief, a paste made from baking soda and water, or an over-the-counter hydrocortisone cream, may soothe itching and discomfort. Oral antihistamines can also be taken if itching persists. If symptoms worsen, such as severe pain, widespread rash, significant swelling, difficulty breathing, or nausea, seeking medical attention promptly is advisable.