The spider caterpillar presents a distinctive appearance, often leading to curiosity. Despite its common name, this creature is not a spider at all. It is instead the larval stage of a particular moth, showcasing one of nature’s more peculiar transformations.
Identifying the Spider Caterpillar
This creature, scientifically known as Phobetron pithecium, possesses a brown, fleshy body. Its most recognizable features are nine pairs of curly, hairy projections (tubercles) extending from its flattened body, resembling spider legs. These tentacle-like appendages vary in length, with the third, fifth, and seventh pairs often appearing longer than the others. This unique morphology gives rise to its common name, “spider caterpillar,” as it can be mistaken for the shed skin of a hairy spider or even leaf debris.
The caterpillar is also widely known by another nickname, the “monkey slug.” This moniker stems from its unusual movement, which is not a typical crawl but rather a smooth, rolling or gliding motion. When fully grown, these larvae measure between 1.5 to 2.5 centimeters (approximately 0.59 to 0.98 inches).
Habitat and Diet
The spider caterpillar is found across the eastern and southern United States. It prefers woody environments like deciduous forests and is also found in gardens. It is a generalist feeder, consuming leaves from various trees and shrubs.
Its diet includes common host plants such as oak, apple, chestnut, and sassafras. Other food sources include ash, birch, cherry, dogwood, hickory, persimmon, and willow. Larvae feed on the underside of leaves.
The Sting and Potential Risks
The spider caterpillar has stinging hairs (urticating setae) that serve as a defense mechanism. These hollow, quill-like hairs connect to poison sacs and can pierce the skin, releasing toxins upon contact. Touching them can cause a burning, itching sensation, redness, and inflammation.
Reactions can vary from mild irritation to more severe pain, swelling, and blistering. Avoid handling these caterpillars to prevent exposure to their irritating hairs. If contact occurs, symptoms typically resolve within 24 hours. Removing any remaining hairs with adhesive tape and washing the area with soap and water can help alleviate discomfort.
Life Cycle From Caterpillar to Hag Moth
The spider caterpillar undergoes a complete metamorphosis, progressing through egg, larval, pupal, and adult stages. After reaching full size, the caterpillar forms a cup-shaped cocoon for its pupal stage, featuring a circular escape hatch for the emerging adult. This non-feeding, immobile stage allows for significant internal reorganization of tissues into adult features.
The adult insect that emerges from the cocoon is known as the Hag Moth (Phobetron pithecium). Unlike the caterpillar’s peculiar, hairy, leg-like appearance, the adult moth is stout and furry, ranging from tan to dark brown with irregular, wavy lines on its forewings. Male moths may have translucent wings, while females are drab brown and gray, often displaying yellow scales on their legs. One generation occurs annually in northern regions; southern United States can see two or more generations a year.