Urticating hairs

Definition

Defensive, barbed setae that detach easily and embed in vertebrate skin, causing mechanical irritation and often chemical inflammation. Found in New World tarantulas (Theraphosidae) and larvae of numerous moths and butterflies (Lepidoptera), these structures serve as passive or actively deployed anti-predator defenses against mammals and other threats. Despite the common name, they are modified setae, not true hairs.

Etymology

From Latin urtica, "nettle," referring to the stinging quality shared with nettle plants.

Example

The Mexican red-kneed tarantula (Brachypelma smithii) flicks urticating hairs from its abdomen using its hind legs, directing the irritating cloud toward predators; processionary moth larvae (Thaumetopoea pityocampa) use their urticating hairs both for defense and as tactile guides during head-to-tail marching.

Synonyms

• urticating bristles
• urticating setae

Related Terms

• setae
• Trichobothrium
• defensive secretion
• Lepidoptera
• Theraphosidae
• caterpillar

Usage Notes

Technically a misnomer—arthropods possess setae, not true hairs—but "urticating hairs" remains the entrenched term in entomological and arachnological literature. In tarantulas, these are abdominal and actively flicked; in caterpillars, they are typically dorsal and passive. Not all urticating setae carry venom; irritation is primarily mechanical, though some species add histamine-like compounds. Contrast with venomous spines (which pierce and inject) and trichobothria (sensory, non-defensive setae).