Urticating hair
This term also refers to certain types of barbed bristles that cover the dorsal and posterior surface of a tarantula's or caterpillar's abdomen.
The most common form of urticating hairs in plants are typified by nettles, which possess sharp-pointed hollow bristles seated on a gland that secretes an acrid fluid.
Exposure to glochidia is an occupational hazard to fruit pickers and other outside workers in areas where Opuntioideae thrive, as the spines can persist in clothing and gloves and can become airborne under the right conditions.
Generally, setae are only externally irritating, but may be more dangerous if contact is made with mucous membranes or if ingested; some can cause severe skin necrosis, eczema-like symptoms and shedding.
For example, many larvae in the family Lasiocampidae bear dense bands of short, stinging bristles across their thoracic segments.
The bristles are normally retracted into folds in the skin, but, if the caterpillar is disturbed, it will display its bristles, which are usually of contrasting colors; in nature, bright and/or contrasting colors are used by many invertebrates, amphibians, reptiles, fungi and plants as visual warnings for predators, indicating the presence of toxicity, venom or poison.
If roughly stimulated or held, lasiocampid larvae are likely to writhe and lash about, forcing the stinging bristles into any vulnerable surface they can.
Many other species of larvae lack any such localized concentrations of bristles and are armed more generally with urticating hairs; even so, they too will lash about frantically if disturbed, making them difficult to handle without suitable equipment.
[7] For brown-tail moths (Euproctis chrysorrhoea) native to Europe and invasive in other parts of the world, bristles are shed or broken off during molts and can be wind-borne, so that direct contact with live or dead larvae is not required to trigger a rash.
Despite this shift, urticating hairs nonetheless retain unique characteristics that render them visually distinct from abdominal bristles, such as their tendency to cover only a portion instead of the entirety of the opisthosoma.
However, there is at least one aviculariine species—Caribena versicolor—which can kick type II urticating hairs off of the abdomen, similarly to species from the subfamily Theraphosinae.
Type II urticating hairs can be found in the genera Avicularia, Iridopelma and Pachistopelma (subfamily Aviculariinae).
[15][16] New World tarantulas will, at the moment of danger, turn toward the attacker and briskly rub their hind legs against the opisthosoma throwing the urticating hairs in the direction of the enemy.
Humans' reaction and the degree of irritation to a defensive urticating hair barrage can vary tremendously, based on the species in question.
They can be found on and around the burrow entrance and in webbing for protection (for example, some subfamily Theraphosinae species include these bristles in cocoon silk).
