Caterpillars of the Pine Processionary Moth, Thaumetopoea pityocampa, have ruined many a vacation in the French countryside. These caterpillars produce urticating hairs that can elicit an allergic reaction in people and animals who contact them. The setae on the posterior segments of these caterpillars are held in place by structures called mirrors. The setae are attached to an internal gland that produces a liquid stored inside the setae. Setae have barbed hooks and sharp ends that penetrate the skin and prevent their removal. The hooks allow the setae to move through the skin and lodge in the vicinity of blood vessels.
The fluid inside the setae is released under the skin. The combination of the physical damage by the setae, the components of the outside of the setae and the liquid components inside the setae can cause an allergic reaction, pain and itching. A group of scientists* investigated the liquid inside the setae and found 70 proteins present. Seven of the proteins elicited allergic reactions in laboratory tests. One of the proteins was found in 61% of patients who experienced and allergic reaction to the caterpillar setae. Perhaps the identification of the allergens will lead to new treatments of these reactions in the future.
*AI Rodriguez-Mahillo & colleagues. 2012. Setae from the pine processionary moth (Thaumetopoea pityocampa) contain several relevant allergens. Contact Dermat. 67:367–74