Spicule Hairs

Type 1 (Euproctis type). These are very small hairs (length 50—200 fim; diameter up to 5 /¿m), each of which has a pointed end that articulates with its own socket. The distal end has multiple small barbs. Upon detaching, the pointed tip of the hair penetrates the skin. They occur as small clusters of 3—15 hairs each in cup-shaped papillae (Fig. 18.5A) on various parts of the caterpillar. Type 1 hairs are characteristic of the brown-tail moth (Euproctis chrysorrhoea) and other Euproctis species in the family Lymantriidae. The number of these papillae can be extremely large, as in E. similislarvae, with an estimated 600,000 spicule hairs, and E. subflava larvae, with more than 6 million.

Type 2 (Thaumetopoea type). These spicule hairs are similar to type 1 in size and shape but are pointed at

Gary Ii. Mullen

FIGURE 18.4 Urticating hairs and spines of North American caterpillars. (A) Pandora caterpillar, Cobra-din pandora (Saturniidae); (B) buck moth, Hemileuca maia (Saturniidae); (C) puss caterpillar, Megaiopyge 0£>£rcB/ara(Megalopygidae); (D) white flannel moth, Norape ovina (Mega-lopygidae); (E) smeared dagger moth, Acronicta, oblinita (Noctuidae); (F) hag moth, Phobetron pitbecium (Limacodidae)-, (G) io moth, Automeris io (Saturniidae); (H) spiny oak-slug caterpillar, Eudea delphinii (Limacodidae); (I) mourningcloak butterfly, Nymphalisantiopa (Nymphalidae); (I) saddleback caterpillar, Sibine stimulea (Limacodidae). (Original by Margo Duncan.)

both ends. They are inserted point-downward into individual cuplike sockets and occur only in third-instar and older larvae. They are typical of the family Thaumetopoei-dae. The processionary caterpillar {Thaumetopoea processioned) is estimated to have over 630,000 of these specialized defensive hairs.

Type 3 (Dendrolimus type). These are relatively long (0.5 — 1,0 mm), slender spicule hairs which have blunt proximal ends and sharply pointed distal tips (Fig. 18.5B). They are loosely articulated with individual sockets and are easily broken off. In addition to mechanical injury to the skin, they can cause localized reactions upon discharge of toxin when the spicule wall is broken. These urticating hairs occur in adults of the Lasiocampidae, notably on the mesothoracic and metathorax of the genus Dendrolimus.

Type 4 (Latvia type). These spicule hairs are relatively short (0.5—1.0 mm) and stout with a pointed distal tip and 3—4 basal barbs. They are multicellular in origin, typically having a large poison-secreting cell surrounded by supporting cells. They are characteristic of slug caterpillars of the genus Latoia, family Limacodidae,

and Illiberis, the arctiid genera Eilema and Lithosia, and first-instar larvae of the gypsy moth Lymantria dispar (Lyman triidae).

Type 11 (Latvia type). These spines are relatively short, stout, and conical, with a bulblike base containing a large poison gland (Fig. 18.5D). There is no opening at the pointed tip. The structure of type 11 spine hairs is more complex than the others, involving not only multiple trichogen cells and poison-secreting cells, but also tracheal and nerve cells. The distinguishing feature is the presence of a diaphragm on which the poison cell rests. When mechanically stimulated on contact with skin, the pressure of the hemolymph acting on the diaphragm causes toxin to be ejected from the tip of each broken spine. The result can be some of the most severe dermal reactions associated with urticating caterpillars. Type 11 spine hairs occur most commonly in the Limacodidae, best represented by Latoia larvae. Other limacodid genera with this type of spine are Microleon, Monema, Parasa, and Scopelodes. Similar urticarial spines are found in larvae of Automeris io and Dirpbia (Saturniidae), Catocala (Noctuidae), and the flannel-moth genera Doratifera and Megalopyge (Megalopygidae).

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