[Illustration: 37. (_b_) Effect of the poison on the blood corpuscles of man. After Tyzzer.] The poison hairs (fig. 37), are found on the subdorsal and lateral tubercles (fig. 38), in bunches of from three to twelve on the minute papillæ with which the tubercles are thickly covered. The underlying hypodermis is very greatly thickened, the cells being three or four times the length of the ordinary hypodermal cells and being closely crowded together. Instead of a pore canal through the cuticula for each individual hair, there is a single pore for each papillæ on a tubercle, all the hairs of the papilla being connected with the underlying cells through the same pore canal, (figs. 39 and 40). [Illustration: 38. Cross section of the larva of the browntail moth showing the tubercles bearing the poison hairs. Drawing by Miss Kephart.] The hypodermis of this region is of two distinct types of cells. First, there is a group of slender fusiform cells, one for each poison hair on the papilla, which are the trichogen, or hair-formative cells. They are crowded to one side and towards the basement membrane by a series of much larger, and more prominent cells (fig. 40), of which there is a single one for each papilla. These larger cells have a granular protoplasm with large nuclei and are obviously actively secreting. They are so characteristic in appearance as to leave no question but that they are the true poison glands. [Illustration: 39. Epithelium underlying poison hairs of the larva of the browntail moth. Drawing by Miss Kephart.] Poisonous larvæ of many other species have been reported from Europe and especially from the tropics but the above-mentioned species are the more important of those occurring in the United States and will serve as types. It should be noted in this connection that through some curious misunderstanding Gœldi (1913) has featured the larva of _Orgyia leucostigma_, the white-marked tussock moth, as the most important of the poisonous caterpillars of this country. Though there are occasional reports of irritation from its hairs such cases are rare and there is no evidence that there is any poison present. Indeed, subcutaneous implantation of the hairs leads to no poisoning, but merely to temporary irritation. [Illustration: 40. Same as figure 39, on larger scale.] Occasionally, the hairs of certain species of caterpillars find lodgement in the conjunctiva, cornea, or iris of the eye of man and give rise to the condition known as _opthalmia nodosa_. The essential feature of this trouble is a nodular conjunctivitis which simulates tuberculosis of the conjunctiva and hence has been called _pseudo-tubercular_. It may be distinguished microscopically by the presence of the hairs. [Illustration: 41. (_a_) Nodular conjunctivitis in the eye of a child. De Schweinitz and Shumway.]