Fig. 215.—A, group of setæ arising from a subdorsal tubercle: cut, the cuticle; hy, the hypodermis; sc, the enlarged and specialized cells of the hypodermis which secrete the spines themselves; pglc, the nuclei which secrete the venomous fluid which fills the cavity of the seta (s), seen at p in a broken spine. B, a short entire, and a long broken seta (s-p); pgle, four poison cells; p, the poison in the hollow of the spine.
These temporary fine glandular hairs are probably the homologues of the larger true glandular bristles and spines of the later stages of certain lepidopterous larvæ, which are brightly colored and lead an exposed life, living through a large part of the summer. In these structures the bristles or spines are hollow, filled with a poisonous secretion formed in a single large, or several smaller specialized hypodermal cells situated under the base of the spine. In the venomous spines of Lagoa crispata the poisonous fluid in the larger spines (Figs. 215, C, 216, b) is secreted in several large cells situated at the base of the spine, and this is the usual form. In the finer spines of a large tubercle (Figs. 215, A, 216) there appears to be a differentiation of the hypodermal cells into two kinds, the large, basal deep-seated, setigenous cells (216, sc) and the poison-secreting nuclei (216, pglc) situated nearer the base of the setæ. The spines being filled with poison and breaking into bits in the skin of the hands or neck, cause great irritation and smarting. These nettling or poisonous hairs or spines are especially venomous in the larva of Orgyia, Empretia stimulea, Hyperchiria io, the larvæ of the saturnians (Fig. 217) and lasiocampids, etc. They rarely occur in insects of other orders, though the skin of Telephorus is said by Leydig to bear glandular hairs.
Fig. 216.—Section of a subdorsal tubercle from a larva in stage 1: sc, the setigenous cells, one for each seta; pglc, nuclei by which the poison is secreted; s, seta; p, poison in middle of a broken spine; cut, cuticle; sd, tub, spinulated surface of the subdorsal tubercle.
Leydig states that in the stout bristles of Saturnia there is, as in the integument of the body, a homogeneous cuticula, under which is the cellular matrix (hypodermis), and the clear contents (hyaloplasma) are secreted from the blood. The cell-structure of the hairs consist, as in the cells of the body, of spongioplasma and hyaloplasma. Leydig has observed the droplets of the secretion of the caterpillar of Saturnia carpini oozing through distinctly observable pores, and states that there are similar openings in the hairs and scales. Dewitz found easily observable openings at the end of the hair of a large exotic weevil (Fig. 130).
The advanced nymph of Psylla is also armed with clavate glandular hairs (Fig. 178).
Fig. 217.—Armature of last four segments of Callosamia promethea: a, a dorsal seta; b, one showing the poison (p) within.
The tubercles are outgrowths of the body-walls; they are either smooth, warty, or spiny, as in many caterpillars. While the armature of insects is of little morphological importance, it is evidently of great biological importance, the welfare or even the life of the insect depending upon it; and it varies in each species of insect, especially in Diptera, where the position of even a single seta characterizes the species.