Fig. 363.—Freshly hatched larva of Hyperchiria io, with its two pairs of eversible glands (g).
Fig. 364.—Young larva of Megalopyge crispata, enlarged, showing the seven pairs of lateral processes (lp): sp, spiracle; abl′, abl6, six pairs of abdominal legs besides the anal pair.
In the Hemileucidæ there is a pair of lateral osmeteria, on the 1st and on the 7th abdominal segments, which however, are not highly colored (Figs. 363, 366, 10). In Megalopyge (Lagoa, Fig. 364) there is a lateral row of singular pale permanently everted processes which appear to be the homologues of the osmeteria of larvæ of other lepidopterous families. As these are repeated on seven segments, their metameric arrangement is obvious. The relation of these curious glands to the viscera is seen in Fig. 297, lgp, and their minute structure in Fig. 365.
At A, the lumen (l) is a deep narrow cavity, with the secretion (secr.), collected at the mouth of the cavity, composed of a thin, mucus-like, coagulated fluid, containing granules of varying degrees of fineness, which take the stain readily. Outside of these are collected fine nuclei (bc), stained dark, and enveloped in a slight, transparent, pale, protoplasmic envelope, which may be blood corpuscles. The glandular cells themselves are simply modified hypodermal cells, as seen at C. In some of the nuclei, indistinct nucleoli are seen, and deeply stained granules, especially around the periphery of the nuclei. At B is represented a section on one side of the middle, but still showing the spacious lumen. In the section represented by C, the knife passed through the process still nearer the outer edge, and near the base; at C1, three of the glandular cells, with their large, deeply stained nuclei, are drawn. A transverse section at D shows the large lumen or cavity (l).
As to the function and homologies of these structures, it is difficult to decide. We have never noticed that they give off any odor, though they may prove to be repugnatorial; they are not visible in the fully grown, living insect, being concealed by the long, dense hairs clothing the body; they are not spraying organs, as they are imperforate at the end, not ending as the lateral, eversible glands of Hyperchiria io, etc., in a crateriform orifice.
They may be permanently everted glands, or osmeteria, which have, by disuse, lost their power of retraction and their crateriform opening, as well as the power of secreting a malodorous fluid.
Fig. 365.—Section of lateral processes of larva of Megalopyge.
In certain of the butterflies, the Heliconidæ (Colænis, Heliconius, Euides, and Dione), there is thrust from the end of the abdomen a pair of large, irregular, rounded, eversible glands, which give out a disagreeable odor, and are consequently repellent, and which seem to be the homologues of the odoriferous glands of other butterflies.