The large, soft, rounded, eversible glands, looking like puff-balls or a rounded pudding (Fig. 366, 12), are everted, when the butterflies are roughly seized, from the dorsal side of the penultimate segment of the abdomen. The males possess two smaller tubercles on the inside of the anal claspers or lobes. Müller also detected, in the females of various species of the Heliconidæ enumerated above, a pair of club-shaped processes like the balancers of flies, which are thrust out on each side of and under the odoriferous puff-balls of the hinder edge of the penultimate segment (Fig. 366, 13). The club or head is armed with hairs or bristles, which, in Heliconius, are like the scales of a butterfly.
Fig. 366.—Scent-glands of insects: 1. Anal eversible glands of Eleodes.—After Gissler. 2. Anal eversible glands of Blaps.—After Gilson. 3. Anal glands (agl) of Carabus hortensis: rs, reservoir; d, excretory duct; i, intestine; r, rectum.—After Kolbe. 4. Prothoracic spraying apparatus of Cerura vinula: gl, the gland; d, its duct, with tænidia; t, the spraying tubes; m, muscles; rm, retractor muscles.—After Klemensiewicz. 5. The thoracic glandular sac of Macrurocampa marthesia: gl, the glandular sac; d, its duct; e, peritracheal epithelium; t, the spiral threads or tænidia. 6. Irregular separate masses of chitinous ridges on the cuticular lining of the wall of the sacs of Macrurocampa marthesia. 7. Osmeterium (os) of the larva of Papilio machaon at rest: rm, the retractor muscles at the ends; m, the numerous oblique muscles; dm, dorsal longitudinal muscles; t, trachea; oe, œsophagus; gang, brain; 1, head; 2, 3, 4. thoracic segments. 8. Osmeterium (os) of one side, enlarged: g, glandular portion at the base; d, depressions in the cuticula of the glandular portion; t, trachea.—This and Fig. 7 after Klemensiewicz. 9. Eversible dorsal glands (ev. gl) of larva of Orgyia leucostigma in Stage II: gc, glandular cells at bottom of the crater-like depression; m, retractor muscle; p, poison gland-cells of the root of the seta (s); c, cuticula; hyp, hypodermis; A, portion of the cuticle and hypodermis enlarged. 10. Lateral eversible gland of Hyperchiria io, Stage II: rm, retractor muscle; oen, œnocytes. 11. The same as Fig. 10, but representing a section through one side of the eversible gland. 12. A, end of body of Colænis julia; ev, eversible anal gland; oa, odoriferous appendages; B, the same in Heliconius apseudes, side view; C, odoriferous appendages of Colænis dido in fresh condition; D, tested with alcohol and benzine. 13. Odoriferous appendages of Heliconius eucrate, head cleansed.—Figs. 12, 13, after F. Müller. 14. Odoriferous glands (ogl) in the pupa of Vanessa io: r, rectum; h, the folds of hypodermis which forms the terminal papilla of the abdomen; ov, oviduct.—After Jackson.
In the caterpillars of certain blue butterflies (Lycænidæ) is an internal osmeterium, being a very minute sac which is everted from a transverse slit on the top of the 7th abdominal segment. Its function is quite the opposite of those of the caterpillars of other families, since the sac exudes a sweet fluid very attractive to ants, which may be diffused more widely by the delicate spinulose bristles crowning the summit. W. H. Edwards states that in several species of Lycæna, besides that on the 7th abdominal segment, there is on the 8th segment a pair of minute dorsal evaginable tubercles.
A pair of small ramose odoriferous glands are said by Siebold, who regarded them as alluring glands, to occur in Argynnis, Melitæa, and Zygæna, to be situated near the orifice of the oviduct, and Scudder has detected them near the anus of the female pupa of Danais archippus. The appearance of the odoriferous glands in the pupa of Vanessa io is well shown by Jackson (Fig. 366, 14). They develop as two tubular ingrowths of the hypodermis, perfectly distinct one from the other, each having its own separate aperture to the exterior. In Fig. 366, 14 the condition of parts is nearly as in the imago, the glands being situated below the rectum and opening of the oviduct. In both sexes of another Brazilian butterfly (Didonis biblis) on the median line of the abdomen between the 4th and 5th segments are two roundish vesicles covered with short gray hairs, which emit a disagreeable smell.
It is possible that the dark-green fluid in Parnassius, secreted by an evaginable gland, and which is moulded into shape by the scimetar-shaped peraplast (Scudder), is formed by the homologues of the anal glands of other butterflies.
Distribution of repugnatorial or alluring scent-glands in insects[[59]]
A. Larval Insects
a. Each thoracic segment; sternal. Phryganea grandis.
b. Prothoracic, sternal, discharging a lateral jet of spray; with a single large internal sack.