Fig. 349.—Cephalic gland of Belostoma.
Bordas has found both the alkaline gland (gland of Dufour) and the acid gland to occur in a hundred species of Hymenoptera, including not only Aculeata, but also Ichneumonidæ (Fig. 348), Tenthredinidæ, and they may be safely said to be of general occurrence. The acid gland consists of three parts, the glandular portion, the reservoir for the poison, and the secretory canal. The alkaline gland is an irregular tube, with a striated surface and without a reservoir. In most Hymenoptera there is still a third gland, which is unpaired, granular, rectangular or lanceolate, with a short filamentous duct which opens beside the orifice of the alkaline glands.
The poison in ants, wasps, and bees consists of two substances, i.e. formic acid and a whitish, fatty, bitter residue in the secretion of the glands; the corroding active formic acid is the essential part of the poison. (Will.)
In Melipona the sting and poison-glands are aborted; in certain ants (Formica, Lasius, etc.) the sting is wanting, but the poison-sac is extraordinarily large.
Bordas finds in various species of Ichneumon three kinds of glands opening into the base of the sting. The first two correspond to the acid (Fig. 348, G.A) and alkaline (G.A) glands of bees and wasps (Vespidæ, etc.), and the third (G.ac) is situated between the two lateral muscular bundles which attach the base of the sting to the last abdominal segment. The poison-reservoir (Fig. 348, V) is recognized by its yellow color and diaphanous and striated appearance. It is situated on the left of the hind-intestine, a little in front of the rectum. The tubular gland (Ga) or alkaline gland of aculeate Hymenoptera is remarkably large; it is situated on the left side of the body. The accessory gland (G.A) is elongated, triangular, flat, its duct opening at the base of the alkaline gland; it is formed of small spherical cells. Bordas has met with well-developed poison-glands in forty species belonging to the Terebrantia, including that of Tenthredo, Emphytus, as well as various genera of Ichneumonidæ, but in all these species the accessory gland was wanting.
Fig. 350.—View from above of the cephalic gland of Belostoma, × 20.—This and Fig. 349 after Locy.
Under the name of cephalic glands (Fig. 349), Locy describes a pair of glands in the head of Nepidæ. The epithelial or secreting cells are 8–sided (Fig. 350). “When these insects are irritated,” he says, “a secretion is freely thrown out around the base of the beak, which produces death very quickly when introduced on a needle point into the body of an insect.” He infers that the cephalic glands may be the source of this poisonous secretion. The poisonous salivary fluid of the larva of Dyticus is referred to on p. 324.
That the mosquito injects poison into the wound it makes has been proved by Macloskie, who discovering fine droplets of a yellow oily-looking fluid escaping from the end of the hypopharynx, afterwards detected the poison-glands. It appears that the two salivary glands are subdivided, each into three lobes, the middle of which (Fig. 351, pg) differs from the others in having evenly granulated contents and staining more deeply than the others. Having examined the preparations, we agree with the discoverer that these lobes secrete the poison. The poison is diluted by the secretion of the salivary lobes, and the two efferent ducts, one from each set of glands, “carry forward and commingle the venomo-salivary products in the main duct; and the stream is then carried by the main duct to the reservoir at the base of the hypopharynx.”