Fig. 328.—Salivary and other glands of Ranatra.—After Locy.
In all the five systems there constantly occur three cellular layers: the intima, epithelial, and propria. As regards their origin Schiemenz states that Systems I and IV are new structures, that System III arises in part, and Systems II and V wholly, from the silk-glands of the larva. As the glands differ much in the sexes, and in different species and genera, Schiemenz believes that their function is very manifold.
In addition to those previously discovered by Schiementz, Bordas has detected two additional pairs of salivary glands in the worker and male honey-bee, i.e. the internal mandibular and sublingual glands, so that in Apis there are in all six pairs, and apparently one unpaired.
The delicate chitinous external layer of the gland is perforated by many very fine pores through which the salivary fluid secreted by the epithelial cells passes into the salivary duct. The glands are externally bathed by the blood.
In many insects, including lepidopterous larvæ, the single median opening of the salivary duct is converted into a spraying apparatus.
In the adult Lepidoptera, according to Kirbach:—
Fig. 329.—Salivary glands of the honey-bee: systems No. 1–3, × 15: sv, salivary valve (of systems 2 and 3) at base of tongue; lp, labial palpus; mx, maxilla; so, salivary opening of system 1 in hypopharyngeal plate; no, openings in plate for termination of taste-nerve; œ, œsophagus; sd, salivary duct; b, junction of ducts of system No. 2; c, junction of ducts of system No. 3; sc, sc, salivary sacs; fl, front lobe; bl, back lobe; a, chitinous duct, with spiral thread. B, single acinus of system No. 1, × 70: n, nucleus; st, salivary tract; d, large duct. C, single pouch, or acinus, from system No. 2: a, propria or outer membrane; sc, secreting cells. D, termination of system No. 3:1,2,3,4, lines marking end of section; d, duct in section; sc, secreting cells in section; n, nucleus.—After Cheshire.
“Its lower half forms a thick chitinous gutter, with a concave cover above, in which the similarly shaped upper half lies encased, so that between the two only a small semicircular opening remains. Powerful muscles extend from the cover to the lower side and to the two ridges of the bottom plate; through their contraction the upper channel is elevated, and presses out of the hinder part of the ducts into the space thus formed a great quantity of the saliva, which by allowing the contraction of the cover-muscle through the crevice-like opening, which is situated in the lower edge of the mouth-opening, becomes squeezed out in order either to mix with the fluid where the 2d maxillæ fuse, passing up into the canal in the proboscis, or to penetrate into and thus dilute the semi-fluid or solid substances taken, into the proboscis.”