Fig. 351.—A, median section of head, showing (du) the venomo-salivary duct, with its insertion in (hy) the hypopharynx; cb, brain; below is the pharyngeal pump, leading from (œ) the œsophagus; lre, base of labrum-epipharynx; m, muscle; n, commissure (other parts removed). B, the venomo-salivary duct, showing its bifurcation, and the three glands on one of its branches; pg, poison gland; sg, the upper of the two salivary glands. C, the bifurcation of the duct, with its nucleated hypodermis.—After Macloskie.
f. Adhesive or cement-glands
Dewitz has discovered in ants and bees, in close connection with the poison-glands, and like them discharging their secretion through the sting, cement-glands. They arise by budding at the base of the poison-glands.
The two glands in these Hymenoptera correspond to the tubular glands of the Orthoptera, which open at the base of the inner sheath of the ovipositor (Fig. 299, sb), so that the secretion flows out through it as the poison of bees, etc., out of the sting. The use of the secretion of these glands is either to glue the eggs together, or to afford material for the egg-case of cockroaches and Mantidæ and the gummy egg-case of the locusts, etc. The contents of the cement-glands serves for the fixture of the eggs after deposition. In the stinging Hymenoptera one of the cement-glands is an accessory gland; the other becomes the poison-sac. The cement-glands are in the Hemiptera only short blind sacs, in the Lepidoptera and Diptera long convoluted tubes, tubular and branched in the Coleoptera, or richly branched in the Ichneumonidæ and Tenthredinidæ. In the cockroach there are two cement-glands, but the right one is probably of no functional importance. The left one is filled with a milky substance, containing many crystals and a coagulable fluid, out of which the egg-capsule (oötheca) is formed. (Miall and Denny.) In the locusts the sebific or cement-gland (Fig. 298, sb) secretes a copious supply of a sticky fluid, which is poured out as the eggs pass out of the oviduct and agglutinates the eggs into a mass, forming a thin coating around each egg, which from the mutual pressure of the eggs causes the tough coating to be pitted hexagonally. In other insects also (Trichoptera, Chrysopidæ, Lepidoptera, etc.) there are similar secretions for the protection or fastening of the eggs when laid.[[57]] The Trichoptera lay their eggs either in or on the surface of the water in bunches or in strings or in annular gelatinous masses on stones or on plants. This jelly-like substance is secreted by two highly developed paired anal glands. (Weltner, in Kolbe, p. 621.) Also in certain dragon-flies (Libellula, Diplax, and Epitheca) the eggs are laid in jelly-like masses.
With a similar secretion, spun from the end of the abdomen, the Psocidæ cover their little bunches of eggs laid on the under side of leaves; and the silk thread forming the egg-sac of the great water-beetle (Hydrophilus) is secreted from such anal glands.
g. The wax-glands
Besides the honey-bee, which secretes wax in little scales on the under side of the abdomen, the bodies of many other insects, such as the plant and bark lice, as well as the Psyllidæ, Cicadidæ (especially Flata and Lystra), are covered with a waxy powder, or as in Chermes, Schizoneura, Flata, etc., with wool-like filaments of wax.
Fig. 352.—Under side of worker honey-bee, carrying wax scales, × 3.—After Cheshire.