among useful secretions are silk and wax. Silk is furnished more or less abundantly by all the caterpillars and many other larvæ. Fig. 19.—Appearance of a Wasp in Flight. The silkworms produce it in large quantities.^[1] Wax is produced by a number of Hymenoptera, which construct cells of it to hold honey. Aphides and cochineals secrete fatty matters, the white tufts of which form a kind of down on their bodies and on the plants they frequent. In other insects the secretions become a defensive armor. Thus the Hymenoptera drop poison in the wound made in animal tissues by their sting, which it causes to swell, An analogous stinging gives rise to the excrescences called galls, with which the leaves of trees are

often covered. Many of the Coleoptera emit penetrating odors. The Cicindela smells of the rose, the Aromia moschata of musk; the anal glands of the carabicus produce butyric acid; while