ANATOMY AND PHYSIOLOGY 75 



under side of the abdomen as four pairs of wax scales (Fig. 104), on 

 the last four of the six evident segments of the abdomen (Dreyling). 

 Plant lice of the genus Schizoneura owe their woolly appearance to dense 

 white filaments of wax, which arise from glandular hypodermal cells. 

 In scale insects, waxen threads, emerging from cuticular pores, become 

 matted together to form a continuous shield over and often under the 

 insect itself, the cast skins often being incorporated into this waxen 

 scale. The wax glands in Coccidae are simply enlarged hypodermis 

 cells. 



Some coccids produce wax in quantities sufficient for commercial 

 use. Thus species of Ceroplastes (and certain fulgorids as well) in 

 India and China yield a white wax that is used for making candles and 

 for other purposes. 



The lac-insect, Tachardia lacca, of India, a scale insect living on 

 more than ninety species of trees and shrubs (Acacia, Ficus, Zizyphus, 

 etc.), furnishes the lac from which shellac, lacquer and other varnishes 

 are made. The raw lac is the scale, or shell, of the female insect (the 

 male producing scarcely any lac) and consists of a yellow to reddish 

 brown resinous exudation containing considerable wax, along with the 

 cast skins of the insect. From this material the commercial products 

 are extracted. 



Silk Glands. Larvae of very diverse orders spin silk, for the purpose 

 of making cocoons, webs, cases, and supports of one kind or another. 

 Silk glands, though most characteristic of Lepidoptera and Trichoptera, 

 occur also in the cocoon-spinning larvae of not a few Hymenoptera 

 (saw flies, ichneumons, wasps, bees, etc.), in Diptera (Itonididae), 

 Siphonaptera, Neuroptera (Chrysopidae, Myrmeleonidae) , and in 

 various larvae whose pupae are suspended from a silken support, as in 

 the coleopterous families Coccinellidae and Chrysomelidae (in part) 

 and the dipterous family Syrphidae, as well as most diurnal Lepidoptera. 



The silk glands of caterpillars are homologous with the true salivary 

 glands of other insects, opening as usual .through the hypopharynx, 

 which is modified to form a spinning organ, or spinneret (Fig. 105). 

 The silk glands of Lepidoptera are a pair of long tubes, one on each side 

 of the body, but often much longer than the body and consequently 

 convoluted. Thus in the silk worm (Bombyx mori) they are from four 

 to five times as long as the body and in Telea polyphemus, seven times 

 as long. In the silk worm the convoluted glandular portion of each 

 tube (Fig. 106) opens into a dilatation, or silk reservoir, which in turn 

 empties into a slender duct, and the two ducts join into a short common 



