362 TEXT-BOOK OF ENTOMOLOGY 



The wax is secreted by minute unicellular dermal glands, which 

 in the lower insects (Hemiptera) are distributed nearly all over the 

 body, but in the bees are restricted either to the under (Apis, Fig. 352) 

 or upper side (Trigona) of the end of the abdomen. 



The wax-glands of Pemphigus, Chermes, etc., lie under the little 

 warts, seen in Laclmus strobi, the white-pine aphis, to be distributed 

 in transverse lines across the back and sides of the abdominal seg- 

 ments (Fig. 353). These warts are surrounded by a chitinous ring, 

 and divided into delicately marked areas. Through the dedicate 

 numerous pits in the chitinous membrane of these areas the little 

 waxen threads project, since under each area ends a duct leading 

 from a large glandular cell, which is a specially modified hypodermis 

 cell (Glaus). The wax threads are hollow, and all those arising from 

 a single glued cell form a bundle, whose threads separate from each 

 other and form a white woolly down or bloom covering the body. 

 Witlaczil also shows that gall-forming Aphids secrete a wax-like 

 substance, which, during the movements of the insects in the gall, is 

 rubbed off, becoming a watery layer mixed with the fluid, excrement, 

 which forms a spherical impervious layer lining the gall, and thus 

 rendering possible the mode of life of the gall-lice. 



In the Psyllidae Witlaczil has discovered wax-glands which also 

 secrete slender waxen threads. They are situated in groups of two 

 or three at the end of the abdomen near the anus, and arise from 

 hypodermis cells. The wax threads surround the liquid excrement 

 as it passes out of the vent, covering it with a continuous layer of 

 wax. The excrement accordingly is discharged very slowly and 

 gradually, in sausage-shaped masses slightly strung together and 

 rolled into close spirals. The body becomes unavoidably smeared 

 with the sticky excrement, since it is not entirely covered by the 

 waxy layer. Moreover, in the larvae of many Psyllidae waxen threads 

 are formed on the upper side of the abdomen ; they are for the most 

 part tightly curled or frizzly, like wool, and form, though partly 

 torn, a waxen coat, chiefly on the side and back of the thorax and 

 abdomen. The insects appear therefore as if covered with dust. 

 The mature animals of many species are also covered with a waxen 

 down. The wax threads rapidly dissolve and disappear in alcohol. 

 From a wax-like substance more or less easily dissolved in alcohol 

 arise peculiar hair-like structures which, in the larvae of Psyllida?, 

 are situated on the side a,nd end of the body and also on the rudi- 

 ments of the wings. They are readily distinguished from ordinary 

 hairs, as they arise from glandular cells, and are of very different 

 lengths, more or less like bristles, but hollow, and very brittle. 



