THE ABDOMEN, WAX GLANDS, AND STING. 71 



one next in front of it and that the intersegmental membrane (Mb) 

 is reflected from the middle of the dorsal surface of each to the 

 anterior edge of the following sternum. By removing an individual 

 plate (fig. 35 A) this is more easily shown. It is also clearly seen 

 that the transverse line of attachment of the membrane (Mb) divides 

 the sternum into a posterior part (Rd), which is merely a prolonged 

 reduplication underlapping the following sternum, and into an an- 

 terior part underlapped by the preceding sternum. The posterior 

 half is, hence, purely external while the anterior half forms the true 

 ventral wall of the segment, its dorsal face being internal and its 

 ventral face external. The anterior part is also very smooth and 

 shiny and somewhat bilobed and for this reason it is sometimes called 

 the " mirrors." Its edge is bounded by a thickened ridge giving off a 

 short apodeme (Ap) on each side. The mirrors of the last four 

 sterna are also, and more appropriately, called the wax plates because 

 the wax is formed by a layer of cells lying over them. It accumu- 

 lates on the ventral side in the pocket between the wax plates and the 

 jjosterior underlapping prolongation of the preceding sternum. Wax 

 is formed only on the last four visible segments, i. e., on segments 

 IV-VII, inclusive. 



In studying any part of the body wall of an insect it must always 

 be borne in mind that the chitin is originally simply an external cutic- 

 ular layer of a true cellular skin or epidermis (erroneously called 

 "hypodermis" in insects), but that in the adult stage the latter 

 almost everywhere disappears as a distinct epithelium. Thus the 

 chitin comes to be itself practically the entire body wall, the cell layer 

 being reduced to a verj- inconspicuous membrane. However, in cer- 

 tain places the epithelium may be developed for special purposes. 

 This is the case with that over the Avax plates which forms a thick 

 layer of cells that secrete the wax and constitute the so-called wax 

 glands. The wax is first secreted in a liquid condition and is ex- 

 truded through minute pores in the wax plates of the sterna, harden- 

 ing on their under surfaces into the little plates of solid wax with 

 which every bee keeper is acquainted. 



The secretion of the wax has been studied by Dreyling (1903), who 

 made histological sections through the glands at different times in 

 the life of the bee. He found that in young, freshly emerged workers 

 the epidermis of the wax plates consists of a simple layer of ordinary 

 epithelial cells. As the activities of the bee increase, however, these 

 cells elongate while clear spaces appear between them and, when the 

 highest development is reached, the epithelium consists of a thick 

 layer of very long cells with liquid wax stored in the spaces between 

 them. In old age most of the cells become small again and in those 

 bees that live over the winter the epithelium degenerates to a simple 

 sheet of nucleated plasma showing no cell boundaries. It is thus 

 evident that the secretion of wax is best performed during the prime 



