THE INTERNAL ANA TOMY OF INSECTS 159 



enclosed in an epithelial sac or egg-follicle, which passes down the tube 

 with the egg (Fig. 179). There is thus a tendency to strip the tube of 

 its epithelium, but a new one is constantly formed. 



The functions of the follicular epithelium. — It is believed that in 

 some cases, and especially where the nurse-cells are wanting, the 

 follicular epithelium serves a nutritive function. Eut the most 

 obvious function cf this epithelium is the formation cf the chorion or 

 egg-shell, which is secreted on its inner surface. The pit-like mark- 

 ings so common on the shells of insect eggs indicate the outlines of the 

 cells of the follicular epithelium. 



The ligament of the ovary. — In many insects, the terminal fila- 

 ments of the several ovarian tubes of an ovary unite and form a 

 slender cord, the ligament of the ovary, which is attached to the dorsal 

 diaphragm; but in other insects this ligament is wanting, the terminal 

 filaments ending free in the body cavity. 



The oviduct. — The common outlet of the ovarian tubes is the ovi- 

 duct (Fig. jj-j, od). In most insects the oviducts of the two ovaries 

 unite and join a common outlet, the vagina; but in the Ephemerida 

 and in some Dermaptera each oviduct has a separate opening. 



The egg-calyx. — In some insects each oviduct is enlarged so as to 

 form a pouch for storing the eggs, these pouches are termed the egg- 

 calyces (Fig. 177, c.) 



The vagina. — The tube into which the oviducts open is the vagina 

 (Fig. 177, v). The vagina differs in structure from the oviducts, due 

 to the fact that it is an invagination of the body-wall, and, like other 

 invaginations of the body- wall, is lined with a cuticular layer 



The spermatheca. — The spermatheca is a sac for the storage of the 

 seminal fluid (Fig. 177,5). As the pairing of the sexes takes place only 

 once in most insects and as the egg-laying period may extend over a 

 long time, it is essential that provision be made for the fertilization of 

 the eggs developed after the union of the sexes. The eggs become full- 

 grown and each is provided with a shell before leaving the ovarian 

 tubes. At the time an egg is laid a spermatozoan may pass from the 

 spermatheca, where thousands of them are stored, into the egg 

 through an opening in the shell, the micropyle, which is described in 

 the next chapter (Figs. 184 and 185). 



In some social insects, eggs that are developed years after the 

 pairing took place are fertilized by spermatozoa that have been stored 

 in the spermatheca. 



The bi'rsa copulatrix. — In many insects there is a pouch for the 

 reception of the seminal fluid before it passes to the spermatheca. 



