INSECTA 397 



extent to which they are divided into follicles , and the form of foUicle, 

 vary in different orders. Thus, in the Diptera, each testis is unifoUi- 

 cular, while in the Orthoptera a multifolHcular condition prevails. 

 Each follicle is divided into a germariiim or formative zone, a zone of 

 growth and maturation, and a zone in which spermatids are transformed 

 into spermatozoa. In multifollicular testes the connection between 

 each follicle and the main duct is known as the vas efferens and each 

 testis leads to the median ejaculatory duct by a vas deferens which is 

 swollen at some point to form a seminal vesicle . The ejaculatory duct 

 opens between the 9th and loth abdominal sterna in association with 

 the external genital plates of copulatory significance. Accessory glands 

 of various kinds and little understood function are usually found 

 associated with the genital ducts. 



The female organs (Fig. 297) consist of ovaries^ oviducts, sperma- 

 thecae, colleterial glands and a bursa copulatrix. 



Each ovary consists of a number of ovarioles, corresponding to the 

 testicular follicles of the male. Reduction of the ovary to a single 

 ovariole occurs in such insects as Glossina, the tsetse fly, where the 

 minimal number of eggs is produced. 



Each ovariole (Fig. 297) is tubular and contains zones corre- 

 sponding to those met with in the follicle of the testis. In addition to 

 the developing ova, nutritive cells are found in association with the 

 latter. Such cells are concerned with the transference of yolk to the 

 growing ova and they or other cells may entirely encircle the ova, 

 round which they secrete the chorion or outer egg shell. 



The ovarioles forming an ovary are connected together anteriorly 

 in the body cavity by their peritoneal coverings, known at this point 

 as terminal filaments, and these are attached either to the body wall 

 or the pericardial diaphragm, thereby maintaining the ovary in 

 position. 



The oviducts leading from the ovaries unite in the middle line to 

 form a common duct which widens to form the vagina immediately 

 before reaching the exterior on or between the 8th, 9th and loth 

 abdominal sterna. 



Colleterial glands providing fluid for the formation of an ootheca 

 (a case surrounding the eggs), or a sticky secretion for fastening eggs 

 to surfaces, usually open into the vagina. The pouch for the reception 

 of spermatozoa is the spermatheca. It is an ectodermal invagination, 

 lined by chitin and provided with a muscular coat. The spermatheca 

 opens into the vagina or into the bursa copulatrix, this being an 

 invagination of the body wall around the genital aperture adapted for 

 receiving the intromittent organ of the male. 



The nervous system of insects (Fig. 298) consists of a dorsal brain 

 and a ventral double chain of ganglia connected by longitudinal and 



