448 THE INVERTEBRATA 



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. 3 13) is tubular and contains zones corresponding 

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

 veloping 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 in- 

 vagination of the body wall around the genital aperture adapted for 

 receiving the intromittent organ of the male. 



The fiervous system of insects (Fig. 314) consists of a dorsal brain 

 and a ventral double chain of ganglia connected by longitudinal and 

 transverse commissures. The anterior three pairs of ganglia of the 

 ventral chain are always fused to form the suboesophageal ganglion^ 

 the nerves from which supply the mouth parts. The suboesophageal 

 ganglion is united by paraoesophageal connectives to the brain. 



The brain consists of three pairs of closely fused ganglia which 

 supply the eyes, antennae and labrum respectively (see p. 425). In 

 addition to this is the sympathetic system (Fig. 314 B, C) which 

 supplies the muscles of the alimentary canal and of the spiracles. 



In the insects, and indeed the arthropods in general, there has been 

 a great advance over the stage of nervous organization in the annelids. 

 The complex nature of the appendages and the necessity of co-ordi- 

 nating groups of these for locomotion, feeding and so on, has led to 

 the association of special parts of the nervous system with these 

 functions. We will call each such part a "functional unit". Each 

 functional unit is to some extent self-regulating and is not dependent 



