NO. 8 INSECT ABDOMEN SNODGRASS 35 



egg passage beyond that formed of the true median oviduct takes the 

 form of a vaginal tube, and may be produced into a large copulatory 

 pouch. 



The egg duct in all Lepidoptera opens at the end of the ninth ab- 

 dominal segment, or on a terminal segment which is evidently the 

 united ninth and tenth somites. The statement made by Petersen 

 (1900) that the single genital aperture in Hcpialiis and other Jugatae 

 is on the eighth abdominal segment has not been substantiated by 

 subsequent investigators. It has recently been shown by Busck (1931) 

 that the posterior aperture is always on the ninth segment (or on the 

 combined ninth and tenth somites) and that the single, posterior open- 

 ing is a feature not only characteristic of the Jugatae but occurring 

 likewise in several of the lower families of the Frenatae. It is thus 

 somewhat surprising to find that in the Lepidoptera the more primitive, 

 anterior opening has been suppressed in the more generalized families, 

 but the explanation lies in the fact that the posterior position of the 

 oviporous is correlated with the tubular extension of the abdomen to 

 form an " ovipositor ". There is no clear distinction in the egg 

 passage of the Lepidoptera between the true oviducal part of the 

 tube and the vagina, but the opening of the spermatheca into the tube 

 probably marks the anterior end of the secondarily added vagina. 



THE GENITAL CHAMBER AND THE VESTIBULUM 



The formation of external cavities at the base of the ovipositor con- 

 cealing the gonopore and the opening of the spermatheca, and some- 

 times the opening of the accessory glands, results from the invagination 

 of the body wall at the base of the ovipositor, usually accompanied by 

 a posterior extension of the eighth or of the seventh sternum. The 

 sternum concealing the cavity is known as the snbgenital plate; in some 

 insects it is the eighth, in others the seventh. 



The cavity here defined as the genital chamber, or bursa copulatrix 

 (fig. 8 B, GC) is that formed by an invagination of the part of the 

 body wall above the end of the eighth sternum in which are situated 

 the openings of the median oviduct and the spermatheca. The ventral 

 wall of the genital chamber may contain the reflected posterior part 

 of the eighth sternum. The oviductus communis (Ode) opens either 

 on the floor of the genital chamber or into its anterior end. The 

 spermatheca (Spt) opens typically in the dorsal wall of the chamber, 

 morphologically posterior to the gonopore. The actual relation of the 

 two openings to each other, however, varies on account of the variable 

 position of the gonopore, the latter, when located on the floor of the 



