FEMALE ORGANS OF REPRODUCTION 137 



has a small cap of undifferentiated cells at its upper pole, which goes 

 to form the third follicle, and so on, the succeeding follicles being pro- 

 gressively smaller in accordance with their age. In the mature ovariole 

 there is, therefore, one fully developed egg ready to be laid, and several 

 follicles at various stages of growth. 



The above account will apply to the Diptera generally. Individual 

 differences are found in the relative degrees of development at which it 

 is usual to find the different series of eggs, and this affects the 

 shape of the ovarian tube as a whole. In the mosquitoes the first 

 follicle becomes nearly mature before the development in the second is 

 much advanced, and while the third and fourth are little more than 

 masses of undifferentiated cells ; when the apical filament is broken and 

 the tube rendered flaccid these undeveloped follicles appear as a little 

 cap on the upper pole of the nearly mature egg. In the Muscidae 

 the follicles after the first are more advanced, so that an isolated tube 

 appears longer. Tabanus occupies an intermediate position. In each 

 case, however, the appearance is accentuated by the shape of the egg ; 

 that of Culex (Plate XXIX, fig. 2) is almost round, while that of 

 Stomoxys (fig. 1) is a long ovoid. 



The paired oviducts and the common oviduct have muscular walls 

 with well developed transverse fibres, which doubtless aid in the ex- 

 pulsion of the ova. They are lined internally with 



J . . Oviducts 



a layer of flat cells with small nuclei, superficial to 



which there is a delicate lamina of chitin continuous with that of the 

 integument. The bursa copulatrix is not as well developed in the 

 Diptera as it is in some other orders of insects, and is not easily 

 distinguished. In Haematopota it is provided with a series of minute 

 spines set on a chitinous framework, which apparently engage with 

 the male organs in copulation. 



The other parts of the reproductive system show considerable vari- 

 ations in the different genera. The spermathecae are small chambers 



with chitinous and pigmented walls, formed as in- 



r . . spermathecae 



vaginations of the wall of the oviduct, and having, 

 therefore, a layer of small cells external to the thick chitinous wall. 

 In Anopheles there is only one such sac, with a long and narrow duct. 

 In Culex and Stegomyia there are three. In Tabanus and Haematopota 

 (fig. 4) the spermathecae are long fine tubes, bent on themselves in the 

 middle of their length ; the slightly expanded blind ends are attached 

 to the wall of the oviduct near the origin of the tube. The chitin 

 forming the internal layer of the wall is thickest near the blind end, 

 18 



