142 MEDICAL ENTOMOLOGY 



organ on the dorsal side of the upper end, is of a similar structure. Im- 

 mediately behind the entry of the oviduct into the uterus there is a 

 conical elevation of the wall, on the top of which are situated the orifices 

 of the ducts of the spermathecae and nourishing glands. At this point 

 the chitinous and muscular layers of the wall are greatly increased in 

 thickness. 



The posterior contracted portion of the uterus forms the vagina. It 

 has a structure similar to that of the uterus, the chitinous lamina 

 of the wall being, however, increased in thickness. It is provided with 

 dilator muscles by means of which the cavity can be widened at the 

 moment of the birth of the larva. 



The spermathecae are small rounded spheres of a yellow colour, the 

 tint being due to the thick chitinous lamina which forms their internal 

 wall. External to the chitinous layer, which in pal- 

 Palis and tachinoides is adorned with small raised areas 

 on the internal surface, there is an investment of a single layer of highly- 

 vacuolated cells. In G. fusca the raised areas are absent, and the outer 

 investment consists of two layers of non-vacuolated cells. 



The two spermathecae are united in a common cellular investment, 

 and open, each by a fine canal, into the dorsal aspect of the uterus. 

 The inner walls of these ducts show an arrangement of chitinous 

 spirals like those of a tracheal tube, but with the rings more separated 

 and less numerous. The external cellular covering is thin and the cells 

 are without vacuoles. The two ducts open into the uterus at the same 

 point, and here the circular muscle fibres are arranged to form a 

 sphincter. 



After fertilization the cavity of the spermatheca and its duct is 

 filled with spermatozoa, which are also found within the cavity of the 

 uterus at the level at which the ducts open. This is also the point 

 at which the mouth of the larva is situated during gestation, and one 

 might suppose, Roubaud remarks, that the spermatozoa which are not 

 used up in fertilization are absorbed by the larva, as has been stated 

 by Berlese to occur in Melophagus, along with the secretion of the 

 nutrient glands. 



The nutrient glands, or milk glands, are characteristic of the genital 

 apparatus of the pupiparous Diptera. In Glossina they are much 



branched tubular glands, which are situated dorsally 



Milk glands . ' 



on each side of the uterus and open into its cavity by a 



common duct just behind the opening of the ducts of the spermathecae. 

 The dimensions and the amount of ramification of the glands depend 



