132 MEDICAL ENTOMOLOGY 



The exact function of the fat body is not known, but it is evidently 

 an important one closely connected with the metabolism of the fly, 

 for it has a rich supply of tracheae. The quantity present in newly- 

 hatched flies and in fully-fed larvae is much greater than that found 

 in older specimens, and the appearance of the cells is greatly altered 

 concurrently with the diminution in amount. The cells lose their 

 nuclei, the protoplasm becomes broken up and replaced by large vacuoles, 

 and sometimes crystals may be found in them. Most probably the fat 

 body is a store of reserve food material accumulated in the larval 

 stage, to assist the insect in the reproductive phase ; there is also some 

 reason to believe that it assists in the elimination from the body of waste 

 products. 



THE REPRODUCTIVE SYSTEM 



The organs of reproduction are situated in the posterior part of the 

 abdomen between the alimentary tract and the ventral chain of nerve 

 ganglia. The external opening lies anterior and ventral to the anus, 

 from wHich it is separated by the sternal plate of the last segment. 

 The separate elements of which the system is composed are paired and 

 situated laterally, only the posterior efferent duct and the opening to the 

 exterior being common to the two sides. The genital opening is sur- 

 rounded by certain modified parts of the exo-skeleton, forming the exter- 

 nal genitalia already described, the purpose of which is to assist in 

 copulation, and, in the female, in oviposition. The sexes are always 

 complete and distinct from one another in the Diptera. The internal 

 organs of reproduction of the male exhibit a wide degree of variation, 

 as is the case with the external parts. 



The essential sexual elements are certain cells derived from the 

 blastoderm, which are contained in the male testes and in the female 

 General structure ovar ^ es - Leading from these organs there are ducts to 

 convey the sexual products to the exterior, known as the 

 vasa deferentia and the oviducts respectively ; in the course of these ducts 

 there may be dilatations or diverticula for the reception of the sperms, 

 prior to copulation in the male, and after copulation in the female. 

 The posterior portions of these ducts are fused with one another, and are 

 formed, together with a portion of the bilateral ducts, by an invagination 

 of the cuticle ; they are therefore lined with chitin continuous with that 

 of the exo-skeleton. The chitin may be present in a dense and pigmented 

 layer in certain parts, as in the spermathecae of mosquitoes. In association 



