THE INTERNAL ANATOMY OF THE HOUSE-FLY 49 



of the fat-body varies considerably ; just before hibernation it 

 seems to fill almost the whole abdominal cavity, and after hiber- 

 nation it is found to have shrunk to almost nothing" (Hewitt). 



The respiratory or tracheal system is developed to a very 

 great extent in the fly and occupies more space than any other 

 anatomical structure. By means of the trachece, which are thin- 

 walled branching tubes, supported by chitinous rings, air is 

 distributed to every organ of the body, in most of which very 

 minute ramifications run in all directions. The system consists 

 of tracheal sacs of varying size having extremely thin walls and 

 the tracliecB, which arise from the sacs, or, in the case of the 

 abdominal tracheae, independently from spiracles. The anterior 

 thoracic spiracles are very large vertical openings in the thorax 

 above the anterior legs. They supply the head, legs and most 

 of the thorax, and a large part of the abdominal viscera. The 

 posterior thoracic spiracles are situated in the posterior margin 

 of the thorax, and only supply part of the thorax. According 

 to Hewitt there are seven pairs of abdominal spiracles in the 

 male, and only five pairs in the female. These communicate 

 with tracheae which ramify among the abdominal viscera, but 

 are not connected with sacs. 



The reproductive system is very greatly developed in the 

 female, the two ovaries being very large and almost filling the 

 whole abdomen. Each ovary contains about seventy " strings 

 of eggs " in various stages of development. They open into two 

 ducts, which unite and form one central duct which passes into 

 the ovipositor (Fig. 5). "Connected with this central oviduct 

 are certain glands and a set of small vesicles which store the 

 spermatozoa received from the male during coitus. The long 

 telescopic oviduct is composed of the last four segments of the 

 abdomen, which can be retracted entirely within the abdomen. 

 When the fly lays its eggs the ovipositor is extended, and when 

 fully extended is as long as the abdomen. The possession of 

 an extensile ovipositor is of great importance as the fly is thereby 

 enabled to deposit its eggs in the crevices of the substance 

 chosen as a nidus for the larvae" (Hewitt, 1912, p. 17). 



" The internal reproductive organs of the male consist of a 

 pair of small brown pear-shaped testes, which open by fine ducts 



G.-s. 4 



