9o6 



THE ANIMAL CARRIERS OF DISEASES 



media i and 2 bound the radial and fifth radial cells, which are separated by 

 the radio-median nerve. Median 3 and cubitus i cut ofE the median and 

 second median cells, which are separated by the medio-cubital vein. The 

 anal vein is incomplete, and therefore does not separate the cubital and anal 

 cells. 



The abdomen consists of eight segments in the male, and nine in the female. 

 The first segment in both sexes is rudimentary, and fused with the second. 

 The second, third, fourth, and fifth are well developed. The sixth, seventh, 

 and eighth in the male are atrophied, and surround the anal and genital 

 orifices. The sixth, seventh, eighth, and ninth in the female form the long 

 ovipositor, which can be retracted into the abdomen. 



The pharyngeal tube mentioned above ends in the pharynx, which has a 

 chitinous wall, the fulcrum. The oesophagus runs into the thorax, where it 

 opens into the proventriculus, and sends a long duct to the trilobed crop which 

 lies in the abdomen. The proventriculus is really a valve which leads to the 

 chylific ventricle. This tube is narrow in front and wide behind, and much 

 coiled. At the junction with the Malpighian tubes it becomes the intestine, 

 which runs to the rectum. There are the four usual rectal glands. 



The salivary glands are two long tubes whose ducts unite in the head and 

 open into the salivary duct of the hypopharynx. The labial glands lie in the 

 proboscis at the base of the labellae, and by their secretion keep the tip moist. 



The reproductive organs consist in the female of ovaries, spermathecae, 

 accessory glands and ducts, and the ovipositor. In the male they consist of 

 testes, vasa deferentia, an ejaculatory duct, and penis. 



Life-History. — This was first studied by de Geer in 1752-78, 

 next by Bouche in 1834, Packard, junior, in 1873, and more recently 



by ourselves and a 

 number of other people. 

 The most recent works 

 are those by Newstead 

 in Liverpool and Hewitt 

 in Manchester. 



The house-flies lay 

 their eggs by preference 

 in human faecal matter 

 when somewhat decom- 

 posed, in horse-manure, 

 goat-manure, cow-dung, 

 and in fermenting vege- 

 table substance, on 

 which the larvae can 

 feed when hatched. 



But these grubs can 

 eat textile fabrics and 

 paper when in need of 

 nourishment. Usually 

 the fly works its way 

 into the rubbish-heap 

 or loose earth to lay its eggs, but this is not invariable. 



The time required for the development to be gone through 

 depends upon the temperature of the air, being most rapid from 

 22° to 35° C. ; upon the food, being more rapid when animal food 

 is available; on moisture, which is most necessary; and upon fer- 



FiG. 472. — Egg, Larva, and Pupa of 

 Musca domestica. 



(After C. J. Martin.) 



