112 GENERAL ZOOLOGY 



importance to man than any other group of insects. The 

 commonest species are the house-fly, Musca domestica, and 

 the stable-fly, Stomoxys calcitrans. The horn-fly of cattle, 

 Hcematobia serrata, is also well known to farmers. Many 

 flies belonging to this family are of value as scavengers, 

 destroying decaying matter through their great numbers 

 and rapid breeding. The horn-fly and stable-fly produce 

 great losses in the dairy and stock industries by their 

 attacks on cattle. The common house-fly, and its near 

 relatives, do untold injury to man. The facility with 

 which these insects take to civilized life and their uncleanly 

 habits make them ideal carriers of disease germs. The 

 house-fly commonly breeds in the excreta of diseased men or 

 other animals, and its roving habits make it a continual 

 source of contamination. Many bacteria will pass through 

 its alimentary canal without losing their virulence. Thus 

 a house-fly not only goes about with dirty body fouling 

 everything it touches, but. is continually defecating and 

 spitting out germ cultures as it goes. Typhoid fever, 

 diarrhoea, and cholera are commonly carried by house- 

 flies. Tuberculosis, infantile paralysis, anthrax, and yaws 

 may spread in the same way. Perhaps syphilis, eye dis- 

 eases, leprosy, diphtheria, small pox, plague, and tropical 

 sores may also be carried by flies in some cases (Fig. 54). 

 Small wonder that the house-fly is the most dreaded hanger- 

 on of civilization and that many communities have taken 

 active measures to keep it down. 



Order 17. Siphonaptera, Fleas (Fig. 55). These in- 

 sects show many adaptations to parasitic life on mammals. 

 They are greatly compressed, so that they are not easily 

 squeezed to death and may easily ramble about among 

 hairs. The mouth parts are suitable for sucking blood. 

 There are no wings, but the powers of jumping are re- 

 markably developed. A flea is thus able, if it drops off 

 one animal, to hop on another when it comes near. Fleas 

 lay their eggs in cracks in floors and similar situations. 

 The larvae eat sweepings, or almost any sort of waste 



