THE ARTHROPODS 397 



330. Flies and Disease. Probably more important than 

 mosquitoes as carriers of disease germs are the common house- 

 flies. It is a well-known fact that these flies persist in walking 

 on food, and long before disease germs were known, careful 

 housewives made strenuous efforts at keeping them from 

 kitchens and dining-rooms. Recent bacteriological studies 

 have disclosed some startling facts which should lead to a 

 general declaration of war against the house-flies. The 

 facts are these : A fly allowed to walk across a sterile gelatin 

 plate ( 255) will leave in its tracks many bacteria pre- 

 viously acquired by walking on filth. Now, if a fly walks 

 on sewage containing germs of typhoid or of other intestinal 

 diseases, or on sputum from a tuberculosis patient, and later 

 walks on food or on dishes ready to be used for food or drink- 

 ing water, it may leave in its tracks dangerous bacteria, which 

 may be taken into the body with the food or water, and then 

 cause disease. It is obvious that in this way a single house-fly 

 may be a very dangerous animal. 



There are several ways of combating this dangerous 

 pest : (1) Manure piles and similar breeding places should 

 be removed. (2) Houses should be carefully screened and 

 fly-poisons, traps, etc., used to kill the few that succeed in 

 entering. (3) All foods should be carefully guarded against 

 flies. (4) Arrangements for sewage disposal should be such 

 that flies cannot distribute bacteria. For this reason sewers 

 and cesspools that discharge into porous drain-tiles below 

 the surface of soil are preferable for country and village 

 homes. (See a pamphlet on " Sewage Disposal," issued 

 (free) by the United States Department of Agriculture.) 



331. Other Insects and Disease. Mosquitoes and house- 

 flies are the most important insects connected with diseases 

 in America, but other insects may likewise affect human 

 health. The germ of the terrible African sleeping sickness 

 is injected into the blood by the bite of a peculiar fly. The 

 bacteria of bubonic plague is probably transmitted by fleas 



