244 ENTOMOLOGY 



typhoid bacillus from the mashed bodies of the flies, the bacilli having 

 been present in the alimentary tract, without doubt. 



Faichnie also obtained cultures of Bacillus typhosus from the intes- 

 tines of flies which had developed from larvae fed on feces containing the 

 bacillus. 



Jordan states that the bacilli survive the passage of the alimentary 

 canal of the fly. 



Ficker recovered typhoid bacilli from flies twenty-three days after 

 they had been infected. 



In fact, a great amount of evidence has accumulated proving that 

 flies transmit not only the bacilli of typhoid fever, but many other bac- 

 teria, and often in enormous numbers. For example, Esten and Mason 

 in their study of the sources of bacteria in milk, collected and examined 

 flies from stables, pig-pens, houses and other places, and found an average 

 of 1,222,570 bacteria per fly; the majority of these being objectionable 

 kinds of bacteria. 



Musca domestica. A single female of the common house fly lays 

 in all some six hundred eggs. In midsummer, in Washington, D. C., 

 the eggs hatch in about eight hours; the larval period is from four to 

 five days and the pupal period five days, making the cycle about ten 

 days in length. In cooler parts of the season the cycle requires more 

 time and in warm climates it may be as short as eight days. The number 

 of generations in Washington is probably not more than nine (Howard). 



Control. One of the best baits for flies in houses is formalin, which 

 is poisonous to flies but harmless to man. This is prepared by diluting 

 formaldehyde with five or six times as much water .and exposing it in 

 shallow dishes, the addition of a little sugar or milk making the solution 

 more attractive to flies, which drink it and quickly die. Pyrethrum is 

 effective against flies, but only when it is pure and has been kept from 

 exposure to the air. Pyrethrum, the chief basis of all the common 

 insect powders, is applied by being puffed through a bellows or by being 

 burned. The powder may be moistened and shaped into cones which 

 when lighted at the top burn slowly and give off fumes that are suffocat- 

 ing to insects. 



Dr. Howard estimates that more than ten million dollars are spent 

 every year in screening houses in the United States. Another enormous 

 sum is spent for fly papers and fly traps. The efficient way to deal with 

 the fly problem, however, is to prevent the insects from breeding. Ex- 

 crementitious substances should be enclosed in such a way as to prevent 

 the access of flies, or should be treated in a way to kill the larvae therein; 



