HOUSE-FLIES 293 



and ticks are responsible for the spread of other 

 diseases. 



NON-BLOOD-SUCKING FLIES. 



The case against the common non-blood-sucking 

 flies is more difficult to prove. That their habits 

 are filthy and that they are constantly contaminated 

 with bacteria is only too certain, but the precise 

 extent to which they are responsible for the spread 

 of disease has yet to be ascertained. We may first 

 consider their habits and the experimental evidence 

 in regard to their capacity for carrying and dis- 

 tributing disease-producing bacteria, and then the 

 evidence relating to the actual production of disease 

 through their agency. 



Most of the common flies pass through the 

 following stages in their life-histories. The females 

 lay eggs in large clusters in decaying animal matter 

 or excrement, and maggots emerge from the eggs 

 in the course of a day or two. The maggots burrow 

 below the surface of their food and grow rapidly. 

 In two to four weeks they have reached their 

 largest size, and then migrate from the food and 

 either burrow in the ground or seek a sheltered 

 position in which' to change into the condition 

 known as the pupal stage. In this stage the maggot 

 ceases to feed or move, becomes barrel-shaped and 

 develops a tough brown covering. During the 

 quiescent stage, which lasts a few days in summer, 

 the maggot within its protective covering turns 



