AGENTS OF DISEASE 289 



sick enough to be in hospital or under observa- 

 tion, and carry the poison so taken up into the 

 very midst of the food and water ready for use 

 at the next meal." 



Professor R. Newstead states that the chief 

 breeding places of the common House-fly are : 



(a) stable middens containing fermenting horse 

 manure, or a mixture of this and cow dung; 



(b) middens containing spent hops ; (c) ashpits 

 containing fermenting matter. It is in the midst 

 of such unsavoury surroundings that the House- 

 fly deposits her pure white eggs, about 120 to 

 140 in number, and from these eggs the legless 

 maggots or larvae emerge in about eight to 

 twenty-four hours, according to temperature and 

 climatic conditions ; temperature playing a very 

 important part in hastening the life-cycle of the 

 House-fly. Though closely resembling the 

 larvae or maggots of the " Bluebottle " and 

 " Blow-fly," the House-fly larvae are much 

 smaller, and, moreover, are essentially vegetable 

 feeders ; they thrive and mature most rapidly in 

 fermenting materials, under such conditions 

 completing the larval stage in five to eight days. 

 From the somewhat barrel-shaped pupa, climatic 

 conditions being favourable, the flies emerge in 

 about five to eight days, but the pupal stage may 

 last from fourteen to twenty-eight days, while it is 

 quite possible that some of the autumn insects 

 remain as dormant pupae throughout the winter. 

 Under the most favourable conditions, the life- 

 cycle from egg to perfect insect is completed in 

 about ten to fourteen days. 



The common Bluebottle Fly (Calliphera 

 u 



