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502 0. GORDON HEWITT. 



normal size. Moisture is necessary for the development; and 

 if the food becomes too dry the result is fatal, as the larvaa 

 perish. 



A fourth and a most important factor affecting development 

 and one intimately connected with the previous factors tem- 

 perature, character of food, and moisture is that of fermenta- 

 tion, to which reference has already been made. This process 

 appears to take place in the substances on which the larvae 

 best subsist. Whether the suitability of the food is deter- 

 mined by the nature of its fermentation is a point which 

 I was unable to determine, but which I am inclined to believe. 

 I feel certain, however, that the calorific property of fermenta- 

 tion is the most important part of this process on account of 

 its direct relation to the time of development ; the endogenous 

 heat of excremental products and decaying substances acting 

 either in addition to, or independently of, the temperature of 

 the surrounding air is of great advantage in accelerating the 

 rate of development. 



The Kate of Development. This was never found to 

 be less than eight days, and was more usually twelve to 

 twenty days owing to the fact that a continuously high air 

 temperature was not maintained for any sufficient length of 

 time; with such a continuous period of hot weather the 

 development would take about ten to twelve days, and in 

 very great heat might be completed in a day or two less as 

 the internal temperature of the breeding places, such as 

 manure-heaps is usually higher than the temperature of the 

 air. It mnst be remembered, however, that except by incu- 

 bation it is difficult to experimentally imitate such natural 

 conditions as occur in a manure-heap or privy midden, where, 

 owing to a larger amount of material, a higher constant 

 temperature is maintained. All experimental results except 

 those of incubation tend to give a long rather than a short 

 rate of development. In many cases where the average tempe- 

 rature was 20 C., but the food material rather dry, the de- 

 velopmental period was about three weeks, and where the 

 temperature was low and the food became dry it extended to 



