502 C. GORDON HEWITT. 
normal size. Moisture is necessary for the development, and 
if the food becomes too dry the result is fatal, as the larvee 
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 larve 
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 Rate 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 amanure-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 
