6 



FARMERS BULLETIN 851. 



The number of eggs laid by an individual fly at one time undoubt- 

 edly is large, probably averaging about 120, and as a single female 

 will lay at least two and possibly four such, batches, the enormous 

 numbers in which the insects occur are thus plainly accounted for, 

 especially when the abundance and universal occurrence of appro- 

 priate larval food is considered. The eggs are deposited below the 

 surface in the cracks and interstices of the manure, several females 

 usually depositing in one spot, so that the eggs commonly are found 

 in large clusters (fig. 8) in selected places near the top of the pile, 

 where a high degree of heat is maintained by the fermentation below. 



FIG. 8. Eggs of the house fly. About natural size. (Newstead.) 



The eggs usually hatch in less than 24 hours. Under the most 

 favorable conditions of temperature and moisture the egg state may 

 last hardly more than eight hours. The maggots which issue from 

 the eggs are very small and transparent. They grow rapidly, com- 

 pleting the growth of the larva stage in four or five days. The larva 

 period may be prolonged greatly by low temperature or by dryness 

 or scarcity of the larval food. As the larvae (fig. 10) attain full size 

 they gradually assume a creamy white color. Just before pupation 

 they become very restless and migrate from their feeding ground in 

 search of a favorable place in which to pass the pupa stage. They 

 will often congregate at the edges of manure piles near the ground or 



