A BULLETIN 14, U. S. DEPARTMENT OF AGRICULTURE. 
edges of piles. Here are some of the figures obtained from a count 
of the pupae collected from different piles: 7,000, 1,500, 10.000, over 
12,000, 4,500, 6,000, 6,700, 30,000, etc. 
In a recent article in the American Journal of Public Health, Levy 
and Tuck make the following statement : " We therefore announce the 
biological fact that the house fly does not pupate in manure if the 
full-grown larvae can find any means of reaching and entering the 
earth." They claim that "the adult larvae regularly leave the manure 
heap' 1 and that they "enter the earth whenever it is possible for 
them to do so." To be sure, larvae may and often do burrow into the 
ground before pupating — witness Dr. Terry's observations at Jack- 
sonville, Fla., where he found larvae and pupae hi the ground of soil- 
floor stables — but that they do so regularly is open to serious question. 
The figures given above are for puparia collected above the surface of 
the ground and in the manure. After the removal of the heaps, 
examination of the ground revealed only a very small percentage 
beneath the surface. The fact that some were found there shows 
that it was not the compact nature of the soil which prevented the 
majority from burrowing and that there was no reason why all could 
not have done so if such were then regular habit. It would seem 
that Levy and Tuck have put too much emphasis on this one point. 
A broader view, including all the phases of the migration of these 
creatures, is necessary and will not detract from the importance or 
value of the "maggot trap" which they have devised. 
It is quite certain that. the migrating habit is deeply ingrained and 
highly characteristic of housefly larvae. A consideration of the known 
facts in the case will enable one to draw some inferences as to "the 
stimuli or particular conditions" which determine this mode of 
action. It has been noted that a sort of "wanderlust" seizes the 
larvae just before pupation. It must be, therefore, that the migration 
is initiated in response to internal stimuli incident to the maturing 
of the larval stage and the onset of the metabolic changes preparatory 
to the transformation to the pupal stage. The course and direction 
of their travels are determined largely by external stimuli. It is quite 
evident that as pupation draws near they flee the very moist regions 
of a manure heap and seek the comparatively dry regions. If no such 
dry places are to be found in the manure, they will leave it to pupate 
in the ground or in cracks or crevices, under boards or stones, in loose 
material of any kind. Dr. Terry found both larvae and pupae in the 
soil of dirt-floor stalls. The larvae were found in that part of the 
floor kept moist by the mine, while the pupae were found in a ring 
in the drier soil outside the moist center. Further proof that moisture 
acts as a stimulus in determining their choice of a place for pupation 
is given below. 
