THE MIGRATORY HABIT OF HOUSEFLY LARVE. & 
pan are of two layers of screen wire 2 inches apart. Now it was 
found in the very first experiments that larvae were escaping from 
the cages, and it was seen that they found their way put through 
the holes in the floor of the cages and also through the screens at 
the sides. The numbers so escaping were surprising. It often 
happened that several hundred crawled out of the cage during 24 
hours. They were found in the vessels placed beneath the cages to 
catch any drippings. By day the light was sufficient stimulus to 
prevent them from crawling out at the sides, but at night they were 
actually seen, with the aid of a flash light, making their way through 
both thicknesses of screen wire and dropping into the vessel below. 
Moreover, in examining manure heaps on the open ground I have 
many records showing this "tendency to congregate" at the edges 
of the piles near the ground. About two cartloads of horse manure 
had been piled out on the open ground for five or six days during 
August, and at the end of this time it was hauled away. I examined 
the ground where the heap had been and found many pupae, not 
in the center* of the area formerly covered by the heap, but around 
the margin. Some were found on the surface, doubtless shaken out 
of the manure at the time of removal; others were found buried a 
half inch or more in the soil, where the larvae had burrowed just 
previous to pupation. 
In another case some 50 cubic feet of manure had been heaped 
up in a pile the base of which covered an area about 4 feet square. 
After the pile had stood three days larvae were found swarming 
in the warm, moist parts of the heap near the top and some distance 
in from the sides. After eight days the entire pile was torn apart 
and gone over carefully in search of pupae. None was to be found 
in the upper parts of the heap where I had previously seen great 
numbers of larvae. In fact none was found until the very lowest 
layers were exposed. Here about 9,000 were collected. Not 
more than 100 were found below the soil. The mass of pupae were 
scattered in little heaps about the margin. They were just outside 
the moist area of the manure, yet sufficiently protected from drying 
and sunlight by the overhanging straw. The explanation of then 
presence in such a position is, of course, that the larvae, just before 
pupating, had migrated from the moist feeding grounds to a drier 
region more favorable to the resting stage. The examination of 
many other piles of manure showed the very same conditions existing, 
the only difference being in the number of pupae collected. 
Altogether some 50 or more heaps of manure on open ground 
have been examined. Each one contained from 40 to 50 cubic 
feet of manure. Some contained much long straw, others very 
little straw or bedding of any kind. The puparia are not hard to 
find nor hard to collect because of their occurrence in masses at the 
