344 • JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 9 



physical stimuli may acquire incidental or temporary dominance, 

 but would have little effect on seasonal distribution. Isolated cities 

 or localities, however, are of undoubted occurrence, where some one 

 physical factor might exert its stimulus continuously for a long period 

 or even a whole season and hence become one of major importance 

 (Munson, 1. c). 



In poorly kept cities files are quite generally distributed, but there 

 are usually areas of unusual abundance due to their advantages as 

 good feeding and breeding grounds. Such areas may be large as a 

 poorly cared for market district or a breeding ground of many acres 

 such as the Miles City Sales Yards, and they may be small as in the 

 vicinity of poorly kept residences or stables. To show how such areas 

 exert a stimulus influencing migration, let us consider Station 216 

 again. The house, a boarding place, was very clean, but a manure 

 pile, privy with open door and uncovered seats, several accumulations 

 of garbage and a privy with an open vault in the next yard (PI. 26, figs. 

 8 and 9) were efficient attractions and because of these, flies were abund- 

 ant in the vicinity of the house. At this station (at the backdoor of 

 the house) 132 stained flies were recovered in twenty days. The 

 total number of days represented in the whole experiment was 680, 

 yet in twenty (one-thirty-fourth of whole time) this station attracted 

 one-eighth the total catch. 



The State Industrial School, which is 700 yards east of the outside 

 limits of Miles City, is another example. Flies were very abundant, 

 especially at the pig house 150 yards to the east and at a shack for 

 mixing garbage for the pigs nearer the main buildings. The pig 

 house was 3,500 yards from the City Dump, 3,070 from the Sales 

 Yards, 2,000 from the Washington School and 1,430 from the Labo- 

 tory. On August 9, nine traps were placed, two inside the pig house, 

 one at the garbage shack and the others at the barns, poultry houses, 

 etc. The traps were collected on the 20th of August and twenty-nine 

 Sales Yard, four Washington School, five City Dump and two Labora- 

 tory flies recaptured. Except for a few in the trap at the garbage 

 shack, all were recaptured at the pig house about which flies were 

 breeding in stupendous numbers. These flies had not only crossed 

 Miles City, but the 700 yards of open country intervening and for the 

 most part had selected the pig house as their goal. The isolated lo- 

 cation of the school doubtless rendered the apparent attracting stim- 

 ulus from the pig house doubly effective, while the flies about the main 

 buildings were perhaps migrants from this breeding ground. 



In presenting these illustrations the object is to show that the fly 

 is essentially a migratory insect, the migration, in the main, seeming 

 to depend upon the two stimuli under discussion, and also that it is 



