June, '16] PARKER: DISPERSION OF MUSCA DOMESTICA LINNAEUS 345 



a false idea to assume that when flies breed out near "food and shelter" 

 they necessarily do not migrate far. Let us further illustrate. It is 

 known that the house-fly deposits several batches of eggs at several 

 intervals. Yet, in general, feeding areas are not breeding areas; 

 large districts suitable for feeding may lack breeding facilities. Hence, 

 must not the fly continually migrate from one to the other? That it 

 does so is evidenced by the fact that 85 to 95 per cent are conceded 

 to breed out in horse manure, yet we do not find that percentage of the 

 adults normally in its vicinity. This indicates that the stimulus from 

 breeding areas may at times transcend that from feeding areas and 

 vice versa impelling consequent migrations. Hutchinson (1915)'^ 

 presents evidence along this line. Recently emerged flies were marked 

 and liberated at a dairy barn about 700 yards distant from a stable 

 and a kitchen. Of two of the lots liberated flies were recovered at the 

 kitchen, but none at the stable. 



Another observation from the summer's work may well be noted. 

 The Miles City Sales Yards afforded extensive breeding facilities. 

 As long as certain physical factors permitted breeding, flies were al- 

 ways present in enormous numbers, but during several periods fac- 

 tors which will be dealt with in another paper practically eliminated 

 breeding, though manure was as abundant as ever. Immediately the 

 number of flies began to decrease, simply due to migration without 

 any compensating emergence of adults. 



The House-Fly Essentially A Migrating Insect 



After due consideration of these results and those of others it is 

 impossible to escape the conviction that the house-fly under Montana 

 conditions is essentially a migratory insect and not necessarily within 

 localized areas. It is constantly moving from the field of one stimulus 

 causing tropic reactions (that is, movement's to or from) to that of 

 another. Looking at Plate 21, which shows the dispersion of flies 

 from the Sales Yards, does not the fact that flies were recaptured at 

 this, that or the other place have an added significance beyond mere 

 locality and distance. It means that each locality capable of attract- 

 ing these flies became a subsidiary center for further distribution. 

 The Sales Yards were an immense breeding ground. An extremely 

 small proportion of their product was stained and recovered as shown, 

 and that seventy-six of the released flies were captured at Station 216 

 means that this was only a small portion of the Sales Yards flies that 

 actually reached this point. From it they spread to other localities 

 and the station thus became a center of distribution subsidiar}^ to 

 that of the Sales Yards. This means that breeding and feeding areas 

 are not necessarily areas which attract flies and retain them, but that 



