June, '16] PARKER: DISPERSION OF MUSCA DOMESTICA LINNAEUS 343 



favoring both more rapid breeding and greater distribution. It may 

 justly be considered a seasonal factor, its influence being present at 

 all times, either in a positive or a negative manner 



Physiographical 



Elevations and Depressions. — Zetek, in discussing his experi- 

 ments at Ancon, Panama, expressed his belief that flies follow de- 

 pressions, rather than to cross elevations. How much importance 

 is to be attached to this point when considering dispersion under 

 city conditions is not entirely apparent. It may have some signifi- 

 cance when considering congested areas and high buildings which 

 offer parallel conditions on a small scale. Whatever stimulus might 

 prompt flies to follow a depression in natural topography, other addi- 

 tional stimuli would be active in a city (presence of food, etc.). 



Bodies of Water. — Hewitt's experiment at Ottawa demonstrated 

 that streams passing through a city are not a barrier to fly dispersion 

 and Hodge's observations at Lake Erie indicate that considerable 

 bodies of water may be crossed. 



Wooded Areas. — The summer's results indicate that wooded areas 

 of small extent and of an open character are not an obstacle to the 

 spread of flies. In order to reach the city from the City Dump re- 

 lease point it was necessary to traverse a wooded area 200 yards in 

 width or to take a roundabout course up the old channel of the Tongue 

 river, shown on the map. In the latter case it was necessary to pass 

 through woodland, but of less extent. City Dump flies captured at 

 the Sales Yards, 800 yards distant, may have followed up the Tongue 

 river, have passed through or over almost continuous woods in the 

 most direct line or have taken a more devious course by way of the 

 city itself. 



Physiological 



To live and reproduce are functions of all animal life, both essential 

 to the perpetuation of the species, but the second highly dependent 

 on the first. As entomologists we are acquainted with many obvious 

 instances where adult life is entirely devoted to feeding and repro- 

 duction, and if there are any two facts that are always in evidence 

 concerning the house-fly they are its presence at feeding and breeding 

 grounds. These facts, then, indicate the two factors of prime im- 

 portance which, given normal conditions, seem to be largely respon- 

 sible for the migratory movements of the house-fly under city condi- 

 tions — namely the stimuli from feeding areas and the stimuli from 

 breeding areas. These apparently are chemotropic and to them the 

 fly seems to react positively to a far greater extent than to any other 

 stimuli which cause movement, though it is obvious that various 



