10 BULLETIN 1369, U. §. DEPARTMENT OF AGRICULTURE 
in 1912 showed that H. bovis occurred in abundance in southern 
British Columbia, and further evidence of the distribution is given 
by Hewitt (42) in 1914. Hence it is problematical as to just when 
that species was introduced, if in fact it is not anative. Records indi- 
cate, however, that it has been introduced rather recently. HZ. linea- 
tum, on the other hand, has been known in this country for many 
years and has been bred from the native bison, suggesting the possi- 
bility that that form may be in reality an American species and not 
introduced from Europe. As indicated by the map, H. bovis is now 
generally distributed in the Northern States from [linois to Maine 
and occurs in more or less isolated areas throughout the Northern 
States to the Pacific. 
As stated by one of the writers (4) in regard to H. bovis, this 
species “must have some well-marked climatic barriers which have 
PNG 
mceyenn 
Fic. 2.—Distribution of Hypoderma bovis in the United States. Each dot repre- 
sents a locality where this species has been collected during this investigation. 
Hee points shown in the Southern States are collections of larvae shipped in in 
cattle 
prevented its general dissemination through the country.” As also 
pointed out in the publication referred to, there are certain areas in 
the United States which are almost, if not entirely, free from grubs. 
The most notable example of this is in the valley of the Red River 
of the North. In the southern two-thirds of Florida grubs are ap- 
parently not to be found except in imported animals. This condi- 
tion may be due to some combination of naturalkcontrol factors, as 
humidity, salt spray, and drainage. In eastern Massachusetts, the 
coastal area of New Hampshire, and parts of southwestern Illinois, 
grubs are relatively scarce. Similar areas of scarcity have been 
noted by Hadwen in Canada, and he has also observed that the 
warbles are more abundant in the western Provinces than in the 
eastern. The writers’ observations and reports from stockmen in- 
dicate that there is a marked variation in the abundance of grubs in 
different localities even when not distinctly separated. Sufficient 
data are not at hand for drawing final conclusions in regard to the 
