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BULLETIN 136&, U. S. DEPAHTMENT OF AGRICULTURE 



in 1912 showed that II. hovis occurred in abundance in southern 

 British Columbia, and further evidence of the distribution is given 

 by Hewitt (^) in 1914. Hence it is problematical as to just when 

 that species was introduced, if in fact it is not a native. Records indi- 

 cate, however, that it has been introduced rather recently, H. 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. hovis is now 

 generally distributed in the Northern States from Illinois to Maine 

 and occurs in more or less isolated areas throughout the Northern 

 States to the Pacific. 



As stated by one of the writers {Jf.) in regard to H. hovis, this 

 species "must have some well-marked climatic barriers which have 



Fig. 2. — Distribution of Hypoderma iovis In tlie United States. Bach dot repre- 

 sents a locality where this species has been collected during this investigation. 

 The 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 natural-control 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 



