" 
The horn fly exhibits a certain preference for red or other dark- 
colored cattle, and that such animals are more thickly infested has 
been frequently noted. When the insects are abundant, however, 
this preference is not so strongly marked. Occasionally sores are 
formed on the animals, which in the South and West may become 
infested with the screw worm (Chrysomyia macellaria Fab.). These’ 
wounds or sores are, as a rule, only indirectly the result of horn-fly 
attacks, but are commonly produced by the rubbing of the cattle in 
efforts to allay the irritation from the bites. 
The loss occasioned by the horn fly to other animals is, as a rule, 
inconsiderable. Sometimes horses are attacked, and especially cow 
ponies, and injury to sheep, as pointed out by Norgaard,* is compli- 
cated with sheep scab. 
LIFE HISTORY AND HABITS. 
The appearance and abundance of the flies is governed by tem- 
perature and rainfall. In the latitude of Washington they are first 
noticed in May, and become most abundant in July, gradually 
dwindling to November or until sharp, frosty nights become frequent. 
Farther south they appear earlier and remain in evidence later. The 
study of this insect in Texas by agents of this Bureau, notably Mr. 
J. D. Mitchell, at Victoria, indicates that the fly reaches its first maxi- 
mum of abundance in May. During the subsequent dry period the 
fly decreases in numbers until fall rains begin, when a second maxi- 
mum is reached in late September, which is checked by the frosts of 
the latter part of October. Continuing on from then until March the 
fly is kept down to comparatively small numbers by low tempera- 
tures. The reduction of the numbers of the fly in Texas by a dry, 
hot summer is sometimes as great as 95 per cent from the maximum 
-of May. 
The characteristic habit of the fly in clustering about the base of the 
horn is developed only when the flies are abundant. When they 
average only 100 or so to an animal, comparatively few will be found 
on the horns. The horn-clustering habit is more noticeable in the 
spring and early summer than in autumn. The horns are not the 
only resting places, and many of the flies cluster upon the back, be- 
tween the head and the fore shoulders, where they can be reached 
by neither head nor tail. When the cattle are feeding, the flies are 
found over the back and flank and on the legs, and during a rain- 
storm they flock beneath the belly. When the animal is lying down, 
a favorite place of attack seems to be under the thigh and back belly 
around the udder. The characteristic appearance of the flies on the 
horn is indicated in the accompanying illustration (fig. 2). 
a Rep. Agr. and Forestry, Hawaii, 1905, pp. 171, 211, 212. 
[Cir. 115] 
