162 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
Various methods are used to reduce the number of flies in 
cow stables and in milk-houses. The cows are sprayed or 
brushed with mixtures of drugs or chemicals known as fly 
repellents, Sometimes sheets of fly-paper are placed about the 
buildings. Milk-houses are very often screened and more rarely 
stables are also screened. Fly repellents are only temporary 
in their action and they are often objectionable on account of 
their odor. Fly-paper is unsightly and inefficient. Fly-traps 
are more useful. Screening milk-houses gives good results and 
is desirable in all cases, but screening stables is not satisfactory. 
Since the doors must be opened frequently and sometimes for 
long periods to remove manure, to take in the feed, and to drive 
the cows in and out, there is abundant opportunity for flies 
to enter. The cow-fly or horn-fly is carried in on the cows. All 
of these methods are fundamentally defective because none of 
them prevents the breeding of flies. The most rational method 
of attacking the fly problem is to remove or abolish, in so far as 
is possible, the conditions which favor the development of the 
insects. To do this intelligently it is necessary to consider their 
habits and life history. 
Three varieties of flies are commonly found in cow stables: 
the common house-fly, the cow-fly or horn-fly, and the stable- 
fly or biting-fly. The house-fly and cow-fly are usually the most 
numerous, but in some sections of the country, especially in the 
grain belt, the stable-fly is present in large numbers. The dif- 
ferent varieties can usually be distinguished by the part of the 
cow which they occupy and by their sitting position. The small, 
black cow-fly is generally located upon the back and sides of the 
cow, and in rainy weather on the under parts of the body, sitting 
with the head downward. The stable-fly usually occupies the 
lower parts of the legs and nearly always sits with the head 
upward, while the house-fly may be found on any part of the 
cow and may sit in any position, but never with the head pressed 
into the hair as though feeding. The stable-fly is about the 
same size as the house-fly but has a more plump appearance 
and has longitudinal lines on the thorax and several dark spots 
on the abdomen. The horn-fly is smaller and black. 
The house-fly seems to prefer to deposit its eggs in horse 
