PARASITES OF ANIMALS. 21 
hooks are used in moving about, and no doubt cause consid-— 
erable irritation. The mouth is small and inconspicuous, 
with small papilla. The posterior end of the body bears two 
small plates in which the spiracles or breathing pores are 
situated ; and this end of the body is kept next to the external 
orifice of the tumor. When the larva is mature, which hap- 
pens from May to July, it is about an inch long (Figure 23). 
It then works its way out of the opening backward, and falls 
to the earth, where, in a day or two, it changes to a pupa, 
the dried-up skin of the larva serving as a covering for the 
pupa. They remain in the pupa state from four to six weeks 
during summer, when the end of the pupa-case comes off, 
like a lid or cap, and the mature fly comes forth. The sores 
caused by them heal very soon after the larve leave. 
Although these insects probably cause some pain, especially 
at first, they do not appear to cause any permanent injury, 
and but little inconvenience, unless in large numbers. They 
can usually be pressed out of their burrows with the thumb; 
but care should be taken not to crush or burst them in the 
burrows. Or the openings of the tumors may be enlarged with 
a sharp knife or lancet, and then they can be easily removed 
and the wounds will soon heal. They attack by preference 
young and healthy or fat animals, perhaps because the skin 
is softer and thinner in these. They frequent chiefly the 
uplands, and especially the vicinity of trees, and seem to 
particularly avoid water or damp localities —a fact that cattle 
scem to learn by experience. Owing to this habit, cattle that 
graze upon meadows are generally free from them, or nearly 
so, although those upon the neighboring hills may be at- 
tacked. The eggs are laid soon after the flies emerge from 
the pupa; but as there is considerable variation in the time 
when the larve mature, the flies may occur from June to 
September. The larve live during the entire winter in the 
tumors, and perhaps some of the pup remain over winter. 
The Sheep Bot-fly (Gstrus ovis Linn.). Figure 24. 
This species somewhat resembles the preceding in form, 
but is considerably smaller, with a rounder abdomen in the 
Digitized by Microsoft® 
