PARASITES OP ANIMALS. 21 



hooks are used in moving about, and no doubt cause consid- 

 erable irritation. The mouth is small and inconspicuous, 

 with small papillas. 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 larvae 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 

 seem 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 larvas mature, the flies may occur from June to 

 September. The larvse live during the entire winter in the 

 tumors, and perhaps some of the pupae remain over winter. 



The Sheep Bot-fly QCEslrus ovis Linn.). Figure 24. 

 This species somewhat resembles the preceding in form, 

 but is considerably smaller, with a rounder abdomen in the 



