260 



NATURAL HISTORY. 



way into the skin and produce suppurative sores, which of course damage 

 1 lie hide for leather. Here they grow until, at last, they attain a length 

 of about an inch, when they escape, fall to the ground, and burrowing 

 beneath the surface, undergo the rest of their changes. These larvae are 

 called ' grubs ' by the cattlemen, while the sores they cause are designated 

 as 'womhV or k wurmals,' which is evidently a derivative of worm-holes. 



A third form of bot is that which infects sheep, and which has habits dis- 

 tinct from those already described. This fly brings forth its larvae alive, 

 and deposits them in the nostrils of sheep ; the larvae then quickly crawl 

 up the air-passages of the head, until they reach the large cavities known 

 as the frontal sinuses, where they pass the rest of their larval life in a 



manner analogous to that al- 

 ready described for the other 

 species. This period lasts for 

 about nine months, and then 

 they crawl out and drop to the 

 ground, where they pupate and 

 change to the imago as before. 

 When a sheep is afflicted by 

 but one or two of these para- 

 sites, the effect is but slight, but 

 the presence of large numbers 

 produce the disease known as 

 ' staggers,' the principal symp- 

 tom of which is indicated by the name. At first there is a discharge from 

 the nose, and later the sheep moves about in a dizzy, staggering manner, 

 and in many cases death may finally ensue. 



It is curious to notice the way in which different animals at once rec- 

 ognize the presence of bot-flies. The deposition of the eggs cannot pain 

 them, and hence this cannot be the cause of their excitement. It must be 

 something connected with their buzzing note, or more probably some pecul- 

 iar smell which warns the horse, cow, or sheep of their proximity. The 

 sheep will crowd together and hide their noses in the dust, cattle run 

 about in the wildest way, while the horses will often become unmanageable 

 in their presence. 



Our common flies, typified by our common house-fly and blow-fly or 

 i blue-bottle,' represent a very large group only a few of which need men- 

 tion. The house-fly is too well known to need description, and although 

 regarded as a nuisance, still does an immense amount of good by its 

 habit of feeding in the larval stage upon decaying substances of all 

 sorts, — offal, carrion, manure, in short, almost any offensive matter which 



Fig. 252. — Sheep bot-fly (CEstrus oris), larva, and pupa, 

 enlarged. 



