DISEASES OF THE OX AND SHEEP. 479 



We now proceed to deal with the CEstrus ovis, or sheep-bot. 

 In the first place we may say that it is in the hot and sultry 

 days of July and August that the bot-fly of the sheep pursues 

 its timid and well-nigh defenceless victim. During the heat of 

 the summer's day, and especially when the atmosphere threatens 

 a thunderstorm, the insect is particularly busy in its attempts to 

 deposit its eggs in the nostrils of the sheep. After the time of 

 its activity is over, the fly may easily be caught and examined. 

 It is a member of the tribe of two-winged insects, and it is a 

 little larger than the ordinary house-fly, and of an ashy grey 

 colour. 



The sight of a flock of sheep being attacked by this fly is a 

 very instructive one. The sheep themselves seem to be quite 

 cognisant of the danger which threatens them. They may hold 

 their heads between their fore-legs quite close to the ground, so 

 that the fly may not be able to approach their nostrils, or per- 

 haps a sheep may be observed lying in a deep rut or hollow 

 with its nose in part protected. Moreover, it appears that the 

 presence of dust aff'ords some safeguard against the pest, and 

 hence we find as a matter of fact that the tormented animals will 

 instinctively resort to a dusty road in order to escape, or, again, 

 they may rush to form a more or less compact assemblage with 

 their noses pushed closely together so as to foil, at least so far 

 as the greater number of the sheep are concerned, the persistent 

 and insidious eff'orts of their insignificant foe. It seems pro- 

 bable that great pain is occasioned by the entry of the fly, and 

 the laying of the eggs. 



As we have said above, it is usually in July or the early part 

 of August that the fly deposits its eggs, which are soon after- 

 wards hatched. The maggots which are set free crawl up the 

 nasal cavity by the help of the hooks with which their mouths 

 are provided. When once they have gained a position of com- 

 parative security, they then fix themselves tightly to the mem- 

 branes, feed upon the pus which is poured out as a consequence 

 of the irritation produced by their presence, or even upon the 

 nasal membrane itself. Occasionally they may actually penetrate 

 to the brain, and even death may result from the effects of their 

 inroads. In the general way, they remain in the higher parts of 

 the nasal cavity until the spring-time, when they are full-grown. 

 Then they fall to the ground, and become pupee. During the 



