DIPTERA. 793 



When the bots are mature they make then- escape, and fall to 

 the ground, burymg themselves in the turf, or hiding underneath 

 stones. During the process of metamorphosis the skin of the 

 hot becomes transformed into a cocoon, and in course of time the 

 pupa or chrysalis stage is completed. In this state it remains 

 as a grub for a month or six weeks, at the expiration of which 

 period the lid of the cocoon comes off, and the perfect insect or 

 imago is set free. 



The common bot-fly of the sheep {(Estrus ovis) neither chooses 

 the stomach nor the back of its bearer as a place of residence 

 during its acquisition of the larval condition, termed the hot. 

 This insect may be regarded as a worse tormentor than either of 

 its common congeners above mentioned. It attacks the nostrils 

 of the sheep, and the distress thus occasioned is so great that 

 the poor animals, in order to avoid the flies, will often bury 

 their nostrils in the dusty hollows of cart-ruts, further protectinf^ 

 their heads with the fore feet. The members of any flock thus 

 attacked will also collect together in groups, and jostle against 

 one another with their heads downwards, so as to avoid the flies 

 as much as possible. When struck by the fly they stamp the 

 ground violently, and exhibit other signs of distress, sometimes 

 amounting to agony. According to Mr. Eiley, as quoted by 

 Verrill, the young larva is itself deposited at the margin of the 

 sheep's nostrils, having quitted the egg whilst yet within the 

 oviduct of the parent insect. Be this as it may, the young 

 larvte having once gained access to the nasal passages, have no 

 difficulty in retaining their hold, at the same time that they 

 cause fresh distress to the unhappy bearer. Within the cranial 

 sinuses they firmly anchor themselves by means of a pair of 

 cephalic hooks, and in this situation they remain until they 

 have perfected this stage of their larval development. Con- 

 sidering the situation of these creatures, there need be no 

 astonishment at the fact that their presence sometimes gives 

 rise to terrible sufferings on the part of the sheep ; the afflicted 

 animals occasionally perishing under the inflammatory action 

 thus set up. Stock-owners and farmers term this disease grub 

 in the head, and it is often asserted by them that the grubs gain 

 access to the substance of the brain itself. They are perfectly 

 sure they have seen maggots in the brain, and no arguments of 

 the veterinarian, derived from a study of the osteology of the 



