DIPT ERA. 



267 



(ESTRID.^. 



The (Estridae have a deep, vertical constriction 

 between the thorax and abdomen, and the latter is 

 joined near the base of the metathorax, giving a more 

 or less pedunculated appearance to this part of the 

 body. In the larval ^ state bot-flies are parasitic on 

 mammals, such as the horse, sheep, ox, and man. 

 The sheep bot-fly, CEstrus ovis, Linn. (Fig. 217, i, 2) 



Fig. 217. 



causes the disease known as "grub in the head," 

 which often proves fatal. The flies place the living 

 larvae (Fig. 217 : 4, dorsal view; 5, ventral view; 6, 

 the same when young) in the nostrils of sheep ; these 

 ascend and attach themselves to the frontal sinuses by 

 means of the two hooks (Fig. 217, 4, a). They have 

 two spiracles (Fig. 217, 6, c) and two horny appen- 

 dages (Fig. 217, 5, b) near the anus. In about nine 

 months the larva is full-grown ; it then drops to 



