THE OX GAD-FLY, 419 



white, and covered with down ; and the thorax is 

 yellowish before, black in the middle, and cinereous 

 behind. The female differs from the male, in hav~ 

 ing a black style at the end of the abdomen. 



This insect deposits its eggs in the back of the 

 ox ; and the larva lives beneath the skin, between 

 this and the cellular membrane. Its sac or abscess 

 is somewhat larger than the insect, and, by narrow- 

 ing upwards, it opens externally to the air by a 

 small apperture. 



The larva is smooth, white, and transparent when 

 young, but, when full grown, is of a deep brown. 

 It is also supplied in this state with innumerable 

 minute hooks, ranged in contrary directions on its 

 body 5 with which, by occasionally erecting or de- 

 pressing them, it is moved about in the abscess; 

 and from this motion, and the consequent irritation, 

 a more or less copious secretion of pus takes place 

 for its sustenance. 



When the larva is full grown, it effects its escape 

 from the abscess by pressing against the external 

 opening. When this becomes of sufficient size, it 

 writhes itself through, and falls from the back' of 

 the animal to the ground ; and, seeking a conve- 

 nient retreat, becomes a chrysalis. After the exit 

 of the larva, the wound in the skin is generally 

 closed up and healed in a few days. 



The Ox Gad-fly is the largest of the European 

 species, and is very beautiful. When the perfect 

 insect leaves the chrysalis, it forces open a very 

 remarkable marginated triangular lid, which is 

 situated on one side of the small end. 



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