376 THE ATHERICERA. 



from attaining its object, which is the deposition of 

 its eggs upon some part of their bodies, whence the 

 larvae may easily attain the position in which they 

 are destined to pass a parasitic existence. The ru- 

 minant quadrupeds are principally subject to the 

 attacks of these insects ; but besides these, the horse, 

 ass, and some wild animals are known to nourish 

 peculiar species, and the occurrence of the larvse in 

 the human body has been repeatedly observed. The 

 larvae are thick, fleshy maggots, with the spiracles 

 placed at the extremity of the body, destitute of feet, 

 but usually furnished with a considerable number of 

 minute spines on their surface. When full-grown 

 they quit the body of their host and bury themselves 

 in the ground, where they pass to the pupa state, the 

 dried larva- skin, as in the other insects of the tribe, 

 forming a hard case for the pupa. 



The habits of the larvae of the various species are, 

 however, very different. Thus the Gad-fly of the 

 Ox {(Estrus Bovis) deposits its eggs upon the backs of 

 our cattle, in the skin of which the larva resides, the 

 irritation caused by its presence producing a large 

 tumour (commonly known to the farmers as a wornil 

 or worble), and causing a flow of purulent matter to 

 the part, which serves as food for the concealed in- 

 mate. The tumour exhibits an opening in the centre 

 of its surface, and to this the flattened hinder ex- 

 tremity of the larva is applied, so that the two large 

 spiracles with which this portion of the body is fur- 

 nished are kept in free communication with the air. 

 The behaviour of a herd of horned cattle at the ap- " 

 proach of this diminutive foe is most extraordinary : 

 the whole herd, throwing their tails into some of those 

 peculiar ungainly attitudes by which the bovine race 



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