THE BIRD-FLY. 379 



a firm hold of any object, a faculty which, as we 

 shall see hereafter, is of considerable importance to 

 them in their mode of life. The wings are generally 

 present, and resemble those of the preceding group. 

 The abdomen, like the rest of the body, is rather 

 soft, but enveloped in a tough leathery skin, which 

 exhibits scarcely any traces of segments, and as the 

 body and limbs are generally clothed with strong 

 hairs, the insects have frequently a tolerably close 

 resemblance to some of the Arachnida, or Spiders, 

 from which circumstance they are commonly known 

 as " Spider-flies/' amongst the French. 



The larvae are retained within the body of the mo- 

 ther until they are full-grown, and either converted 

 into pupae, or ready to undergo their transformation. 

 They are produced one at a time, and at the moment 

 of their exclusion are nearly as large as the abdomen 

 of their parent, but their skin is then soft and white, 

 and it is not until the lapse of some little time that 

 they acquire the hard consistence and brown colour 

 common to most of the pupa-cases of the coarctate 

 Diptera. 



The species belonging to this group, which are few 

 in number, are parasitic upon various species of birds 

 and mammals, running about amongst their hairs and 

 feathers, piercing their skins, and sucking their blood 

 by means of the curious rostrum above described. 

 The most abundant and generally-distributed species 

 is the Ornithomyia avicularia, or Bird-fly, which in- 

 fests various species of small birds, and often torments 

 them to such an extent that they appear quite mad- 

 dened, sometimes striking themselves against walls 

 and other objects with such force as to stun or kill 

 themselves. Young birds appear to be either favourite 



