THE WARBLE-FLY 



151 



becomes deeply wrinkled, while the warble-cavity gets a special 

 wall, derived from the connective tissue of the ox. The larva 

 has no limbs, and only a vestige of a head, which is ordinarily 

 retracted into the body. A pair of forks, 

 answering to the hard and pointed hooks of 

 the blow-fly larva, can be protruded from the 

 mouth - opening. The prominent transverse 

 ridges of the body bear broken rows of small 

 hooks, and it is by means of these that the 

 maggot executes such limited movements as 

 are indispensable. In the middle of the flattish 

 surface which ends the body are two dark spots ; 

 these are the spiracles; they lead into longi- 

 tudinal air-tubes, which supply every part of the pjg^ g6._Larva of 

 body. The body is always so placed that the warbie - fly. After 

 spiracles are directed towards the opening of ^^'^'^ 

 the warble cavity. Just beneath the spiracles is the outlet of 

 the alimentary canal. 



In the next summer, the maggot, being now nearly a year 

 old, is about 35 mm. long, fat, wrinkled, and of a leaden or 

 deep-brown colour. It squeezes through the skin, and falls to 

 the ground, where it passes the next few weeks, with only such 

 slight protection as it can get by hiding in a crevice or beside 

 a stone. Its skin contracts, hardens, and acquires a deeper 

 colour. The body becomes flat and then concave along one 

 side, towards which both head and tail are directed. The 

 transformation to a pupa is completely hidden, as in the blow- 

 fly, by the retention of the larval skin, which acts as an ex- 

 ternal shell or cocoon. After four 

 or five weeks on the ground, the dry 

 skin cracks near the head, a circular 

 cap surrounding the mouth. becomes 

 detached, and the fly escapes. 



The fly is seldom seen, except 

 when it is reared from a captive 

 pupa; it is only about 13 mm. long. 

 In most points of structure it re- 

 sembles the blow-fly, but is greatly 

 disguised by its coloured hairs, which make it superficially like 

 a humble-bee. The face is yellow, the wings dusky yet trans- 

 parent, the legs black, except for the tarsi, which are pale 



Fig. 87. — Female warble-fly. 

 After Bracy Clark. 



