344 THE DIPTERA, 



In the structure of the abdomen, the Diptera pre- 

 sent nothing remarkable. This region of the body is 

 always composed of several distinct segments, usually 

 of a horny texture, and united together by rings of 

 flexible membrane, which are concealed by the sliding 

 of each segment into that preceding it ; in some cases 

 the apex of the abdomen of the females is furnished 

 with an ovipositor, frequently of telescopic structure, 

 whilst the males occasionally exhibit singular forceps- 

 like appendages at the same point. The base of the 

 abdomen contains a pair of large air-sacs, which some- 

 times appear to occupy the greater part of its interior, 

 and this vacuity renders some of the more gaily- 

 coloured species (of Syrphus, Volucella, &c.) so ex- 

 ceedingly transparent, that, by holding them up to 

 the light, the movements of the internal organs may 

 be pretty distinctly traced. 



The metamorphosis of the Diptera is one of the 

 most complete. The larva is a footless, worm-like 

 grub or maggot, sometimes fleshy, sometimes co- 

 vered with a firm skin, and frequently destitute of a 

 distinct head. The stigmata are sometimes placed 

 on the segments of the body, as is usually the case 

 in insects, but in a great many instances, only two of 

 these organs are efficient, and these are placed quite 

 at the hinder extremity. The pupa is quiescent, and, 

 like that of the Hymenoptera, enclosed in a delicate 

 skin, which leaves all the organs free and recog- 

 nizable ; but in a great many species, the larva skin is 

 not cast, the insect undergoing its transformation 

 within this integument, which then hardens and 

 forms a firm case for the protection of the creature 

 in its helpless condition. This form of pupa is called 

 coarctate. 



