154 MEDICAL ENTOMOLOGY 



mistaking the several moults of one species for different larvae. In 

 mosquito larvae the difference lies mainly in the length and ramifi- 

 cations of the hairs of the exo-skeleton, the hairs tending to become 

 more complex at a late stage in the life of the larva. In the Muscidae 

 the form of the spiracles changes in a characteristic manner, the 

 openings into the tracheae becoming much more convoluted. 



The pupae of Diptera are of two main kinds. In one the larva 

 casts its skin at the moment of pupation ; in the other it pupates 

 within the larval skin, which is then termed the puparium. In either 

 case the pupae absorb no food, but remain passive while their tissues are 

 reorganized to fit them for the final and reproductive stage of their life 

 history. The larval skin is cast in the Orthorraphic flies. In them 

 the pupa is often active, especially in the aquatic forms, but more usually 

 it is motionless, and may be fixed to some object in its environment. 

 Such a pupa is termed obtecta. In the higher Diptera the larval skin 

 remains as an outer covering, within which pupation takes place, and 

 the pupa is said to be coarctate. The distinction between the words 

 pupa and puparium should be noted. The puparium has generally an 

 elongate ovoid shape, with a rounded contour in cross section ; the 

 segmental markings can be faintly traced. The anterior and posterior 

 stigmata can still be seen, and show the same markings as in the last 

 larval stage. 



The method of emergence of the imago from its pupa marks an im- 

 portant distinction between the tw r o main groups of the Diptera. In 

 the simpler forms it emerges through a T-shaped slit on the anterior 

 part of the dorsum of the thorax, while in the higher forms the anterior 

 end of the puparium is removed as a small circular cap, a method which 

 has given to the group the name Cyclorrapha. 



In some Diptera the life history is profoundly altered by the reten- 

 tion of the early stages within the body of the parent, where they 

 undergo a part or the whole of their period of growth. When the larva 

 is thus retained and nourished until it is mature, the fly is said 

 to be pupiparous, a term which, though sanctioned by long usage, is 

 a misnomer, for the larva must necessarily leave the body of the parent 

 to pupate. The habit of producing larvae instead of eggs is found 

 scattered somewhat indiscriminately throughout the Diptera, and occurs 

 in varying degrees, as has already been pointed out in Chapter II. It 

 is highly developed in the debased parasitic forms included in the 

 Suborder Pupipara. 



Because of the enormous size of the order!and the additions which 



