viii] THE LIFE-STORY AND THE SEASONS 91 



(1870) who saw pupae of Harlequin-midges (Chiro- 

 nomus) lay unfertilised eggs, which developed into 

 larvae. Here the imaginal stage only is omitted from 

 the life-history. Not always however is it the imaginal 

 stage of the life-history which is shortened. Refer- 

 ence (p. 18) has already been made to the case of the 

 virgin female aphids, whose eggs develop within the 

 mother's body, so that active, formed young are 

 brought forth. Among the Diptera it is not unusual 

 to find similar cases, the female fly giving birth to 

 young maggots instead of laying eggs. Such is the 

 habit of the great flesh-fly (Sarcophaga), of some 

 allied genera (Tachina, etc.) whose larvae live as 

 parasites on other insects, and occasionally of the 

 Sheep Bot-fly (Oestrus). In such cases we recognise 

 the beginning of a shortened larval period, and 

 Bruce's investigations in 1895, summarised by E. E. 

 Austen (1911), have shown that females of the dreaded 

 African Tsetse flies (Glossinia) bring forth nearly 

 mature larvae, which pupate soon after birth. In 

 another group of Diptera, the blood-sucking parasites 

 of the Hippoboscidae and allied families, the whole 

 larval development is passed through within the 

 mother's body, and a full-grown larva is born the 

 cuticle of which hardens and darkens immediately to 

 form a puparium ; hence these flies are often called, 

 though incorrectly, Pupipara. Still more astonishing 

 is the mode of reproduction in the allied family of 



