45 2 DIPTERA CHAP. 



obtecta. A good name for the whole structure of this instar 

 has not been found. Older authors called it " pupa coarctata," 

 or " nympha inclusa " ; Brauer speaks of it as a " compound 

 pupa " ; ordinarily in our language it is called a " puparium," a 

 term which is more applicable to the case alone. 



In species having a pupa obtecta the larval skin is cast after 

 the chief processes of the external metamorphosis have occurred, 

 and then an exudation of chitin hardens the general surface. 

 In the " compound pupa " of the Blow-fly there is for a consider- 

 able period no formed pupa at all, but merely a shell or case 

 containing the results of histolysis and the centres for regenera- 

 tion of new organs ; the chitin-exudation to the exterior of the 

 larval skin occurs in the early part of the series of metamorphic 

 changes, and the organism breaks down to a cream within the shell 

 thus formed, and then gradually assumes therein the condition of a 

 soft, nymphoid pupa. The exceptional conditions previously re- 

 ferred to as exhibited by a few forms are certain cases in \vhich a 

 more or less perfect pupa obtecta is found within the last larval 

 skin, as is the case in Stratiomys. Another highly remarkable 

 condition exists in the Hessian fly, and a few other Cecidomyiids, 

 where the Insect apparently makes an exudation which it uses 

 as a covering case, independent of the larval skin ; this latter 

 being subsequently shed inside the case, so that this condition 

 of coarctate pupa differs from that we have described as exist- 

 ing in Cyclorrhaphous flies, although the two are superficially 

 similar. In the Pupipara the larval stage is passed in the body 

 of the mother, which produces a succession of young, nourished 

 one at a time by the secretion of glands ; this young is born as 

 a full-grown larva that becomes at once a pupa. 



Metamorphosis. As it is in Diptera that the phenomena 

 of Insect-metamorphosis have reached their highest development 

 we endeavoured to give some idea of their nature in the previous 

 volume, therefore we need give only a brief sketch of the chief 

 features of Dipterous metamorphosis. The Blow-fly undergoes a 

 rapid embryonic development, the later stages of which are, on 

 the whole, of a retrogressive nature. On the emergence of the 

 young maggot it feeds up rapidly, the rapidity varying greatly 

 according to circumstances, and then when full-grown rests. 

 While resting, a process of internal liquefaction, called histolysis, 

 is going on, and the maggot contracts and exudes an excretion 



