METAMORPHOSIS 453 



that hardens its skin. At the time this hard skin has become 

 complete, or soon after, the maggot inside has dissolved into a 

 cream contained in a sac inside the shell ; this cream becomes 

 reconstituted into a fly by a gradual process of growth and 

 development of certain minute portions of the body — the 

 imaginal discs or folds, the histoblasts and neuroblasts that 

 were exempt from the histolytic process : in the early stages 

 of the reconstitution the general structure is, of course, altogether 

 vague, and this condition — purely one of transition — is called 

 the pronymph ; the nymph becomes gradually developed : it 

 corresponds vaguely with the pupa obtecta of the early groups 

 of Diptera, but is soft like the pupa of Hymenoptera. This 

 nymph gradually develops into the fly itself, the external 

 parts being first completed and the internal organs elaborated 

 subsequently. The sexual organs do not undergo metamorphosis 

 like other internal organs, there being a gradual (though ir- 

 regular or interrupted) growth of them in the young larva, till 

 they are completed some time after the emergence of the perfect 

 fly. The processes in the Blow-fly have been studied by numerous 

 able histologists of various nationalities, and have recently been 

 described by Lowne in our own language.^ Comparatively little 

 has been done in studying the corresponding phenomena in other 

 Diptera. Weismann has investigated the development of Corcthra, 

 and Miall that of Chironomus. These two flies belong to a division 

 of Diptera different from that which includes the Blow-fly, and 

 they display a condition of the metamorphic processes allied to 

 what occurs in Lepidoptera, as well as to that which takes place in 

 the Blow-fly. Imaginal folds are formed, but they only appear 

 much later in the life, and they are much less distant from the 

 positions they will, when developed, occupy in the imago. In 

 Chirono7nus, according to Miall, the imaginal folds only appear 

 in the last larval instar, but they grow wdth such rapidity that 

 the legs and wings of the future fly can be distinguished in the 

 larva, even before pupation ; thus when the activity of the larva 

 ceases but little change is required to complete the obtected 

 piipa. In the Blow-fly some of the imaginal folds have been 



•' Since our brief and imperfect sketch of metamorphosis appeared iu YoL V. of 

 this series, Packard has treated the subject more fully iu his Text-hook of Entovwloyy, 

 New York, 1898 ; and Pratt has summarised the state of knowledge as to imaginal 

 discs in Psijclie, viii. 1897, p. 15, etc. 



