THE HIDDEN TYPE OF WING-GROWTH 145 



air-tubes of the enclosed pupa, being placed far forwards, 

 often on prominent processes (Fig. 84 g). A sub-circular lid 

 at the front dorsal region of the puparium often breaks away 

 neatly to allow the fly when developed to escape, and the 

 suture or surrounding margin of this a narrow depression 

 along which the cuticle is thin and weak can in some cases 

 be seen in the cuticle of the final larval stage, affording an 

 interesting example of a specialized provision preformed for 

 the needs of the latest stages of the insect's development. 



(c) SOME FEATURES OF INTERNAL CHANGE 



The transitions from larva to pupa and from pupa to imago 

 in the insect life-histories sketched in the preceding pages, 

 are necessarily accompanied by marked changes in the outward 

 form. The cuticular envelope is remade at every stage of 

 the life-cycle even if, as in the successive instars of a beetle 

 grub, caterpillar, or maggot, no conspicuous transformation 

 be apparent ; and at what may be regarded as the crises of 

 transformation pupation and final emergence the change 

 revealed is often startling in its extent. Some reference has 

 already been made (pp. 63-4), to the inward modifications 

 which accompany these outward changes, and it should be 

 clear that while some larval systems grow directly into the 

 corresponding structures of the imago, others are broken 

 down during the pupal stage, and the corresponding parts of 

 the perfect insect are at the same period built up from the 

 minute imaginal discs of the larva. 



Among the organs that grow continuously through the stages 

 of the life-history, the essential reproductive structures are 

 especially typical. The germ-cells of an insect are recognizable 

 in the early embryo. In a leaf -beetle (Cyltia) they are differen- 

 tiated from the newly-formed blastoderm. 1 In a common 

 midge (Chironomus) two conspicuous cells which are the 

 primordial germs can be seen even before the formation of the 

 blastoderm. 2 From these are derived the ovaries or testes 

 of the female or the male respectively, the cells multiplying 



1 A. Lecaillon : " Recherches sur 1'Oeuf et sur le Developpement de 

 quelques Chrysomelides ". Arch. d'Anat. Microsc., II. 1898. 



2 E. G. Balbiani : " Contribution a 1'Etude de la formation des Organea 

 Sexuels chez les Insectes ". Rec. Suisse Zool., II. 1885. 



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