1 68 METAMORPHOSIS 



CHAP. 



appropriate the products of the disintegration so as to "produce 

 the perfect fly. 



Our Fig. 88, taken from Graber, represents a longitudinal 

 median section of a full-grown larva of Musca, in which the 

 processes of metamorphosis are taking place. The position of 

 some of the more important imaginal rudiments is shown by it : 

 &\ &2, h^, rudiments of the three pairs of legs of the imago ; an, of 

 antennae ; between an and w, rudiment of eye ; w, of wings ; h, of 

 halteres ; /, fat-body ; d, middle of alimentary canal ; n, ventral 

 chain ; st, stigma ; 6, 7, sixth and seventh body segments. 



Physiology of Metamorphosis. 



Many years ago, Harvey perceived the probable existence of 

 a physiological continuity between the earlier and later stages 

 of the Insect's life. Modern investigation has shown that in the 

 blowfly a remarkable analogy exists between the conditions of 

 the pupa and the Qgg. The outer shell of the pupa corresponds 

 to the chorion or egg-shell, and the delicate outer membrane of 

 the pronymph to the oolemn or lining membrane of the Qg^ ; the 

 creamy matter corresponds with the yolk, and the regenerative 

 buds are analogous to the formative portions of the developing 

 Q^g. The process of histolysis as carried out by the phagocytes 

 of the later life appears also to find a parallel in the vitellophags 

 of the embryonic life.^ It appears probable that the physio- 

 logical processes of the post -embryonic metamorphosis may be 

 essentially a repetition or an interrupted continuation of 

 those of the embryonic period. 



The inquiry as to what are the determining causes of the 

 metamorphic changes of the blowfly and other Insects has as 

 yet but little advanced. Why does the larva grow up to a 

 certain period with great rapidity, then cease its appropriating 

 power and break up the parts that have been so rapidly and 

 recently formed ? And why do the imaginal buds remain 

 quiescent till the other tissues are being disintegrated, and 

 then, instead of sharing the general condition of disintegra- 

 tion, commence a career of development ? To these questions no 

 satisfactory answer has yet been given, though the remarkable 

 studies, already referred to, of Bataillon on the later larval life 

 1 Wheeler, in J. Morphol. viii. 1893, p. 81. 



