IMAGINAL DISCS. 



653 



gradually ensheath the lining epithelium of the larval digestive 

 tract. This epithelium is finally split off and lies in the lumen 

 of the newly formed enteric tube. 



The respiratory system undergoes a similar process of dis- 

 solution and reformation. The stigmata and the larger tracheae 

 of the imago correspond neither with those of the larva nor with 

 those of the pupa, but the system is at no one moment totally 

 destroyed and it continues to function throughout the pupal 

 life. The larval fat-bodies also undergo histolysis and are re- 

 constructed from certain imaginal discs. 



A 



FIG. 409. Diagram illustrating the transformations that take place in the pupa of Musca 

 before it hatches (adapted from Kowalevsky and Van B-ees) . The wing-rudiments are not 

 drawn, as optic disc ; at antennal rudiment ; ft 1 , b 2 , 6 3 rudiments of the three thoracic 

 limbs ; bg ventral chain of ganglia ; g brain ; k cephalic vesicle (formed by the union of 

 the pharynx with the brain appendage ) ; oe oesophagus ; r rudiment of proboscis ; * 

 frontal disc ; /, //, /// the three thoracic segments. 



The phagocytes of the blood which are the active agents in 

 the dissolution of the various tissues, do not themselves form 

 constituent parts of the new system of organs. They however 

 doubtless act as distributing agents for the nutriment they 

 have absorbed. Some of them become the blood corpuscles of 

 the imago, but many of them degenerate and are finally absorbed 

 within the newly formed tissues of the adult. 



No evidence has been obtained of the existence of Insects 

 before the Carboniferous epoch. In the coal measures they 



