652 



CLASS IV. 1NSECTA. 



an embryonic power of growing into new organs. Their number 

 varies in different families and probably reaches a maximum in 

 the Diptera. In the Lepidoptera the head with its mouth parts 

 and the thoracic legs of the caterpillar pass with some modifica- 

 tions direct into the corresponding parts of the moth, but the 

 compound eyes and the wings arise from imaginal discs. In the 

 Diptera however there are always imaginal discs for each of the 

 legs and in many cases, i.e. the Muscidae, special discs for the 

 reconstitution of the head. The whole epidermis is also renewed, 

 a layer of new cells arising around the base of the limbs and 



I at> 



FIG. 408. Diagrams illustrating the position of the imaginal discs in the larva (A) and pupa 

 (B) of Musca (taken from Van Rees). The wing-rudiments are omitted, as optic disc ; 

 at antennal rudiment ; &1, b 2 , & 3 rudiments of the three thoracic limbs ; bg ventral chain of 

 ganglia ; g brain ; h so-called " brain-appendage " ; m peripodal membrane ; o aperture of 

 the brain-appendage into the pharynx ; oe oesophagus ; p so-called " pharynx " ; r rudi- 

 ment of the proboscis ; ss frontal disc ; st stalk-like connexion between the peripodal 

 membrane and the hypodermis ; I, II, III the three thoracic segments. 



insinuating itself between the cuticle and the larval epidermis. 

 The latter is finally destroyed by the phagocytic activity of the 

 blood-corpuscles. In the abdomen there are four imaginal 

 discs in each segment destined to form new epidermis. Other 

 centres form the muscles of the adult which replace the disin- 

 tegrated bundles of the larva. The stomodaeal and proctodaeal 

 portions of the intestine are reconstituted from ring-like imaginal 

 discs which occur at the inner end of these in vagina tions, and 

 numerous imaginal discs occur along the course of the alimentary 

 canal. These growing and finally fusing with one another 



