150 THE INTERNAL STRUCTURE OF THE FULL-GROWN LARVA 



association witli thf corresponding prothoracic and mesothoracic 

 ventral discs. The smaller and more posterior disc (d.mt.) will 

 develop into the remaining portion of the much reduced meta- 

 thoracic segment, including the halteres. 



Reference has already been made to other imaginal rudiments 

 which occur in the abdominal region as regular patches of em- 

 bryonic cells. The abdominal segments develop from numerous 

 segmentally aiTanged plates of a similar nature which are found 

 during the early pupal stage. 



Fig. 68. Transverse section of one of the salivaiy glands of the mature larva. 



(Camei'a lueida drawing.) 



During pupation the imaginal rudiments increase in size and 

 are not destroyed by the phagocytes in histolysis, as is the case 

 Avith most of the larval structures. The cephalic discs are evagi- 

 nated by the eversion of their sacs by way of the anterior end of 

 the larva, a cord of cells attached to the dorsal wall of the anterior 

 end of the phar3rnx marking the path of eversion. A similar 

 process takes place in the case of the thoracic imaginal discs, 

 which, by their eversion, build up the whole of the skeletal case 

 of the thorax and its dorsal and ventral appendages, the wings> 

 halteres and legs. 



