146 THE INTERNAL STRUCTURE OF THE FULL-GROWN LARVA 



the larva by means of these imaginal rudiments, which are 

 gi-adually formed during the later portion of the larval life. 

 They do not all appear at the same time, for whereas some may 

 be in a well-developed state early in the third larval instar, others 

 do not appear until the larva reaches its resting period or even 

 later. The imaginal discs appear to be hypodermal imaginations, 

 though their origin is difficult to trace in all cases ; in many 



d.ms 



and 



drnt. 



OS. 



Fig. 65. 

 Fig. 64. Internal aspect of the posterior thoracic imaginal discs of the right side. 

 d.ms. Dorsal mesothoracic or alar imaginal disc. d.mt. Dorsal metathoracic 



disc. l.tr. Lateral tracheal trunk of the right side of larva, 

 metathoracic imaginal disc. 



v.mt. Ventral 



Fig, 



65. Longitudinal sections through the major cephalic imaginal discs of 

 mature larva to show the position of the individual imaginal rudiments. The 

 dextral section is more dorsal than the sinistral. (Camera lucida drawings.) 



an.d. Imaginal disc of the antenna. f.d. Facial imaginal disc. i.s. Sheath 



of imaginal rudiments. o.d. Optic imaginal disc; o.g. Imaginal disc of 



the optic ganglionic structures. o.a. Optic stalks. s.f/. F'undament of the 

 imaginal supra-oesophageal ganglionic. .•^h. Sheath of cerebral lobe. 



instances they are coimected with the hypodermis by means of a 

 stalk of varying thickness. The imaginal disc or rudiment may 

 consist of a simple or of a folded lamina of deeply-staining columnar 

 embryonic cells, as in the wing discs, or of a number of concentric 

 rings of these cells, as in the antennal and crural discs. They are 

 usually closely connected with the tracheae and in some cases are 



