INSECTA 



403 



wings carrying parallel longitudinal veins with a reticulum of 

 cross veins between them — the orders are clearly defined. They 

 are not easily linked together by intermediate forms and the story of 



Fig. 301. The internal development of a wing in the larva of the butterfly 

 Pieris rapae as seen in transverse sections. A, Instar i. B, Instar 2. C, 

 Instar 3. D and E, Instar $• ch. chitin; /iy. hypodermis; m.l. middle lamella; 

 p.m. peripodal membrane ; tch. tracheo]es within the veins ; tel. tracheole cells ; 

 tra. trachea; v. vein; wr. wing rudiment. 



evolution within the subphylum consists rather of disjointed sentences 

 than a continuous theme. The two divisions already mentioned, how- 

 ever, the Exopterygota and Endopterygota, are natural groups which 

 we may for convenience call the " generalized " and the " specialized '* 



26-2 



