460 



THE INVERTEBRATA 



Fossil record. Though the insects form an undoubted natural group 

 — all its members being referable to some generalized form, possess- 

 ing among other things mouth parts similar to those of the cockroach, 



ivr.- 



Fig. 320. 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, In- 

 star 3. D and E, Instar 5. ch. chitin; hy. hypodermis; m./. middle lamella; 

 p.m. peripodal membrane ; tch. tracheoles within the veins ; tel. tracheole cells ; 

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



efficient for chewing solid food, an ii-segmented abdomen, a 3-seg- 

 mented thorax and a 6-segmented head, and two pairs of membranous 

 wings carrying parallel longitudinal veins with a reticulum of cross 

 veins between them — the orders are clearly defined. They are not 



