STRUCTURE AND DEVELOPMENT OF INSECTS 



29 



FIG. 20. Mouth-parts of a plant- 

 louse: a, the jointed beak; b, the 

 lancets, much enlarged ; c, antenna; 

 d, foot. (After J. B. Smith.) 



FIG. 2 1 . Diagram of tracheal 

 or breathing system of an 

 insect: sp., spiracles; tr., 

 trachea. (After Kolbe.) 



FIG. 22. Ideal section through an insect: a, alimentary canal; h, heart; 

 n, nerve cord; s, spiracle; t, tracheal tubes; I, legs; w, wings. (From 

 Riverside Natural History.) 



