FIELD BOOK OF PONDS AND STREAMS 



conspicuous on full-grown nymphs of dragonflies, damsel- 

 flies and mayflies and usually look thick, often black, for a 

 few days before the adults emerge (PI. XV). 



mK 



Fig. 140. — Diagram of the principal veins of an 

 insect wing, showing their types of branching, cross- 

 veins omitted: C, costa; Sc, sub-costa; R, radius; 

 M, media; Cu, cubitus; A, anal. (After Needham.) 



When the stonefly finally emerges from its nymphal skin 

 it pulls the fully developed wings from the wing-pads and 

 unfolds them to the air. They are thin membranous sails 

 supported by rods of chitin, the veins. Toward the front 

 edge of the wing the veins are stronger and set more closely 

 together than they are further back. This strengthens the 



> C 5e, 



3 



Fig. 141. — Front (A) and hind (B) wings of the 

 stonefly, Perla capitata. The principal veins are 

 marked by capitals; cross-veins, by small letters. 

 (After Needham.) 



182 



