9i6 



FRESH-WATER BIOLOGY 



These latter larvae are red in color and are known as ■' blood worms." 

 The color is due to haemoglobin in the blood plasma; the capacity 

 of this substance for oxygen gathering seems to enable these blood 

 worms to live in water that is poor in oxygen. 



In the preceding pages the principal groups of aquatic insects 

 are briefly characterized, and typical forms are figured. Hints are 

 given for the recognition of the nymphs of Plecoptera on page 885, 

 and of the larvae of aquatic Lepidoptera on page 903 . In the fol- 

 lowing pages keys are given for determining the adults of Trichop- 

 tera and Hemiptera, and for both adults and immature stages of 

 the other orders. An understanding of the venation of the wings 

 is essential to the study of adult insects of most orders, and the 

 following figure (Fig. 1386) is given to illustrate the wing venation 

 and explain the terminology used. 



Fig. 1386. The venation of the wings of a stonefly, Chloroperla. The designation of veins is given 

 here for all succeeding wing figures: 



C, Costa M, Media 



Sc, Subcosta Cm, Cubitus 



R, Radius A, Anal veins. 



The radius has a main stem (Ri) and a principal branch {Rs) on the posterior side. 

 Media is often twice forked, and Cubitus once forked; the recognizable branches are numbered from 

 front to rear: three anal veins are likewise recognized. 



