10 



ELEMENTARY LESSONS ON INSECTS 



and radius, followed by two, media and cubitus, that spread 

 out across the middle of the wing and then several anal 

 veins that radiate to the hind border. Of these veins only 

 radius, media and cubitus are conspicuously branched. 

 These veins are joined together by cross- veins in a very 

 light and strong supporting framework; and the joinings 

 and branchings in the outer half of the wing confuse their 

 identity; but they may be easily recognized in their basal 

 unbranched stems. Our diagram, Fig. 4, omits the cross- 

 veins for the sake of clearness; but the next figure, Fig. 5, 

 shows the venation as it actually appears in the adult stone- 



fly- 



3dA 



2dA IstA 



Fig. 4. — Diagram of the principal veins of an insect wing, showing their types of branching, 



cross-veins omitted. 



The abdomen of the stonefly consists of ten similar 

 rings or segments, and a terminal cluster of three minute 

 plates, one median and dorsal, and two lateral, the latter 

 bearing the long tails. The segments are less rigid than in 



