PLECOPTERA OF NORTH AMERICA 27 



of bracing of the wing — arculus, cord and anal cell. We 

 have found a very dependable character that has scarce- 

 ly been noticed hitherto in the relation between the cubi- 

 to-anal crossvein of the forewing and the anal cell. This 

 crossvein in a series of genera in the difficult family of 

 Perlidae, shifts its position from the front of the anal 

 cell out beyond the apical angle along the base of the first 

 anal vein as indicated for a series of genera in the ac- 

 companying diagram. This has furnished a clue to af- 

 finities in some of the difficult cases. 



Genital Characters. 



Male. — The terminal abdominal segments of male stone- 

 flies are variously modified to serve as copulatory organs. 

 Since the sperm ducts open at the apex of the 9th seg- 

 ment, the 9th and 10th segments are always involved, and 

 the 8th, 7th and 6th and even the 5th may be as well. As 

 is well known, there are 10 segments that usually form 

 complete rings in the abdomen, and what is supposed to 

 be an 11th segment is represented by three plates, con- 

 vergent over the terminal anal opening — a median dorsal 

 supra-anal plate and a pair of ventral subanal plates that 

 bear the cerci or tails. These are the same in both sexes. 

 In copulation the male climbs upon the back of the fe- 

 male, drops the tip of his abdomen below the level of hers, 

 and recurves it, turning the end segment upward and for- 

 ward, grasshopper fashion, beneath hers, bringing the 

 hollow up-turned end of -the genital scoop of the 9th seg- 

 ment into apposition with the subgenital plate at the apex 

 of the 8th segment in the female. The commonest fea- 

 ture of the male copulatory apparatus of the order is the 

 development of recurved genital hooks that can be thrust 

 beneath the subgenital plate of the female to draw it 

 downward and open the vulva. These are most simply 

 developed as a pair of processes arising from the subanal 

 plates and curving forward. In the Nemouridae there is 

 a very great range of form assumed by the subanal lobes. 

 In the genus Nemoura they may be as simple as in the 

 female, or they may be very large. In Leuctra they are 

 divided into two processes, one slenderer than the other 

 and in Taeniopteryx (in part) they are unsymmetrical. 



A single median hook, in some cases becoming a long 

 probe, and sometimes exceedingly complicated, is often 

 developed from the supra-anal lobe. Paired hooks may 

 be developed from the apex of the 10th segment, and this 

 segment may become deeply cleft and the hook arising 

 from the hind angles of the cleft may become very large. 



