lo University of Michigan 



the amount of variation in the form of this triangle due to the 

 shifting about of the cross-vein between M^ and Cu, which in 

 the ordinary triangle forms its anterior side. This however 

 is what might be expected. Prior to the formation of the usual 

 three-sided triangle the cross-vein, which forms its anterior 

 side, would have no particular attachment point on AI^. It 

 might be expected to be as variable as any other cross-vein in 

 an enclosed area. Held posteriorly at the angling of Cu, its 

 anterior end would be free to swing about. If it swung far 

 enough distally a three-sided triangle would result in which 

 none of the enclosure would be formed by M^. It might 

 swing basally so that it would be shorter than the portion of 

 jNI^ forming the distal part of the anterior side of the triangle, 

 which would now be four-sided. Any position the cross-vein 

 might have between these two extremes would result in a four- 

 sided triangle. The more proximal the attachment of the an- 

 terior end of this cross-vein with AI^, the better developed the 

 four-sided triangle is.^ 



The triangles of the front wings of Jiamatits may be ar- 

 ranged in a series of four groups; I, the distal part of the 

 anterior side (M^) is about one-half the combined lengths of 

 the distal and proximal (the cross-vein from M^ to Cu) parts of 

 the anterior side; II, distal part more than two-fifths the com- 

 bined lengths; III, distal part about two-fifths; and IV, dis- 

 tal part about one-third. Two males are in I ; two males 

 and two females in II ; one male in III ; one female in IV ; and 

 one female with one wing in III and another in IV. Thus, in 

 the front wings, no individual varies in itself more than from 

 one group to another. A very dift'erent condition is found in 

 the hind wings. Here five groups may be arranged in a series : 



" For a fuller discussion see Needham, A Genealogic Study of Dragon-fly 

 Wing Venation. Proc. U. S. Nat. Mus., Vol. XXVI, 1903, PP- 703-764- 



