NORTH AMERICAN ODONATA. l65 



In ihe Anisoptera, an area will be seen occupying nearly the same 

 position as the quadrilateral in the Zygoptera. This is the hyper- 

 trigonal, or supratrlangvlar space, for the reason that it is immediately 

 above a distinctly triangular area, tJie triangle, discoidal triangle, or 

 cardinal cell. Lying on the basal side of the triangle is frequently 

 another triangular area, the internal triangle or subtriwigular space. 

 The sectors of the triangle arise from or near the lower angle of the 

 triangle ; on the outer side of this last between the short sector and 

 the upper sector of the triangle is a varying number of rows of 

 cells — the post-tria)igular cells or discoidal areolets. 



Between the two families of the Anisoptera a difference exists in 

 the triangle of the front wings. In most Aeschnidse the triangles 

 of front and hind wings are similar in size and shape. In the Li- 

 bellulidse the triangle of the hind wings remains similar to that of 

 the Aeschnidie with its longer axis parallel to the long axis of the 

 wing, but in the front wings it is elongated from above downwards, 

 so that its long axis is at right angles, or nearly so, to the long axis 

 of the wing. 



The lower boundary of the supratriangular space is formed by a veiu brokeu 

 into two parts at the point whence the inner (basal) side of the triangle diverges, 

 Walsh (Proc. Eat. Soc. Phila. II, p. 208) held that of these two parts, the basal 

 one corresponds to the lower side of the quadrilateral of the Zygoptera, the distal 

 part to the outer side of the quadrilateral, and that the supratriangle and quad- 

 rilateral are homologous. Consequently the outer side of the triangle would be 

 the homologue of a cross-vein between the short sector and the tirst of the 

 triangle in the Zygoptera; the upperside of the internal triangle would be the 

 correspondent of the downward prolongation of the apical side of the quadri- 

 lateral, while that side of the internal triangle nearest the anal angle would be 

 a portion of the second sector of the triangle of the Zygoptera. 



Qu the other hand De Selys had earlier stated (Mon. Calopt. p. 279, 1854) that 

 the quadrilateral corresponds to the triangle and supra-triaugle taken together, 

 and that the distal part of the lower boundary of the supra- triangle is a "supe- 

 rior branch of the submedian vein" which is wanting among the Zygoptera. 

 From this view it would follow that the outer side of the triangle is homologous 

 with the apical (outer) side of the quadrilateral, the inner side of the triangle 

 is a part of the lower side of the quadrilateral, and in either view, is a prolon- 

 gation of the submedian veiu and continued by the upper sector of the triangle ; 

 finally, the side of the internal triangle nearest the anal angle is a part of the 

 post-costal vein. 



The difference between these two views depends upon the homology recog- 

 nized for the "upper branch of the submedian vein." The view of De Selys is 

 here adopted for the two following reasons : 



1. The apical side of the quadrilateral is a convex vein, a condition satisfied 

 by the outer side of the triangle, but not by the " upper branch of the subme- 

 dian veiu." 



TKANS. AM. ENT. SOC. XX. JULY, 1893. 



