164 PHILIP p. CALVERT. 



If the distribution of these veins be now compared with that of 

 such a Cah)pterygine as Calopte7'i/x or Hetcerina, it will be seen that 

 the principal and subnodal sectors apparently arise from the median 

 vein, but a closer examination will show that the principal sector 

 here also arises from the upper sector of the arculus by a short vein 

 which runs from the upper sector (at a short distance from the ar- 

 culus) obliquely upwards and outwards to the median vein, thence 

 alongside of, and in contact with it, then again separating and con- 

 tinuing on its course as already described for the other groups. The 

 subnodal sector parts from it in the region of its contact with the 

 median vein, while the nodal sector is unchanged. This arrangement 

 of the principal sector does not prevail among all Calopteryginse ; 

 in many exotic genera the positions of principal and subnodal sec- 

 tors is as described for the order in general, and even some individ- 

 uals of Calopteryx show a comj)lete separation between principal 

 sector and median vein. 



DeSelys aud Hageu have considered the principal vein to extend from the 

 arculus to the apex of the wing, but for the sake of convenience, it is here pro- 

 posed to regard the principal sector as beginning at the point of its separation 

 from the median sector, and to designate the vein from the arculus to that point 

 of separation, as the upper sector of the arculus. 



The lower sector of the arculus is quite constant in position, and 

 runs behind and fairly parallel to the median sector. It is also 

 termed, or at least as much as lies beyond certain areas "hereafter 

 described as the quadrilateral or the triangle, — the short sector. 



The quadrilateral is a space on the wings of the Zygoptera, bounded 

 by the lower sector of the arculus, the submedian vein, a cross-vein 

 between these two, and the lower part of the arculus. It may or 

 may not be cross-veined, but in the former case, the cross-vein which 

 forms its outer (distal) boundary is thicker than any of the veins 

 within. Beyond the quadrilateral, the submedian vein is prolonged 

 as the first or ujiper sector of the triangle. The post-costa extends 

 outwards below and parallel to the submedian as far as the outer 

 side of the quadrilateral ; its prolongation is the second or lower 

 sector of the triangle. 



In the Agrioninse, Baron de Selys has considered the lower sector of the tri- 

 angle in some genera as "arising in front of the basal post-costal nervure (see 

 fig. 1)." It would seem more correct, judging from a comparison with the Calop- 

 teryginse, to say "the post-costal vein separating from the hind margiu in front 

 of," etc. 



