The Imago. 53 



where there is a thick cross-vein — the node. At the node 

 the sub-costa terminates. Near the base of the w'wv^ a 

 bent cross nervure joins the median and sub-median : 

 this important Httle nervure is called the arculus, and 

 from the centre of its distal side start the upper and 

 lower sectors of the arculus. 71ie upper sector after 

 proceeding" for a short distance divides into the principal 

 sector above and the median sector below (nothing to do 

 with the median nervure). Between these two are the 

 nodal sector, branching from the principal at or near the 

 node, and the sub-nodal, leaving the principal near its 

 basal extremity. The lower sector of the arculus bends 

 down till it meets the sub-median nervure at the 

 termination of the latter, from which point starts the 

 short sector. Between the sub-median and the post- 

 costa are three spaces — .r., v., and /. of Fig. 24, A ; 

 the distal one, /., is the triangle. From its aj:)ex, where 

 the sub-costa terminates, start two sectors — the upper 

 and lower sectors of the triangle. The ultra-nodal 

 sector is found between the principal and sub-nodal. 



Such is a general description of an Anisopterid wing ; 

 that of a Zygopterid is fundamentall}^ \ery similar, only 

 that the triangle and supra-triangle (7. and //. of 

 Fig. 24, A) are united to form the cjuadrilateral, O , 

 of Fig. 24, B and C."^ In the British Caloptciygiiut; 

 the principal sector is for a time near its basal end in 

 contact with the median nervure, which it shortly lea\es 

 again. A lens of a very low power will, however, reveal 

 the two l>'ing side by side. Individuals may perhaps be 

 found in which they do not quite touch. 



.Some say that ti alone corre-ponds to the (juadrihiteial. 



