58 



THE WINGS 



[CH. 



condition essential to further specialization by sub-division, as in 

 the Anisoptera. The tendency towards broadening the base of the 

 wing in this suborder not only put a strain on the discoidal cell. 



Fig. 20. Phylogenetic diagrams of the discoidal cell. a. Essential condition of 

 the wing-tracheae in order that the discoidal cell (d) may be formed. 6. The 

 corresponding archaic wing-venation. Stages in the formation of the triangle 

 (<), supra-triangle (,), and sub-triangle (st) of the Anisoptera are shewn from 

 c to <7 2 5 x * ne cross-vein destined to divide the discoidal cell into two ; d the 

 Tetrathemis-st&ge; e incomplete and / complete Gomfhine-st&ge ; g t Libellulid- 

 stage, hind-wing; g z the same, fore-wing. Stages in the formation of the 

 quadrilateral (q) of the Zygoptera are shown from cf to /" ; c' archaic type, 

 somewhat irregular quadrilateral: d' , e' Calopterygidae ; d"-f" Lestidae 

 and Agrionidae; sq sub-quadrangle. Original. 



