. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 58 THE ^^^XGS [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 Fig. 20. Phylogenetic diagrams of the discoidal cell. a. Essential condition of the wing-tracheae in order that the discoidal ceU (d) may be formed, h. The corresponding archaic wing-venation. Stages in the formation of the triangle (t), supra-triangle (,«), and sub-triangle (st) of the Anisoptera are shewT
. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 58 THE ^^^XGS [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 Fig. 20. Phylogenetic diagrams of the discoidal cell. a. Essential condition of the wing-tracheae in order that the discoidal ceU (d) may be formed, h. The corresponding archaic wing-venation. Stages in the formation of the triangle (t), supra-triangle (,«), and sub-triangle (st) of the Anisoptera are shewTi from c to gfj; X the cross-vein destined to divide the discoidal cell into tv/o; d the Tetrathemis-stage; e incomplete and/complete (?'J?w/A^?^e-stage; g-^ LibeUidid- stage, hind-wing; g^ the same, fore-'.ving. Stages in the formation of the quadrilateral (q) of the Zygoptera are shewn from c' to /"; c' archaic type, somewhat irregular quadrilateral; d', e' Calopterygidae; d"-f" Lestidae and Agrionidae; sq sub-quadrangle. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Tillyard, Robin John, 1881-1937. Cambridge [Eng. ] : University Press
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