fo. 1331. 



DRA GON-FL Y WING 1 '/;X 1 TION—NEEDII. I M. 



7U) 



3. The ascent of vein Oii^. — The cubital fork is in till Odonata at the 

 lind angle of the triangle. When the distal end of the anal vein 

 Ijneets the cubitus squarely at this point, a struggle ensues between 

 fhe branches of the cubitus for the maintenance of this strong point 

 [)f support. In a series of genera 6^?^^ wins, and Cu^ is starved and 

 •rowded out, becomes much the weaker vein, and is forced to ascend 

 die outer side of the triangle. This has happened in both fore and 

 kind wings of Pseudophlehia minima (Plate XLV, tig. i). The suc- 

 ■essive positions it assumes are epitomized in the diagram; tlicy may 

 Ije verified in the hind wings of such genera as Agrioncyptera (Plate 

 jXLIV, fig. 1), Mesothemis (Plate XLV, fig. 3), Dlplacodes (Plate XLV, 

 lig. 2), and Mierodlplax (fig. 13). 



j ^. The descent of vein On^.^A happier solution of the struggle just 

 fiientioned is found when vein Ott^ is left in possession of the hind 

 ingle of the triangle, vein Ou^ descending at a right angle from the 

 fork, carrying the tip of vein A^^ 

 with it for its own support. We 

 have already seen that a very 

 perfect adjustment of this sort 

 exists throughout the Caloptery- 

 gidte, where vein Ou,^ appears as 

 the direct continuation of vein A^. 

 Our diagram illustrates the manner 

 in which this adjustment is brought 

 about in the Lil)ellulidtv'. This will 

 be better understood by examining 

 the fore wings of such genera as 

 Agrlonoptera (Plate XLIV, fig. 1), 

 Atiatya (Plate XLIV, fig. 2), Ra- 

 pkkmia (Plate XLIV, fig. 3), and Ephldatla (Plate XLVII, fig.^ 2). 

 I Differentiation hetweenfore and hind wings at the triangle.^Vvimi- 

 tively, fore and hind wings of Odonata were alike, and the arculus 

 jwas a little beyond the triangle in both, with the anal vein extending 

 to the hind angle of the triangle (conditions still preserved in Cordu- 

 legast<n' (fig. 25)). The above diagram of these parts (fig. l-t) will 

 therefore stand for either wing. 



In the space between the cubital and anal \ cins before the triangle 

 are two crossveins which may fairly be considered typical, and which, 

 because of the frequent necessity for reference to them, we will des- 

 ignate as the first and second cubito-anal crossveins (« and }> of all the 

 figures). The part of the cubital space beyond the first cubito-anal 

 crossvein will be seen to lie directly beneath the quadrangle (which it 

 much resembles). It may be conveniently designated as the ^- sul)quad- 

 rangle" (.svy), and the part of this ])eyond the second cubito-anal cross- 

 vein as the subtriangle {t' of all the figures; called also, elsewhere, 

 "subtriangular space" and ''internal triangle'^). 



Fig. 11.— Diagram showing base of typical 

 deagon-fly wing; showing at (t and h first 

 and second cubito-anal crossveins; show- 

 ing also quadrangle ((/), subquadhangle 



(Sg), SUPER-TRIANGLE (.<), TRIANGLE (0, AND 

 SUBTRIANGLE (<'). 



