728 PROCEEDINGS OF THE NATIONAL MUSEUM. 



I 



The accompanying diag-ram (fig. 21) shows the steps by which cross- 

 veins may be brought into line. All these steps may be seen in the 

 wiiu's of Agrionina? represented in the plates and better still in fig. !). 



In Caloptervgi(hi> crossveins are so numerous it seems to have l)een 

 of more moment (at least in Calopteryx and its nearest allies) that the 

 longitudinal lines of crossveins should become straight, allowing the 

 longitudinal furrowing of the base of the wing to be extended to the 

 margin. 



SECTORS AND SUPPLEMENTS. 



The areolae, or cells, of the wing membrane are potential hexagons. . 

 Where bordered on one side by a straight vein they are converted into ; 

 pentagons by the elimination of one angle; where included in a single 

 row between two veins they become quadrilaterals, through the elim- 

 ination of two angles. Even in the broader areas of the wing their i 

 hexagonal regularit}' is early disturbed by two more or less distinct t 

 means: (1) ])y the development of new tracheal branches, which pene- 

 trate into the broader spaces, straightening out certain liroken lines of 

 crossveins and converting their bordering hexagons into pentagons; 

 (2) by the development of longitudinal veins, independently of the 

 tracheae — veins which traverse the same spaces in a direction generally 

 more or less transverse to that of the accessory trachea? and complete 

 the havoc of the hexagons. By the first means are produced extra 

 branches of principal veins, which so appear in the adult wing. Some 

 of the most important of these are designated J/ja, ^-Sa, Cn.^a.^ etc. , in 

 the figures. B}^ the second means are developed curved veins, which 

 are generally opposed in position to the principal veins bounding the 

 wider spaces anteriorly. These may ha called SKjiplciuents. Both are 

 well shown, and the difl^erence between them is clearly demonstrated by, 

 Plate XXXII, fig. 1 of Anax j mines. The principal trachea shown i 

 is the radial sector, whose branches are accessory; the strong, develop- 

 ing vein that sets across them, bending toward the radial sector at both : 

 its ends, is the radial supph-inent {B. su.ppl.., of all the figures). It will 

 be seen to be a purely cuticular vein, without trachea of its own. .< 

 Behind vein 3f^ is an exactly similar vein, the median supplement {M. .' 

 suppL, of all the figures). Behind vein If^ and proximal to vein J^a,.' 

 though often joined to the latter, is another, the apical supplement {ap. ] 

 suppL), well shown in Tramea (Plate XLIX, fig. 3) and its allies. From 

 the outer side of the triangle there starts another, the trigonal supple- 

 ment {fr/'(/. snpj}].) well shown in ILujcnlm (fig. 28), and in most ^Esch- ' 

 ninte (Plates XXXVII-XL). 



Both supplements and extra l)ranches, being relatively recent 

 acquirements, are well developed only in the more specialized mem- 

 bers of the several groups. 



