736 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv 



and not angiilatcd, in the male. These characters, together wit 

 others found in other organs, give cause for setting apart Aiid.r (wit 

 Ifeiiiiaiuhv, Phite XXXV 1, tig. 3) as constituting a diti'erent develof 

 mental line from the one in which ^E^chui is found. 



Then there is the imi)ortant Brachytron group of genera {Brach^ 

 tron (Plate XXXV 1 11, tig. 2), ^EschnopJdebla (Plate XXXVllI, % 

 3), Planxschnd^ Caliaischna^ JSfasimschmi (Plate XXXIX, tig. 1 

 Epi;vfich))(f, etc.), which seems at first sight so closely allied to ^Exchih 

 which was set apart by Karsch" for systematic purposes, on excellci 

 venational characters, the manner and the purport of whose develoj 

 ment remain to be indicated. The radial sector is forked in these, bi 

 after the anterior branch is acquired the whole sector does not, a 

 in yEsehna^ bend anteriorly, arching away from the radial supplenienii 

 which becomes bent in the opposite direction, but both remain straighil 

 and the tcndeniy seems to be toward strengthening and deepening th' 

 fork. As a consequence, vein J/ja is allowed in the Brachytron grou 

 to occupy its accustomed place and to bear its share in the stress c 

 the wing stroke, not being crowded out, as in the ^^^schna group, 1) 

 the encroaching distal end of vein 31^^. 



Boyer'm seems to stand near where was the parting of the ways ths 

 led to the development of these two groups. One may see that eitht 

 form of radial sector might readily have been derived from that ( 

 Boyena (Plate XXXVII, fig. 3).- 



The study of the trigonal supplement adds force to such opinioi 

 In Boyeria it might be interpreted as joining the median supplemeni 

 or as passing below it with a single row of cells between; its positio 

 is not decided. But in the ^Eschna group it distinctly joins tt 

 median supplement, and in the Brachytron group it distinctly exten( 

 below b}^ one row of cells. 



The l)chavior of the distal ends of veins J4 and M^ oft'ers sti 

 further corroborative testimon3\ Beginning with a condition lit 

 that found in Boyeria, where there are several rows of cells include 

 between these two veins for the distal third or fourth of their lengtl 

 the veins themselves being parallel, development takes place aloii 

 two distinct lines. In the Brachytron line these veins remain paralle 

 and specialization consists in the progressive reduction of the eel 

 between them to a single row. In the ^Eschna line, vein M^ becom» 

 bent away from \ein J/3 at a point opposite the ])i'oximal end of tl 

 radial supplement, while reduction proceeds as usual on either side th 

 place. (See yE. calif ornica^ Plate XL, fig. 1, and ^E. im/ens., Pla 

 XL, fig. 2.) It will be observed that the vein M^ finally appears to ri 

 into vein J/3. This is wholly due to the increasing deflection of ve 

 M^ and to the straightening out of two cross veins which happen | 

 lie opposite the point of deflection. These cross veins first arq 



« Kritik des Systems der yEschniden, Eiit. Nachr., XVII, i^p. 273-290. 



