718 PROCEEDINGS OF THE XATIONAL MUSEUM. vol. xx^ 



structure. The increasing angulation at this point may he followe' 

 in the fore wings of a series of Libclhiline genera, such as Mierodlplu 

 (tig. 13), Anatya (Plate XLIV, tig. 2), J/ew^Ac^/yi/.v (Plate XLV, tig. 3 

 and Perithemis (Plate XLIII, tig. 3). It is only .a little less evidei 

 in such Goniphine genera as AgrlogoniphMH (tig. 37), Gomphoid,^ 

 (Plate XXXIII, tig. 2), and Gomphm (Plate XXXIII, fig. 1). It wr 

 be observed that this deflection of the cubitus results in the widenini 

 of the space between veins M^ and 6\ beyond the triangle. Th:lj 

 may be seen in the aforementioned genera. 



2. The deflection of the iniddle crosft vein of tlie quadrangle. — It 

 this process that makes the distal half of the quadrangle triangulai; 

 Successive positions of the cross vein are shown at 1, 2, 3, 4, and 5 c 

 figure. Such genera as Tetrathentk (tig. \iS)^ Anatya (Plate XLI\ 

 tig. 2), Macrothemis (Plate XLVI, fig. 1), and Ephidatia (Plat' 

 XLV II, tig. 2) exhibit these stages. A similar but less striking seriti 

 will be seen in such Gomphine genera as Agriogomj^hus (fio-. 27 



Fig. 13.— Wings of Microdiplax delicatula Selys. 



Gomphoides (Plate XXXIII, tig. 2), and Ilagenins (fig. 23). Thi 

 cross vein has become greatly elongated in most ^Echnidfe, resultin; 

 in the elongation of the whole triangle, as seen in such genera a 

 Gomph^schw (Plate XXXVII, fig. 1), Brachytron (Plate XXXVIIl 

 fig. 2), and Nasixschna (Plate XXXIX, fig. 1). Thus this cross vei 

 has come to lie in such position that it appears in the adult wing U 

 be a continuation of the cubital trunk, and it has generally been s 

 interpreted. « 



^Pentathemismemhranulata Karsch (Ent. Nachr., XVI, 1890, pp. 33-35) presents 

 somewhat unusual condition of this cross vein, in that it is dechned to the maximui 

 and bent upward in the middle at its junction with a cross vein in the supertriaugl 

 which has migrated outward upon it half the length of the triangle. This conditior 

 very like that seen in the hind wing of Tetrathemis (fig. 10), only a little i 

 advance of that, has resulted in the triangle, after once attaining triangular forn: 

 becoming again four sided. Clearly, it is not five sided as Karsch thought. Hi 

 alternative exi)lanation (same reference, p. 35), rejected because the triangle woul( 

 not reach vein M^ and the supratriangle would be in contact with the discoida 

 areolets— a condition seen in the hind wing of NecorduUa and Hemicordidla (Plat 

 XLII, figs. 1 and 3) and in many Libellulina?— is the right explanation, and th 

 name Pentathemis is the result of a misinterpretation. 



