924 FRESH-WATER BIOLOGY 



1 8 (17) Sexes strikingly unlike in color; a bifid process arising from the 



apical margin of the loth abdominal segment in the male 

 and the superior abdominal appendages strongly directed 

 downward and inward. . . . 19 



19 (20) Males chiefly green and black, with normal rhomboidal stigma; 



females with the orange of the abdomen covering something 

 less than the three basal segments (becoming wholly densely 

 pruinose with age). . . . ... Ischnura. 



20 (19) Males yellow or orange, with ovoid stigma which does not reach the 



costal vein; females with the four basal segments of the 

 abdomen yellow or orange Anomalagrion. 



21 (i) Fore and hind wings dissimilar, the latter broader at the base 



(dragonflies proper) . . . Suborder Anisoptera 22 



22 (49) Triangle (Fig. 1388) about equally distant from arculus in fore and 



hind wing; stigma with a brace vein at its inner end (except 

 in Cordulegaster) 23 



23 (24) Stigma unbraced Cordulegaster. 



24 (23) Stigma braced at its inner end against an inclined cross vein in the 



space below it (Fig. 1362) 25 



25 (36) Eyes widely separated on the top of the head 26 



26 (27) Basal subcostal cross vein (Fig. 1388, b) present; a hnear or spatulate, 



median, sternal process on the first abdominal segment; 

 legs very short, the hind femora hardly reaching the apex 

 of the first abdominal segment Progomphus. 



27 (26) Basal subcostal cross vein usually wanting; no median sternal 



process on the first abdominal segment; legs longer, the 

 hind femora reacliing or surpassing the middle of the second 

 abdominal segment. ... 28 



28 (31) Hind wings with a distinct anal loop (Fig. 1388, a) consisting of several 



cells 29 



29 (30) Anal loop normally consisting of three cells; first and fifth antenodal 



cross veins matched in position and hypertrophied; stigma 

 broad with both sides convex; triangles not traversed by 

 cross veins. . . ■ .... Ophiogomphus. 



30 (29) Analloop consisting normally of four cells; first and seventh antenodal 



cross veins matched in position and hypertrophied; stigma 

 long and narrow with parallel sides; each triangle divided 

 by a cross vein . . Hagenius. 



31 (28) Hind wings with no distinct anal loop, or with one consisting of a 



single cell. 32 



32 (33) Triangle of the fore wing one-third shorter than that of the hind 



wing; generally a single cell between the bases of veins A2 

 and As Lanthus. 



33 (32) Triangle of the fore wing less than one-fourth shorter than that of the 



hind wing; generally, two or more cells between A2 and A, 

 at their origin 34 



34 (35) Hind femora naked, or with numerous short spines. . . Gomphus. 



35 (34) Hind femora with five to seven long, strong spines. . Dromogomphus- 



