AQUATIC INSECTS IN THE ADIRONDACKS 445 



12 Two cells between veius Ai and A2 at their origin ; leugth less than 



45 mm b revis 



A single cell between veius Ai and A2 at their origin ; length more than 

 45 mm 13 



13 Midlateral thoracic stripe complete : length about 46 mm ....... adelphus 



Midlateral thoracic stripe interrupted, not extend ing above the spiracle : 



length about 54 mm vastus 



14 Ninth abdominal segment little longer than the eighth, ground color 



brown '. plagiatus 



Ninth abdominal segment much longer than the eighth; ground color 

 black.. spiniceps 



SYNOPTIC ARRANGEMENT OF THE GENUS, IMAGOS AND NYMPHS 



Subgenus gomphus 



Imago. Generally with two cells between veins Ai and Ag at their 

 origin, when with but one, that one generally longer (in the axis of the 

 wing) than wide, and never so thickened in its bordering veins as to con- 

 stitute a distinct anal loop; hind femora similar in the two sexes; pos- 

 terior genital hamule in the male generally nearly vertical in direction, 

 at least not directed anteriorly; eighth abdominal segment generally 

 squarely cut, rarely a very httle longer on the dorsal than on the ventral 

 side. 



Nymph. Abdomen wider than the head; lateral spines on abdominal 

 segments 6-9 ; an impressed middorsal line on abdomen more or less 

 evident, often appearing to divide the segments longitudinally, present 

 even on the bases of segments which may bear dorsal hooks apically ; 

 median lobe of labium straight on anterior border, or very slightly con- 

 vex, and not bearing a median tooth. 



A synthetic group, offering evident points of departure for the three 

 subgenera which follow it. 



KEY 



1) Ninth abdominal segment a little shorter than the eighth; two cells be- 



tween veins Ai and A2 at their origin; male with the fork of the in- 

 ferior abdominal appendage not extending laterally beyond the superiors ; 

 female with a low carina on the vertex, at whose extremities arise black 

 thornlike spines; small species. 



a) Iviago Face yellow : Xymph Length when full grown 24 mm ; 

 lateral spine on the sixth abdominal segment less than half the length 

 of the one on the seventh segment G. abbreviatus 



aa) Imago Face transversely lineate with black. Nymph Length when 

 full grown 26 mm ; lateral spine on the sixth abominal segment more than 

 half as long as the one on the seventh segment G. brevis 



2) Ninth abdominal segment as long as the eighth, or often a very little 



longer; inferior abdominal appendage of the male widely forked, its 



