1911] Muttkowski, Studies in Tetragoneuria. 93 



the increase of the latter we find the postnodal-radial space (Wil- 

 liamson) lenghtening and the substigmal series of two to four 

 veins retreating from the third to the fifth postnodal. 



The origin of the cubito-anal crossvein — coincident with A 4 

 or distal thereto — seemed a constant factor in Wisconsin material; 

 the tables showed otherwise for other regions. 



The crossing of the triangle of the forewings may be set down 

 as a generic feature, for it is constant, with rare exceptions, in 

 all specimens seen by me. Only mspinigcra and canis are 

 the triangles of the hind wings found crossed, but this is the 

 case in only 50% of the specimens so that no specific value can 

 be attached to this character. 



Structure. — The form of the abdomen is an uncertain feature. 

 Here also a progression from parallel to broadly spindle-shaped 

 abdomens takes place, with stella and spinigera at the two 

 extremes. As a rule, the constriction of segment three is fairly 

 well pronounced and segments nine and ten are markedly nar- 

 rower than the preceding ones, and the species so labeled (as 

 spindle-shaped) are therefore easily recognizable. 



The form of the male appendages is the chief character of 

 distinction, though subject to some variation, at least in the 

 cynosura series. The female appendages show little variation in 

 a species and this character has formed the chief distinction 

 between females of the various species. If taken in conjunction 

 with other characters, I believe, a species cannot be easily 

 mistaken. 



Colors. — The general color is subject to little variation. All 

 have practically only one type of color pattern, with slight indi- 

 vidual variation. The chief feature is the T-spot, which appar- 

 ently is constant in most species by its presence or absence, though 

 extremely variable in cynosura. 



The question of wing coloration is taken up more fully under 

 cynosura and semiaquea. 



Distribution. — The genus Tetragoneuria covers a greater por- 

 tion of North America. Few specimens have been collected in 

 the west, though the species undoubtedly occurs in the western 

 states, as proven by material obtained by Dr. Osborn and others 

 in Washington and Britsh Columbia. 



