6 E. B. Williamson 



edge where the abrupt widening takes place. But if this angle warps or 

 rolls up, as it seems to do frequently, then the angle is lost, and the passage 

 from the basal stem of the appendage to the expanded apical portion is less 

 abrupt. For example I have specimens of nervosa, which so far as I can 

 detect, agree well with both of Ris' figures (35), figure 19 representing G. 

 bifida and figure 20, G. nervosa. As in the trifida group body colors here 

 are practically uniform in the various species, or, if dififerences exist, these are 

 not well enough known to be of any service at this time. But fortunately in 

 the males of the neri'osa group, contrary to the condition found in the tri- 

 fida group where all the males have constricted abdomens, constriction of 

 the abdomen of the males has progressed in different degrees. And at the 

 same time some striking wing patterns have been developed or have survived 

 within the group, a thing which does not exist in the trifida group. By the 

 use of these two characters, form of abdomen and color of wings, a fairly 

 satisfactory classification of the material before me has been possible, but 

 there are some individual cases which present problems I cannot positively 

 solve at this time. See text under G. litoralis. 



Eyes and flight : Dr. Calvert (9) has written an interesting account of 

 Ihe relative size of the eyes in various insects, especially in dragonflies. wit'i 

 reference to the varying amount of light in the different environments in 

 which the insects spend their lives, and he points out the great relative 

 advantage of the eyes of Gynacantha over some other species. In such pre- 

 daceous insects as dragonflies keenness of sight and power of flight might 

 be expected to vary directly. In Gynacantha, for example, it is probable that 

 the increased size and efficiency of the eyes, make possible the swift and 

 often erratic flight, which would otherwise often terminate fatally. In dark 

 forests many genera of agrionines with eyes relatively very small, compared 

 with Gynacantha, live in great numbers, apparently living in such environ- 

 ments more successfully even than Gynacantha. Their eyes are amply keen 

 enough to detect food and mates, and to avoid the pitfalls spun by design- 

 ing spiders. In fact, were they gifted with all of the powers of sight and 

 flight of Gynacantha, and at the same time remained as small as they are, it is 

 possible they might not survive in the environment where they now flourish. 

 For Gynacantha escapes the ever present danger of spider webs not by 

 keenness of sight but by sheer bulk and momentum. It is unusual to find a 

 fully adult Gynacantha with no bits of spider webs attached to wings or 

 body, strands which would have proved the undoing of a Heteragrion or a 

 Palaemnema with equally unguarded flight. In fact Gynacanthas hawking 

 in the evening may often be observed colliding violently with grass or sedge 

 stems and leaves, and once a nervosa struck a motionless insect net handle 

 with a resounding whack. Crepuscular flight is probably only an adaptation 

 to the food supply and many aeshnines often practice it, though it seems 

 more confirmed in certain species of Gynacantha and Triacanthagyna than 

 in other genera such as Coryphaeschna and Aeshna. In Aeshna I have seen 

 certain sun-loving species hawking late in the evening when it was so dark 

 they were visible only against the sky or against the reflection of the sky 

 in the pool over which they were flying. At this time their flight was as 



