3l6 A YEAR OF COSTA RICAN NATURAL HISTORY 



it was. During this hour of stalking one gracilis, however, 

 I caught two trifida in the same spot. 



Gynacantha gracilis is similiar to G. memhranalis mentioned 

 in the preceding chapter but a little larger (three and one- 

 half inches long, wing-spread four and three-quarters inches). 

 The reddish-brown spot at the base of the hind wings is 

 much smaller, being adjacent to the front margin only in- 

 stead of extending halfway across the wing, as it does in 

 memhranalis. Its eyes, in life, were blue above ranging 

 through green and yellow to reddish-brown below, instead 

 of the metallic green of its congener. Previously, gracilis 

 had not been reported north of Surinam. Gynacantha trifida 

 is a smaller species (two and three-eighths inches long, 

 wing-spread three and one-quarter inches) with uncolored 

 wings, its eyes brilliant green, darker above. 



The Gynacanthas have the absolutely largest eyes of all 

 dragonflies, possibly of all insects. They fly in such dark 

 places as these virgin forests or, if in the open, at dusk and 

 they have occasionally been known to fly to artificial lights 

 at night. At Poyntelle, Pennsylvania, in June, 1904, I 

 found two species of Cordiilegaster along the same brook; 

 one species with smaller eyes (C diastatops) was found only 

 where the brook flowed through open pasture; the other 

 (C maculatus) with larger eyes, occurred only in the 

 forest courses of the brook where there was much less 

 light. All these observations suggest an inverse correla- 

 tion of size of eyes and quantity of light in which they are 

 used, at least in the case of swift-flying species. But there 

 are other species occurring in equally dark places in these 

 Banana River forests with eyes no larger than those of 

 their light-dwelling brethren. Such are Protoneura re- 

 missa and Philogenia carrillica, which I caught this day 

 in the forest with the Gynacanthas. These two species, 

 however, seem to travel only short distances — a few 



