THE BANANA RIVER COUNTRY 317 



yards — and to float through the air instead of moving 

 rapidly. 



The Protoneiira and the Philogenia are much smaller 

 insects, being one and one-half and two and one-quarter 

 inches long respectively, of slighter build and with narrower, 

 feebler wings. Their right and left eyes are separated from 

 each other on the upper surface of the head by a distance 

 equal to, or greater than, the width of either eye. The 

 right and left eyes of the Gynacanthas meet, for a relatively 

 long distance on the top of the head, so that they cover a 

 larger part of the surface of the head than is the case in the 

 two smaller species. The heights (in millimeters) of the 

 eyes of these four dragonflies are: Protoneura remissa i.ii, 

 Philogenia carrillica 2,88, Gynacantha trifida 5.76, G. gracilis 

 7.44. It is well known that the surfaces of the eyes of in- 

 sects are made up of a number of facets, or lenses, usually 

 hexagonal in shape, fitting against each other. The diam- 

 eters of these facets are usually greater on the upper surface 

 of a dragonfly's eye than on the lower surface of the same 

 eye, and the size is different in diff"erent species, Protoneura 

 having the lenses smaller than in any of the other three. 

 The distinctness of vision of an insect's eye is proportional 

 to the number of these surface lenses whose inner nerve 

 fibers are stimulated by light. Other things being equal, 

 therefore, the greater the number of facets the better the 

 eye for distinctness. The smaller the diameter of the lenses, 

 of course, the greater is the number in a given area. In 

 spite of the smaller diameter of Protoneura's lenses, a rough 

 calculation shows that its eyes contain only one-half, or less 

 than one-half, of the number of facets of the eyes of Gyna- 

 cantha trifida^ and between three-sevenths to one-third as 

 many facets as the eyes of G. gracilis. On the other hand, 

 the intensity of the light stimulation which the inner nerve 

 fibers (corresponding to each facet) receive increases with 



