3l8 A YEAR OF COSTA RICAN NATURAL HISTORY 



the diameter of the facet; the larger facets of Gynacantha 

 gracilis must give it an advantage in this respect over its 

 three associates. While these eyes are not portions of 

 spheres but of much more complicated shapes, we may, 

 for ease of comparison, say that the eyes of Protoneura and 

 of Philogenia correspond to much larger segments of a sphere 

 than is the case in Gynacantha where the eyes are much 

 flatter on the upper surface especially. Taking the greater 

 absolute size of the last into account, it results that the 

 radius of curvature of the eye surface of Gynacantha is 

 greater, and this in turn means that the divergence of the axes 

 passing through the centers of the facets is less than in the 

 two smaller insects. The greater this divergence the less 

 distinct the vision. These considerations tend to show 

 that the eyes of the Gynacanthas are more efficient organs 

 than those of Protoneura and of Philogenia. But since other 

 dragonflles {Anax, jEshna) with eyes almost as large, both 

 absolutely and relatively, as those of Gynacantha, are found 

 in well-lighted places, and as Protoneura and Philogenia 

 live chiefly in the shade, we cannot suppose that the large 

 eyes of Gynacantha are a result of shadow-haunting habits. 

 Rather must we look on their large eyes as giving them an 

 advantage over insects with less efficient visual organs when 

 they took to living in darkness. 



These reflections on the eyes of shade-dwelling dragon- 

 flies suggest a question as to the condition of the eyes of 

 the shade-dwelling butterflies which have been mentioned 

 in various preceding chapters. A comparison of dried speci- 

 mens furnishes the following results. Callitcera menafider 

 of Turrialba and Peralta, observed only in dark woods, 

 has eyes not as large as those of Callidryas argante and 

 Pieris ilaire, creatures of about the same total size, bask- 

 ing in the brightest sunshine of Guaclmo. The eyes of 

 the Ithomiine butterflies, which we took in the shade in va- 



