DIVIDED EYES OF CERTAIN INSECTS 477 



In an eye like that of Anax where the large elements in one 

 part of the eye pass gradually over into smaller elements in 

 another part of the eye, both kinds of elements seem to develop 

 from the same center — the smaller elements being the last 

 formed. 



As has been shown in the 2 divided eyes studied {Callibcetts 

 and Synvpetrum) the large ommatidial elements begin develop- 

 ment after the pigmented lateral eye is complete. In this case 

 the optic ganglion which has already been formed for the pig- 

 mented eye appears to bud or enlarge to receive the nerve fibers 

 of the new eye elements. To support statements already made 

 and for further reference the accompanying table of measure- 

 ments of the eye elements of the different eyes studied is given. 



SIGNIFICANCE OF THE DIVIDED EYE CONDITION. 



Exner, 1891, has shown that an eye with a structure like that 

 of the turban eye of Callibcetis (adult) is capable of forming an 

 image of superposition upon the proximal retinulas as well as 

 an image of apposition upon the distal retinulae. By means of 

 this repeated formation of images upon the retina, the eye with 

 the superposition image is enabled to see, even if somewhat 

 indistinctly, in dim light where the small facetted deeply pig- 

 mented eye could not see at all. Zimmer has shown that this 

 is of advantage to the mayflies in mating, since the males seek 

 the females on the wing in the twilight. 



In the case of all the other large facetted eyes discussed in 

 this paper, an image of superposition would be impossible, since 

 the retinulae in every case lie rather close together and are not 

 divided into proximal and distal parts. In everv eye however, 

 the increase in the size of the dioptric apparatus accompanies 

 the decrease in pigmentation. Both of these conditions favor 

 the admission of more light. This would admit of a better appo- 

 sition image being formed in dim light. The small dioptric ap- 

 paratus and dense pigmentation accompany each other and both 

 favor the formation of a distinct apposition image in extremely 

 bright light. Whatever the special adaptation then, the divided 

 condition of the eyes may be regarded as an adaptation of dif- 

 ferent parts of the eye to suit different intensities of light. 



