RETINAL IMAGE TO ANIMAL REACTIONS. m 



euthyscopic or direction eyes for the reason that they have to do with 

 light only so far as the direction of its source is concerned and not 

 with the possible formation of images. Animals that possess this 

 type of photoreceptor to the exclusion of other types exhibit perhaps 

 the most striking of all instances of phototropism. The larval stages 

 of many insects are excellent examples of this kind. The maggots 

 of blowflies creep with great precision away from a source of light 

 or take a balanced course between two lights of different intensities 

 or at varying angles of incidence. When one photoreceptor is cov- 

 ered, circus movements result. In short, the animal possessed of 

 direction eyes shows a phototropism that is probably purest in its 

 type in the sense that it is least complicated by extraneous factors. 



From the direction eye as a point of departure two chief types 

 of eyes have evolved, both characterized by the capacity to form 

 images. Hence they have been called eidoscopic or image eyes. On 

 the one hand, photoreceptive units, each more or less like a direction 

 eye, have become arranged as a spherical system, thus giving rise to 

 the compound or mosaic eye so common on the optic stalks of crus- 

 taceans and on the heads of insects. On the other hand, by enlarging 

 the cavity of the direction eye and providing it with a wall, and a 

 pupil or lens, or both, a camera eye has been produced such as is 

 seen in many snails and higher mollusks like the squids, devilfish, and 

 so forth, and in the vertebrates from fish to man. These two types 

 of eyes produce images that are often remarkably rich in detail, the 

 image in the compound eye being upright and that in the camera eye 

 inverted. 



When the light reactions of animals that possess compound eyes, 

 like the insects, for instance, are studied, they are found to be by no 

 means simple tropic responses. The mourning-cloak butterfly, when 

 liberated in a room illuminated by a single, bright light, flies toward 

 the light and behaves in a way to justify the designation positively 

 phototropic. When, however, it is watched in the open field, its 

 reactions are very different. After flying about in the sunlight for 

 a while, these butterflies come to rest definitely oriented to the direc- 

 tion of the sun's rays, but instead of being headed toward the sun, as 

 a positive animal should be, they head away from the sun in the 



