214 PRINCIPLES AND CONCLUSIONS. 



object in the form of a mosaic, due to the fact that each facet receives a 

 different part of the light rays coming from the object. It follows necessarily, 

 as Muller pointed out, that, since the clearness of vision, the localization 

 of the light rays, is rendered possible only by the combination of the separate 

 action of each facet, it is the number of facets that will determine the 

 degree of definition. The smaller the facet, the longer the crystalline, the 

 fewer the rays the retinule will receive and the smaller or more localized will 

 be the external object from which the rays proceed. On the contrary, the 

 larger the facet and the shorter the crystalline, the more light the retinule 

 will receive and it will see in a manner more intense and diffuse, that is, less 

 clearly. Thus, many small facets will diminish the intensity of the light, 

 but will increase the clearness of vision. When in addition the entire eye 

 is not flat but strongly convex, it will receive light from more diverse 

 points, which increases the common field of vision. If this be true, it ought 

 to be possible to prove by observation that insects with many small facets 

 and with the compound eyes strongly convex have the best vision. 



Forel brought forward a number of observations to support this view, 

 the most significant, perhaps, being with reference to eyes of male and female 

 ants. The eyes of the former are more convex and may contain twice 

 as many facets, since they require greater vision to discern and follow 

 the females in flight. He also confirmed the opinion of many investigators 

 that the loss of the ocelli was without effect, but that this was not the case 

 with the compound eyes. When the latter were varnished in the case of 

 flies, these no longer flew if placed on the ground and once in the air they 

 flew about aimlessly. Wasps and bumble-bees similarly treated behaved 

 in like manner, flying even more rarely and finding their way on the ground 

 more slowly. They searched for cracks in which to hide, as though it 

 were night. 



The following summary was given of the existing knowledge of vision 

 in insects. 



1. Insects guide themselves almost wholly in flight and partly on the ground by 



means of their compound eyes. The antennae and the sensory organs of 

 the mouth can not serve for direction in the air, their extirpation in no wise 

 diminishing the ability to guide themselves on the wing. 



2. The mosaic theory of vision proposed by Muller is alone sound. Each retinule 



of the compound eye does not receive an image, but merely a pencil of light 

 rays more or less distinct from that of its neighbors. 



3. The greater the number of facets and the length of the crystallines, the more dis- 



tinct the sight, as well as further. 



4. Insects perceive the movements of objects particularly well, that is, the displace- 



ments of the visual image relative to the compound eye. They thus see better 

 in flight than at repose, for the image of immobile objects is then displaced 

 with respect to the eye. Perception of the mobility of objects decreases as the 

 distance becomes greater. 



5. Insects distinguish the contours and form of an object only in a manner more or 



less indistinct, as much more indistinct as the number of facets is less, the crys- 

 tallines shorter, the object more distant or smaller. Those insects that have 

 large eyes with several thousand facets see quite distinctly. 



6. In flight insects recognize the direction and distance of objects very clearly by 



means of their compound eyes; at least this is true for short distances. Even 

 when in repose they can also recognize the distance of immobile objects. 



