LT.(i TEX.T-BOOK OF i:.\T<>MLOGY 



the Orthoptera generally, also, the simple eyes are. in consequence of the de- 

 pressed position of the head, directed downwards towards the surface upon 

 which the insects are moving." l Lowne considers that in the ocellus of Kris- 

 talis, the great convexity of the lens must give it a very short focus, and the 

 comparatively small number of rods render the picture of even very near objects 

 quite imperfect and practically useless for purposes of vision, and that the func- 

 tion of the ocelli is " the perception of the intensity and the direction of light, 

 rather than of vision, in the ordinary acceptation of the term." 



lie'aumur, Marcel de Serres, Duges, and Forel have shown by experiment, 

 that in insects which possess both ocelli and compound eyes, the former may be 

 covered over without materially affecting the movements of the animals, while if 

 the facetted eyes are covered, they act as if in the dark (Lubbock). 



While Plateau regards the ocelli as of scarcely any use to the insect, and 

 Forel claims that wasps, humble bees, ants, etc., walk or fly almost equally well 

 without as with the aid of their ocelli, Lubbock demurs to this view, and says 

 the same experiments of Forel's might almost be quoted to prove the same with 

 reference to the compound eyes. Indeed, the writer has observed that in caves, 

 eyeless beetles apparently run about as freely and with as much purpose, as their 

 eyed relatives in the open air. 



Plateau has recently shown that caterpillars which have ocelli alone are veiy 

 short-sighted, not seeing objects at a distance beyond one or two centimetres, 

 and it has been fully proved by Plateau and others, that spiders, with their well- 

 formed ocelli, are myopic, and have little power of making out distinctly the 

 shape of the objects they see. 



On the whole, we are rather inclined to agree with Lubbock and Forel. that 

 the ocelli are useful in dark places and for near vision. They are, as Lubbock 

 states, especially developed in insects, such as ants, bees, and wasps, which live 

 partly in the open light and partly in the dark recesses of nests. Moreover, the 

 night-flying moths nearly all possess ocelli, while with one known exception 

 (Pamphila) they are wanting in butterflies. 



Finally, remarks Lubbock, "Whatever the special function of ocelli may be, 

 it seems clear that they must see in the same manner as our eyes do that is to 

 say, the image must be reversed. On the other hand, in the case of compound 

 eyes, it seems probable that the vision is direct, and the difficulty of account ing 

 for the existence in the same animal of two such different kinds of eyes is cer- 

 tainly enhanced by the fact that, as it would seem, the image given by the 

 medial eyes is reversed, while that of the lateral ones is direct" (p. 181). 



Mode of vision by facetted eyes. The complexity of the facetted 

 eyes of insects is amazing, and difficult to account for unless we 

 accept the mosaic theory of Miiller, who maintained that the dis- 

 tinctness of the image formed by such an eye will be greater in 

 proportion to the number of separate cones. His famous theory is 

 thus stated : " An image formed by several thousand separate points, 

 of which each corresponds to a distinct field of vision in the external 

 world, will resemble a piece of mosaic work, and a better idea cannot 

 be conceived of the image of external objects which will be depicted 

 on the retina of beings endoAvcd with such organs of vision, than by 

 comparing it with perfect work of that kind.'' 



1 J. Miiller, Physiology of the Senses. Trans, by Baly, copied from Lubbock, p. 176. 



