MODE OF VISION BY OCELLI 255 



primary importance, as it so generally occurs, but in the young of that most 

 generalized of all pterygote insects, the cockroach (Periplaneta), Hickson states 

 that the optic nerve-fibrils which leave the periopticon pass without decussating 

 to the ommateum, and in the adult there is only a partial decussation. In 

 Nepa there is no decussation, but the anastomosis is complicated by the pres- 

 ence of looped and transverse anastomoses. 



Looking at the eye as a whole, Hickson regards all the nerve 

 structure of the eye lying between the crystalline cone-layer and the 

 true optic nerve to be analogous with the retina of other animals. 

 With Ciaccio, Berger, and others, he does not regard the layer com- 

 posed of the retinulae and rhabdoms as the equivalent of the retina 

 of vertebrates, etc. 



Origin of the facetted eye. The two kinds of eye, the simple and 

 the compound, are supposed to have been derived from a primitive 

 type, resembling the single eye (ommatidium) of the acone eye of 

 Tipula. As stated by Lang, " an increase of the elements of this 

 primitive eye led to the formation of the ocellus; an increase in 

 number of the primitive eyes, and their approximation, led to the 

 formation of the compound facet eye." This view is suggested, he 

 says, by the groups of closely contiguous single eyes of the myrio- 

 pods, considered in connection with the compound eye of Scutigera. 

 Grenacher loots upon simple (ocelli) and compound eyes as " sisters," 

 not derived from one another, but from a common parentage. 



Immature insects rarely possess compound eyes ; they are only known to 

 occur in the nymphs of Odonata and Ephemeridse, and in the larvae and pupa of 

 Corethra. 



Mode of vision by single eyes or ocelli. In their simplest condi- 

 tion, the eyes of worms and other of the lower invertebrates, prob- 

 ably only enable those animals to distinguish light from darkness. 

 The ocelli of spiders and of many insects, however, probably enable 

 them, as Lubbock remarks, to see as our eyes do. The simple lens 

 throws on the retina an image, which is perceived by the fine ter- 

 minations of the optic nerve. The ocelli of different arthropods 

 differ, however, very much in degree of complexity. 



Mliller considered that the power of vision of ocelli " is probably 

 confined to the perception of very near objects." 



"This maybe inferred," Miiller states, "partly from their existing princi- 

 pally in larvae and apterous insects, and partly from several observations which 

 I have made relative to the position of these simple eyes. In the genus Enipusa 

 the head is so prolonged over the middle inferior eye that, in the locomotion of 

 the animal, the nearest objects can only come within the range. In Locusta 

 cornuta, also, the same eye lies beneath the prolongation of the head. ... In 



