PHYSIOLOGY OF THE INVERTEBRATA. 339 



entering optic nerve surrounds the soft parts, and reaches as 

 far as the cornea.* 



Almost all the Inseda have a pair of these compound eyes; 

 but they are sometimes replaced by simple eyes, and at other 

 times both kinds are present. In a few cases there are 

 neither compound nor simple eyes ; among these are certain 

 species of Pt'diuvi, that live under the bark of trees ; the 

 NyctcHhia, which is parasitic on the skin of certain animals ; 

 the Anojihthalmus,^ which lives in dark caves ; and the 

 Clavigcr, which dwells in the nests of ants. The larvi© of 

 the Diptcra and Hymcnoptera, and most of the apodal larvie 

 of the Golcoptcra are also blind. 



The second form of .eye occurring in the InsMa is the simple 

 eye, ocellus, or stemma. It contains the following parts : — 

 sclerotica, cornea, lens, vitreous humour, and choroid ; and it 

 is of globular form. " But the lens appears to be always a 

 mere thickening of the cuticle, which constitutes the cornea, 

 and the so-called vitreous humour is partially or wholly made 

 up of crystalline cones, analogous to those which are found in 

 the compound eye. In this respect the ocellus of the insect 

 resembles the simple eye in the Arachnida and Crustacea." 



The larvis of the Liyidoptera, Neuroptera, ColcojJtera, and 

 some Hymcnaptcra and Diptera have only ocelli. Two or three 

 of these ocelli remain, but with superadded compound eyes, 

 in the majority of the winged orders except the Colcoptcra, in 

 which only compound eyes are present in the perfect state. 

 Simple eyes are present in the following Inseda: — Pcdiculida^, 

 Coccidcv, FodiirkUv, Nirmidm, and the larvee of the Phryganidw^ 

 Hrmcrohida', Mynneleonida:, and Ilaphidida\ 



The sense of sight must be keen in the Inseda, but their 

 mode of vision is essentially different from that of the higher 

 Vertchrata. On this point. Professor C. Lloyd Morgan :j: says : 

 " Remember their compound eyes, with mosaic vision, coarser 



* CIaxxs' Lehrbuchder Zoolorjie. t See Darwin's Or?Y7i;iq/";S^cctes, p. 11 1. 

 { Animal Life and Intelligence (1891). 



