PHYSIOLOGY OF THE INVERTEBRATA. 359 



entering optic nerve surrounds the soft parts, and reaches as 

 far as the cornea.* 



Almost all the Insecta 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 Ptilmm, that live under the bark of trees ; the 

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

 the Anophthalmus,\ which lives in dark caves ; and the 

 Claviger, which dwells in the nests of ants. .The larvae of 

 the Biptera and HymcTioptera, and most of the apodal larvae 

 of the. Goleoptera are also blind. 



The second form of jeye. occurring in the Inseda is the simple 

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

 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 larvae of the Zepidqptera, Heuroptera, Goleoptera, and 

 some Hymenoptera and Biptera have only ocelli. Two or three 

 of these ocelli remain, but with superadded compound eyes, 

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

 which only compound eyes are present in the perfect state. 

 Simple eyes are present in the following Inseda: — Pediculidm, 

 Goccidce, Podtoridce, Nirmidce, and the larvae of the Phryganidm, 

 Hemerobidce, Myrmeleonidw, and Baphididce. 



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

 mode of vision is essentially different from that of the higher 

 Vertebrata. On this point, Professor C. Lloyd Morgan if says : 

 " Remember, their compound eyes, with mosaic vision, coarser 



* Clans' Lehrbuch der Zoohgie.. t See Darwin's Origin of iS^eeie8,p, III. 

 I Animal Life and Intelligence (1891). ' 



