ARTHROPODA 



313 



embryonic stages of centipedes and certain insects, an appendage 

 which co-exists with the eye. 



In most compound eyes, the pigment, both in retinular and in pig- 

 ment cells, flows to and fro, being in dim light retracted towards the 

 inner or outer ends of the cells so as to leave the sides of the om- 

 matidia exposed, and in bright light extending so as to separate the 

 ommatidia completely. In many diurnal insects it is permanently in 

 the latter position. Vision takes place in two ways according to the 

 situation of the pigment. When the latter is extended, in each omma- 



B 



n.ji. 



f>opUga. 



-w. 



opt.n. 



Fig. 212. The eye of Astaciis. A, The left eye. B, A portion of the cornea 

 removed, to show the facets. C, A longitudinal section of the eye. w. muscles 

 which move the eye; n.fi. nerve fibres; oynm. ommatidia; opt.ga. optic 

 ganglion; opt.n. optic nerve. 



tidium there falls on the retinula a narrow pencil of almost parallel 

 rays. There is then mosaic vision, an apposition image, composed of as 

 many points of light as there are ommatidia, being formed on the 

 whole retinal layer. When the pigment is retracted, each ommatidium 

 throws a complete image of the greater part of the field of vision, and 

 the images together form a superposition image, falling in such a way 

 that their corresponding parts are superposed. Superposition images 

 are less sharp than apposition images, but are formed with less loss of 

 light. Compound eyes are especially adapted for perceiving the move- 

 ments of objects, owing to the way in which such movements affect 

 a series of ommatidia in succession. 



