210 



COLLEGE ZOOLOGY 



the ommatidia are represented by A-E, and 

 the fibers from the optic nerve by A'-E'. 

 The rays of Hght from any point, A, B, or C, 

 will all encounter the dark pigment cells sur- 

 rounding the ommatidia and be absorbed, 

 except the ray which passes directly through 

 the center of the cornea as D or E; this ray 

 will penetrate to the retinulae and thence to 

 the fibers of the optic nerve. Thus the 

 retinula (plural, retinulae) of one om- 

 matidium receives a single resultant impres- 

 sion from the light which reaches it. But the 

 adjacent ommatidia, being directed to a dif- 

 ferent though adjoining region of the outer 

 world, may transmit a different impression, 

 and the stimuli from all of the ommatidia 



which make up a compound eye will corres- 

 pond in greater or less degree to the whole 

 of the visible outer world which subtends 

 their several optic axes. This means that 

 each visual unit responds to a fragment of 

 the total field and that these fragmentary 

 images are fitted together into a single gen- 

 eral picture. However, the image formed by 

 this type of eye is never very good; it func- 

 tions best at short distances— the arthropod 

 is always near-sighted. 



When the pigment surrounds the om- 

 matidia (Fig. 118, A), vision is as described 

 above; but it has been found that in weak 

 light the pigment migrates partly toward the 

 outer and partly toward the basal end of the 



Facet 



Cornea (lens) 

 Corneagen ce 



Distal retina 

 pigment cells 



•Crystalline cone 



Rhabdom 



Retinula 



Basal retinal 

 pigment cells 



Rhabdom- 



Nerve fibers 



B 



Figure 118. Longitudinal sections of two ommatidia of the crayfish. A, one ommatidium in 

 the light; pigment extended so that it completely surrounds the ommatidium, isolating it from 

 its neighbors. B, ommatidium in the dark; pigment in the basal pigment cells is withdrawn. 

 (After Bernhards. from Parker.) 



