332 



PHYLUM ARTHROPOD A. 



connected. Thus the retinula of one ommatidium receives a 

 single resultant impression from the light which reaches it. 

 But the adjacent ommatidia being directed to a different, though 



adjoining, region of the outer world, 

 may transmit a different impression, 

 and the stimuli from all the ommatidia 

 which make up a compound eye will 

 correspond in greater or less degree to 

 the whole of the visible outer world 

 which subtends their several optic axes. 

 The sum of the resulting images which 

 we may thus suppose to be transmitted 

 to the brain may be compared to a 

 mosaic in which the effect is given by 

 a large number of separate pieces, of 

 one size and each of uniform colour. 

 It is evident on the one hand that the 

 smaller the angle of each ommatidium 

 and the larger the number of omma- 

 tidia in an eye, the more perfectly will 

 the resulting stimulus correspond with 

 the details of surrounding objects. On 

 the other hand the loss of light by 

 absorption in the pigment of such an 

 eye is very great and increases for each 

 unit of surface with the number of 

 ommatidia it contains. 



Our knowledge of the functions of 

 the compound eyes of Arthropods has 

 been extended by the work of Exner, 

 Szczawinska, Chun, Parker and others. 

 It has been shown that in a variety of 

 Arthropods inhabiting shallow water, 

 or the land, the pigment contained in 

 the iris pigment cells and the retinulae 

 occupies very different positions in 

 accordance with the degree of illumina- 

 tion (Fig. 233). In bright light the 

 pigment invests the ommatidia in the 

 manner described above, and though a 



no: 



FIG. 233. Longitudinal sections 

 of two ommatidia of Astacus 

 fluvitlilis showing the ar- 

 rangement of the pigment as 

 influenced by light (A) and by 

 darkness (B) (after Parker) ; 1 

 cornea ; 2 nucleus of corneagen 

 cell ; 3 nucleus of vitrella ; 4 

 nucleus of iris pigment cell ; 5 

 crystalline cone ; 6 nucleus of 

 cell of tapetum ; 7 rhabdom ; 

 8 nucleus of retinal cell ; 9 

 basement membrane ; 10 reti- 

 nal nerve fibre. 



