THE COMPOUND EYE 



career he contributed lavishly to many branches of biology but perhaps the 

 conception for which he is best remembered is the law of specific yierve energies 

 which lays down that each organ, however stimulated, gives rise to its own 

 characteristic sensation. ^^ His enunciation of the Mosaic Theory to explain the 

 optical properties of the compound eye has stood the test of time, and was the 

 first scientific explanation advanced on this subject ; Fig. 156 is a characteristic 

 illustration from his book. His classical textbook on human physiology - 

 crystallized the knowledge of his day in a vast compendium which stimulated 

 work in every field for more than one generation. 



The compound eye, an organ peculiar to Arthropods, has evolved 

 along different lines from the ocellus. In the former, instead of being 

 independent of each other, the sensory elements are structurally and 

 fnnctionaUy associated in groups. For this purpose complexity has 

 been attained by the division of the indi^•idual sensory cells of a simple 



155 



Fig. 134. — The Compound Eye. 



Diagram of a compound eye of an insect with a sector excised. 

 a, corneal facet ; h, crj'stalline cone ; c, surface epithelium ; d, matrix 

 cells of cornea ; e, iris pigment cell ; /, cell of retinule ; g, retinal pigment 

 cell ; h, rhabdome ; ;', fenestrated basement membrane ; _;, nerves from 

 retinular cells ; k, lamina ganglionaris ; /, outer chiasma. 



eye to form a coordinated colony, a process first shown to occur in the 

 development of the stalkefl eyes of the shrimp, Crangon, by Kingsley 

 (1886) and confirmed by others in many different species. Moreover, 

 optical imagery has been attained not by the single large lens charac- 

 teristic of the ocellus (or of the vertebrate eye) which by attaining an 

 adjusting mechanism reached its highest development in Cephalopods, 

 but by ensheathing each individual group with pigment, thus convert- 

 ing the eye into a series of blackened tubes so that the multiplicity of 

 images increases the acuity of vision by a mosaic effect. In this 

 arrangement each separate element is called an ommatidium {ofifnx, 



' Zur veryleicheiulen Physiologie <hr GesiclitNtilnnes, Leipzig, 1826. 

 * Handbuch der Physiologic der Menschen, 18.34-40, translated into English in 

 Baly's Elenieitts of Physiology, London, 1838-42. 



