150 



COENEA— CRYSTALLINE CONES. 



The wonderful complexity is well shown in the pre- 

 ceding figure, which represents a section through the 

 eye of a fly, after Hickson.* 



In illustration of the finer structure, I may take the 

 eye of the bee (Apis) (Fig. 102), as described and 

 figured by Grenacher in his beautiful 

 work.t Fig. 102, the general accuracy 

 of which has been confirmed recently 

 by Dr. Hickson, represents two of the 

 elements of the faceted eye. 



The structure of the eyes varies 

 considerably in different groups. They 

 may be said to consist of the following 

 principal parts : — 



1. The cornea {Lf, Fig. 102). 



2. The crystalline cones {Klc), of 

 which there is one immediately behind 

 each facet. The development of the 

 crystalline cone has been carefully 

 studied by Claparede. It consists of 

 from four to sixteen original, but com- 

 pletely combined segments, secreted 

 by cells which lie immediately behind 

 each facet, but of which, when the eye 

 is completely developed, only the 



nuclei, known as Semper's nuclei {n), finally remain. 



3. Next comes the retinula (rZ), which stands in 

 more or less intimate connection with the pointed inner 

 end of the crystalline cone. It is generally composed 

 of seven, but sometimes of as few as four, or as many 



Jim. 



Fig. 102.— Two sepa- 

 rate elements of the 

 faceted eye of a bee 

 (after Grenacher). 

 Lf, Cornea ; n, nu- 

 cleus of Semper ; 

 Kk, crystalline cone ; 

 Pg, Pg\ pigment 

 cells ; Jil, retinula ; 

 Em, rhabdom. 



* " The Eye and Optic Tract of Insects," Quarterly Journal of 

 Microscopical Science, 1885. 

 t " Untersucliungen iiber das Sehorgan der Arthropoden." 1879. 



