64 BARKER : 



remember that the woi'ker ant of Rciton is said to have at 

 most a single facet on each side of the head. 



In Folsom's Entomology,* a text-book accepted by uni- 

 versities and colleges of the United States, we find the subject 

 amply treated, the structure of the compound eye being 

 described as follows : — 



"As regards delicacy and intricacy of structure, the compound eye 

 of an insect is scarcely if at all inferior to the eye of a vertebrate. In 

 radial section a compound eye appears as an aggregation of similar 

 elongate elements or ommatidia, each of which ends externally in a 

 facet. The following structures compose or are concerned with each 

 ommatidium : — 



1. Cornea 



2. Crystalline lens or cone 



3. Rhabdom and retinula 



4. Pigment (iris and retinal) 



5. Fenestrate membrane 



6. Fibers of the optic nerve 



7. Tracheae 



"The cone cells may contain a clear fluid, or else as in most insects, 

 are solid transparent copes. 



"The rhabdom is a transparent chitinous rod or a group of rods 

 (rhabdomeres) situated in the long axis of the ommatidium and sur- 

 rounded by greatly elongated cells which constitute the retinula. 



"The nerve fibrilhe, which are ultimate branches of the optic 

 nerve, pass into the retinal cells — the end organs of visioil. 



" Physiology. — After nuich experimentation and discussion upon the 

 physiology of the compound eye — the subject of the monumental works 

 of (Trenacher and Exuer — IMiiller's ' mosaic ' theory is still generally 

 accepted, though it was proposed early in the last century. It is thought 

 that an image is formed by thousands of separate points of light, each of 

 which corresponds to a distinct field of vision in the external world. Each 

 onunatidium is adapted to transmit light along its axis only, as oblique 

 rays are lost by absorption in the black pigment which surrounds the crys- 

 talline cone and the axial rhabdom. Along the rhabdom then light can 

 reach and affect the terminations of the optic nerve. Each ommatidium 

 does not itself form a picture ; it simply preserves the intensity and 

 color of the light from one particular portion of the field of vision, and 



* Entomology, with Special Reference to its Biological and Econo- 

 mic Aspects, by Justus Watson Folsom, Sc. D. (Harvard). 1906. 



