250 TEXT-BOOK OF ENTOMOLOGY 



to form the vitreous body (vitrella) and retina. Each retinal cell 

 (re) is connected with a fibre from the optic nerve, contains pigment, 

 and ends in a rod directed outwards towards the lens. The cells at 

 the end of the pit or depression are, next to the lens, without pig- 

 ment, and, growing in between the retina and the lens, fill it up, and 

 thus form a sort of vitreous body. 



The ocellus appears to be a direct heirloom from the eyes of worms, while the 

 many-facetted compound eye of the crustaceans and of insects is peculiar to 

 these classes. The compound eye of the myriopod Scutigera differs structurally 

 in many respects from the compound eye of insects, and that of Limulus still 

 more so. 



It should be observed that in the young nymph of Ephemera, as well as in the 

 semi-pupa of Bombus, each of the three ocelli are situated on separate sclerites. 

 In Bombus the anterior ocellus has a double shape, being broad, transversely 

 ovate, and not round like the two others, as if resulting from the fusion of what 

 were originally two distinct ocelli. 



The ocelli are not infrequently wanting, as in adult Dermaptera, in the Locus- 

 tidse, and in certain Hemiptera (Hydrocora). In Lepidoptera there are but two 

 ocelli ; in geometrid moths they are often atrophied, and they are absent in 

 butterflies (except Pamphila). 



The compound or facetted eye (ommateum). The facetted arthropod 

 eye is wonderfully complex and most delicately organized, being far 

 more so than that of vertebrates or molluscs. The simplest or most 

 primitive facetted eye appears to be that of Lepisma. As stated by 

 Watase, the compound eye of arthropods is morphologically "a 

 collection of ectodermic pits whose outer open ends face towards the 

 sources of light, and whose inner ends are connected with the central 

 nervous system by the optic nerve fibres." 



The facetted eye is composed of numerous simple eyes called 

 ommatidia, each of Avhich is complicated in structure. The ele- 

 ments which make up an ommatidium are the following : (1) The 

 facet or cornea, which is a specialized portion of the cuticula ; and 

 (2), the crystalline lens or cone ; (3), the nerve-ending or rethmla, 

 which is formed out of the retinula cells and the rhabdom or rod 

 lying in its axis ; and (4) of the pigment enclosing the lens and rod ; 

 the last three elements are derived from the hypodermis. The 

 single eyes are separated from each other by pigment cells. 



The facet or cornea. - - This is biconvex, clear, transparent, usually 

 hexagonal in outline, and refracts the light. The corneal lenses are 

 cast in moulting. 



The corneal lenses are circular in most cases where they are very convex, as 

 in Lathridius and Batocera. The hexagonal ones are very irregular. When 

 they are very convex the eye has a granular appearance, but when not greater 

 than the convexity of the eye itself, the eye appears perfectly smooth (Bolbo- 



