NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 69 



gans that any theory which would derive an ocular sensillum from 

 that of a sensory hair, as that proposed by Patten (1890), is too far- 

 fetched to be convincing. The number of sense cells in a single eye 

 varies from two cells to many thousand in the different types of 

 insect eyes. All the sense cells of one eye constitute what is gener- 

 ally regarded as the retina of the arthropod eye. The optic lobes of 

 the brain (fig. 4, OpL), upon which the compound eyes rest, are 

 parts of the central nervous system and do not contain the precipient 

 elements of the eye, as in vertebrates. Each retina cell is traversed 

 by fibrils which are continuous proximally with the ocular nerve, 

 and which end distally in a definite marginal part of the cell, which 

 part becomes the sensitive area of the cell. This area commonly has 

 the appearance of being vertically striated or formed of a fringe of 

 minute thread-like rods. It is known as the rhabdomere because it 

 often combines with the corresponding borders of neighboring cells 

 to form a crystalline visual rod called a rhahdom. Retina cells thus 

 grouped about a rhabdom constitute a composite retinal element 

 known as a retinula. The retinal cells rest upon the basement mem- 

 brane, which is perforated by the fibers of the optic nerves. 



The same qviestion arises regarding the innervation of the eyes 

 as with the other sense organs ; i. e., whether the ocular nerve fibers 

 grow outward and penetrate the retinal cells, or whether they origi- 

 nate in the retinal cells and grow inward to the optic lobes. Some 

 writers are positive in asserting that the second process of growth 

 takes place with the compound eyes, that the growth of the nerves 

 can be followed in the development of the eye from the retinal cells 

 inward to the ganglia of the optic lobes. If this is true, then the 

 hypodermal retinal cells are the cytons of the peripheral ocular nerve 

 fibers. 



THE COMPOUND EYES 



The well-known lateral compound eyes of Crustacea and Insecta 

 consist, in their typical form, of groups of ocular sensilla or onima- 

 tidia. The number of ommatidia in a single eye in different insects 

 is usually between a few hundred and 20,000, but in some the num- 

 ber is estimated to be as high as 30,000. In aberrent or degenerate 

 types there are but a few ommatidia or even only one ommatidium. 



The outer surface of each ommatidium consists of the cuticular 

 cornea (fig. 32 A, B, Cor), which is commonly thickened to form a 

 lens. Beneath the cornea of some of the more primitive insects and 

 in crustaceans are two corneagenous cells (A, CorCl), but in the 

 mature eye of most insects (B) these cells are usually withdrawn 



