Eyes 



39 



the tiny nerve-fibres which pass through a finely- 

 perforated membrane situated at the base of the rods 

 (fig. 29 B). They are believed to form part of the 

 retina of the eye — that portion which receives the 

 impressions conveyed through the transparent facets, 

 cones and rods. 



A crystalline cone of the cockroach's eye, though 

 a simple transparent body, is known to be formed 

 out of four primitive " cone-cells." In Earwigs, 

 Bugs, Craneflies, Midges and some Beetles, these four 

 cells retain their original condition ; no crystalline 

 cone is formed, and the eyes of such insects are 

 known as acone eyes. In the eyes of most two-winged 

 Flies, the four cells are for the most part filled with 

 a transparent fluid, but do not become fused together 

 to form a true cone ; such eyes are called pseudocone. 

 Other insects have eucone eyes, in which, as in the 

 cockroach, the cones are perfectly developed though 

 they vary much in form (2, 2o). 



It has been mentioned that the optic tract of the 

 brain of the cockroach consists of two swellings, 

 an inner (opticori) and an outer (epiopticon) on either 

 side. Many naturalists consider that these swellings 

 belong to the retina of the eye rather than to the 

 brain. Absent in the cockroach, but present in the 

 vast majority of insects, between the epiopticon with 

 which it is connected by nerve fibres crossing each 

 other (decussating), and the membrane at the base of 

 the rods, is a third swelling. This is called the 

 periopticon, and is, without doubt, a retinal structure. 

 It consists of a series of rod-like pigmented nerve- 

 cells, surrounded by small " chaplet cells " with 

 large nuclei, and connected with the exceedingly fine 

 nerve-fibres which pass outward through the per- 

 forated membrane to the retinuls surrounding the 

 eye-rods (fig. 30) (20). 



