402 MR. B. THOMPSON LOWNE ON THE COMPOUND VISION 



absent when the cornea is continuous and without facets. I suspect it is shed after the 

 development of the cuticular layers beneath. 



I have named this type of cornea the simple non-facetted cornea. It is apparently 

 the permanent condition in Gammarus and Branchippus amongst the Crustacea. 



The second type of cornea (fig. 30) is seen in the functionally active eye of the larval 

 Cockroach. It is divided into lenticular facets. I shall speak of this type as the 

 facetted cornea. 



The facetted cornea exhibits two distinct layers — an outer nuclear layer and an inner 

 laminated layer. 



This form differs from the continuous cornea in the persistence and further develop- 

 ment of the cellular layer. The large flat nuclei of the cells enlarge at the expense of 

 the cell-substance until they form the whole surface of the cornea, one nucleus forming 

 each facet. This layer, when fully developed, is very hard and brittle. It is no longer 

 stained by eosin, as in its semideveloped condition, but assumes a yellow colour in 

 specimens which have been treated with chromic acid or its salts. 



The structure of the facetted cornea is worthy of study in dried insects. I have a 

 beautiful section from the eye of an African Carpenter Bee (fig. 30). The lenticular 

 nuclei are firmly cemented together, and exhibit no trace of minute structure. The 

 laminated portion beneath these consists of from fifty to sixty layers. These follow the 

 contour of the inner surface of the lenticular facets. 



Fine lines are also seen running perpendicularly to the lamina?. These correspond to 

 the divisions between the facets. There are also distinct lacunse between the laminse, 

 with minute teeth on each side of the lacuna ; indicating that the laminated structure is 

 probably due to the existence of finely serrated fibres interwoven with each other. 



The third modification of the cornea in the Cockroach is only seen in the final stage 

 of development : I shall speak of it as the histoid cornea * (figs. 3 & 10). 



I have not met with any description of the kistoid cornea, although it is preeminently 

 characteristic of the most highly differentiated forms of compound eye. It consists of a 

 chitinous (?) cuticular membrane folded on itself so as to resemble a piece of honeycomb, 

 the openings of the hexagonal cells being turned inwards. The closed ends of the cells 

 are segments of spheres : these form the surface of the cornea. 



The hexagonal walls of the cells are often highly chitinized and deeply pigmented ; 

 the curved ends which form the corneal facets are thin, membranous, and very trans- 

 parent. The material of which they are formed is usually soft, flexible, and elastic, and 

 the two surfaces of the membrane are parallel to each other. 



The cavity of the corneal cell is occupied by the oil-like lens which I have already 

 described. 



This lens is certainly developed from the substance of the cornea. I was fortunate 

 enough to obtain sections of the eye of a Blow-fly which throw light upon this point. 

 Fig. 15 shows three segments of the dioptron of an immature imago, which had not long 

 before escaped from the pupa. 



* KtVrr,, a capsule or small case. 



