AND THE MORPHOLOGY OF THE EYE EN INSECTS. 10:5 



Each corneal facet consists of a chitinous membrane overlying a soft substance which 

 is easily stained with logwood or carmine solution. 



As development progresses the deeper portion of the cornea becomes more highly 

 refractive, and loses its property of taking up status. It appears as if the albuminous 

 substance of the young cornea becomes gradually converted iuto a stroma infiltrated 

 with oil. I have never seen the oil in drops, like those in a young fat-cell. In the 

 rudimentary lens it appears rather as if the oil were evenly distributed throughout the 

 whole substance of the lens. 



In the condition which I have just described I have seen no traces either of division 

 of the albuminous portion of the cornea into four, or of four nuclei; but in a still 

 earlier stage there are distinct indications of four nuclei in each corneal facet, and in a 

 later stage the appearance of the "nuclei of Semper" on the addition of ether, or by 

 drying, points to the origin of the corneal lens from four original cells. 



In the immature insect, during the development of the eye, a number of capsules, 

 filled with cells containing an abundance of oil in large drops, are seen under the mem- 

 brana basilaris. Similar cells are shown in some of Claparede's figures, and arc named 

 by him basal cells * . These are entirely absorbed during the development of the eye, 

 and it appears probable that they afford the oil required for the formation of the refractive 

 media. I have no doubt they belong to the system of fat bodies in which the larvae and 

 pupse of insects are so rich. 



The fourth form of cornea is apparently confined, amongst insects, to the imago form 

 of some Gnats, but is also probably the condition in Mysis, in its perfect state, and in 

 some other Crustacea. I propose to distinguish this type of cornea as the lenticular 

 cornea. 



The lenticular cornea consists of a number of well-developed, almost hemispherical 

 lenses, which are nothing more than the united crystalline cones of the larva and nymph. 



Dr. Grenacher has correctly figured and described the eye of the larva and pupa of a 

 Gnat; but be adds : " I have only studied the eye of this insect (Corethra plumicornis) 

 in the larva and pupa, not in the imago;" and says, "This is a matter of no importance, 

 as the eye undergoes no change in passing from the pupa to the imago " t. 1 was at 

 once struck by this remarkable statement, as the eye is covered by a smooth non-facetted 

 cornea in the nymph, whilst the corneal facets in the imago are remarkable for their size 

 and very great convexity (fig. 74)— a fact well known to all who have made the most 

 cursory examination of the compound eyes in this family of insects. 



A comparison of a section of the eye of the adult larva (fig. 70) or nymph (fig. 75) 

 with that of the imago (fig. 74) shows that the great convex facets of the cornea of the 

 perfect insect are really the ovoid crystalline cones (" spherocones") of the larva, covered 

 by a thin cuticular membrane, which dips down between them, as the membrane of the 

 kistoid cornea dips between its lenses. 



In some of the Gnats the outer nearly hemispherical segment of the cone becomes 

 converted into a nearly globular oil-lens, exactly bke the oil-lens of a Crane-fly. The 



* L. c. t /- ■ p. 94. 



