I 



404 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



inner segment of the cone then hears the same relation to the other refractive structures 

 that the cone hears in the ordinary Diptera and Hymenoptera, or in insects and Crus- 

 taceans generally. 



The formation of a fluid lenticulus from the suhstance of the cornea is not confined to 

 the compound eye, hut is also seen in the very different simple ocelli of some Arachnids. 

 In a section of the simple ocellus of a Spider (Salticus scenicus) (fig. 34), which I pre- 

 pared in the usual way and mounted in Canada balsam, the corneal lens is absolutely 

 hollow. It evidently contained a fluid during life. 



The sudden scintillations, which the bright reflex of the fundus of the eye of this insect 

 exhibits in the living state, seem to indicate a power of accommodation, possibly from 

 the action of muscle fibres which are inserted around the edge of the cornea. 



Dr. Landois * traced the development of the subcorneal lens in the compound ocellus 

 of a Caterpillar to a very different source. Soon after an ecdysis he found three spheroidal 

 bodies very similar to those which I have described as a tetrasome in the eye of a larva 

 of Acridiwm f . These unite to form the lens. 



From the manner in which the cornea is formed in Gnats, I think it probable that this 

 subcorneal lens becomes the cornea at the next ecdysis, and that a new lens is then 

 formed from subcorneal cells or nuclei. Although I have not observed either subcorneal 

 nuclei or cells in the eyes of caterpillars, both are very frequently seen in the eyes of 

 larval insects. 



I suspect that the compound cornea is developed in a similar manner in some Ortho- 

 ptera, from the indications I observed in Acridmm, but I am uncertain on this point; 

 neither can I do more in the case of the Isopods than point out the very strong resem- 

 blance of the subcorneal lenses to those of the compound ocelli of caterpillars. 



B. Modifications of the Crystalline Cone. 



The outer portion of each segment of the dioptron, between the cornea and the external 

 layer of pigment, may be conveniently termed " the chamber " (fig. 10 ch.). It is formed 

 by the cuticular sheath of the segment, often thickened by a deposition of chitin ; and is 

 lined by fringes, pigment-cells, or ciliary rods. 



Tlie chamber contains the crystalline cone (cc.), which in many insects is replaced by a 

 coagulable fluid, contained in four vesicles, and in others by four nucleated cells. Dr. 

 Grenadier % classifies the eyes of insects in three groups : those with a cone he calls 

 euconic, those with the albuminous fluid pseudoconic §, and those with four nucleated 

 cells aconic. 



Up to a certain point my observations correspond with those of this distinguished 

 author. In all insects the contents of the chamber are developed from four primitive 

 cells || . This condition persists in the Crane-flies ( Tipula), even in the recently formed 

 imago (figs. 10 & 77) ; but I Lave failed to find these cells in the perfectly developed 

 insect, in which the chamber is filled with an albuminous fluid, and I believe that the 

 presence of the four nucleated cells is always evidence of immaturity. Dr. Grenadier's 



* Zeitschr. fur w. Zool. Bd. xvi. f L. c. Phil. Trans. J L. e. 



§ Hydroconic, rnihi, I. e. Phil. Trans. || This was observed by both Claparede and Weismann. 



