AND THE MORPHOLOGY OF THE EYE IX INSECTS. 395 



In most insects a lens lies under each corneal facet ; I shall show hereafter that this 

 3ns is sometimes formed from the outer portion of the crystalline cone, and in other 

 ases from the cornea itself, which then assumes the appearance of a honeycomb, the 

 ells containing the lenses. 

 The consistence of the lens seems to vary very much ; in some cases it is apparently 

 uid, enclosed in an elastic capsule ; in the Earwig and the Cockroach I have even been 

 ble to separate such lenses from the cornea by pencilling its inner surface ; the capsules 

 an then be ruptured, and the fluid seen to escape by pressing on the cover glass ; the 

 uptnred capsules exhibit a single tear and fine wrinkles (fig. 21) *. 



The refractive index of the fluid with which the lenses are filled is very nearly 2 ; 

 his fluid appears like an oil, but it undergoes slow solution and decomposition by the 

 ction of water and weak saline fluids ; a comparatively lowly refracting fluid and a fine 

 eddish molecular substance result from their action. 



The oil-like fluid is rapidly dissolved by ether and is blackened by osmic acid. Only 

 he red molecular precipitate remains in specimens prepared in the usual way with clove 

 til. I believe the great brilliancy of the cornea of many insects during life is due to 

 his fluid lens immediately beneath its surface, and that the loss of brilliancy which 

 tccurs soon after death, is due to the decomposition of the fluid, or its escape from the 

 ens-capsule. 



I am inclined to regard this fluid as an oil of complex constitution, which is possibly 

 ■ich in sulphur or phosphorus, to which it owes its high refractive index and ready 

 lecomposition. Observations with a micro-spectroscope have given negative results. 



In other cases the lens, when separated, breaks up like a viscid body ; the lens-capsule 

 dways appears to contain a stroma, the meshes of which the fluid permeates ; and the 

 consistence of the lens depends on the relative amount of the stroma. 



Every one who has examined the compound eye since improved methods have been 

 idopted, must have been puzzled by the " nuclei of Semper " which figure so promi- 

 nently in the descriptions of Claparede f. I formerly supposed with Dr. Grenadier that 

 Claparede so designated the nuclei which are frequently seen in the immature eye 

 between the crystalline cone and the facet of the cornea (en. tig. 3). But such a view is 

 by no means satisfactory. 



I was very much surprised on one occasion, to see the " nuclei of Semper," which are 

 really nothing but the shrivelled segments of the lens-stroma, appear suddenly in a 

 compound cornea treated with ether on the stage of the microscope ; as the oil dis- 

 solved out of the lenses the contents of the capsules split into four parts. I regard 

 this as the result of the shrinking of the stroma. A similar appearance is seen when the 

 cornea of an insect is examined after having been allowed to become partially dried. I 

 have never observed this in specimens prepared in the usual manner with alcohol and 

 clove oil. 



In the simple eyes of caterpillars , " ocelli coitijjosili " (figs. 36-10), which are un- 



* This lens was described by Miiller ; but its existence has been completely overlooked bv reeont authors, 

 t Zeitschr. f. w. Zool. Bd. x. 



