266 



Mr. B. T. Lowne on the Modifications of [Mar. 28, 



insects are exceedingly difficult to make out ; but I think I have suc- 

 ceeded in working out the retinal structures of Eristalis and Agrion in 

 considerable detail. In the other insects in which I have examined the 

 eye, the knowledge which I have been able to obtain of this portion of 

 the nervous system, must be considered at present as fragmentary. In 

 FJristalis there are from without inwards : — 1. A double layer of large 

 stellate ganglion cells ; 2. A layer of small round nucleated cells ; 

 3. The facelloid layer already referred to ; and 4. A layer of stellate 

 ganglion cells. 



These parts are connected with a deep ganglion, which consists of 

 several layers of fusiform cells by a decussating optic nerve, the fibres 

 of which cross each other from above downwards, and from without 

 inwards. The deep ganglion is connected' by a distinct peduncle with 

 the supra-cesophageal ganglion. All the structures of the ganglionic 

 retina are supported by a fine neuroglia, which extends from a thick 

 outer to a fine inner limiting membrane. 



In Agrion the ganglionic retina differs from that of jEristalis in the 

 absence of a facelloid layer, which is replaced by a triple layer of 

 prismatic cells : the investigation of the nerve structures of this 

 insect is rendered very difficult by the presence of a large quantity of 

 dark pigment in the stellate connective cells of the neuroglia. 



In Vanessa the facelloid layer of the retina is also absent, but in its 

 place there are numerous layers of fusiform cells. In noctuid moths, or 

 at least in some species, the nervous structures are obscured by the 

 large quantity of deep black pigment which they contain. In the 

 semi-compound, or, as I have termed it, the microrhabdic eye of Tipula, 

 the nervous retina consists of, (1,) a layer of stellate ganglion cells ; 

 (2,) of several layers of round cells ; and (3,) of several layers of 

 fusiform cells. The greatest simplicity exists in the eye of Formica, 

 in which all the structures of the nervous retina are absent except 

 numerous layers of small round cells. I have not at present been 

 able to make out any decussation of the nerve fibres connecting the 

 deeper parts with the ganglionic retina in the insects with microrhabdic 

 eyes, but the investigation is very difficult, owing to the great change 

 of the plane in which the nerve tracts lie. I do not think, however, 

 that any decussation exists, or I think I should have found indications 

 of it. 



The extent and curvature of the cornea and the size and curvature 

 of the facets afford the most important indications as to the manner 

 in which vision is accomplished. In the true compound eye, I think 

 the structure indicates that J. Miiller's theory of vision is the most 

 probable ; this is also Dr. Grenadier's view, and it is supported, as I 

 shall now endeavour to show, by the curvature of the cornea and the 

 size of the corneal facets in different insects, as well as in different 

 parts of the same eye. 



