1878.] 



the Simple and Compound Eyes of Insects. 



265 



Lepidoptera, renders it highly probable that the four bodies are the 

 nuclei of the primitive cells of the cone, and that the remainder of 

 these cells undergoes liquefaction. The four bodies in question are 

 elipsoids, with their long axes at right angles to the plane of the 

 cornea ; they exhibit fine longitudinal striations. I have called the 

 compound organ formed of these four bodies, the tetrasome. 



In the other type of eye, the chamber contains a fluid formed by 

 the liquefaction of a portion of the cells of the primitive cone ; but 

 a remarkable body is developed in the interior of these cells of a 

 very complex nature. It consists of a tetrasome, formed of four minute 

 highly refractive spheres, supported on a tetrapliore. I think it probable 

 that the tetrasome is formed from the nuclei of the four superficial 

 cells of the cone, whilst the tetraphore appears to be a highly modified 

 scleral cone formed in the interior of the deeper cells ; but the whole 

 question of the development of these parts is very difficult, and requires 

 further investigation. 



I have used the terms hydroconic and tetraphoric to designate these 

 two forms of eye. The first is characteristic of the heterocorous Diptera 

 and Libellulidae ; the second occurs in Acriclkim, and in the diurnal 

 Lepidoptera, Vanessa, Pieris, Colias, and Goneptrix. 



The rhabdion is either prismatic or cylindrical. In the eye of the 

 pupa it is seen to be formed of four cells ; but in the imago these are 

 so closely united that they can no longer be recognized as separate 

 structures. It contains axial longitudinal stride, which appear to be 

 the internal prolongations of the highly refractive cone or tetrasome. 



In the Lepidoptera the rhabdion rests on a facellus, formed of seven 

 fusiform cells, or, in some cases, of as many cylindrical rods, but in the 

 Diptera, Dragon flies, and some saltatorial Orthoptera it is in imme- 

 diate relation with the outer stellate ganglion cells of the ganglionic 

 retina. In the Diptera the rhabdia of the two peripheral rows of 

 facets are, however, united into bundles of four or more at their inner 

 extremities ; at least, this is the case in the Syrphidso ; and these 

 bundles are surrounded by fusiform pigmented cells in such a manner 

 that they somewhat resemble a facellus. 



The Diptera ha^ve, however, a very remarkable layer in the gang- 

 lionic retina itself, which apparently represents the facellus in function 

 at least ; I have termed it the facelloid layer. 



The accompanying diagram represents the axial structures connected 

 with a single rod-cell, in the semi-compound eye, and with one segment 

 of the cone or tetrasome in the true compound eye. 



The rhabdia in the compound eye are surrounded in most insects by 

 a close network of tracheal tubes, but in the Diptera these are replaced 

 by sac-like trachea which fill the interspaces between the prismatic 

 rhabdia ; this arrangement has been described by Leydig. 



The nervous structures of the retina and optic ganglia of the eyes of 



