VISION IN INSECTS. 137 



as their radiated disposition will permit ; but they 

 are much smaller in diameter than the cylinders, 

 and, notwithstanding their sleriderness, appear, under 

 the microscope, somewhat opaque and of a fibrous 

 texture. Surrounded by a dark choroid secretion 

 (i), these filaments, on account of their great tenuity, 

 cause the pigment to appear much thicker and 

 darker, when regarded en masse, than that portion of 

 it represented as passing between the cylinders. 

 These latter are almost in immediate contact with 

 each other: the nervous filaments, on the contrary, 

 are separated by spaces much exceeding in size their 

 own diameter. 



" In the centre of the eye is the optic ganglion (j), 

 which, however pulpy and homogeneous it may ap- 

 pear at first sight, exhibits nevertheless a fibrous and 

 radiated structure when submitted to moderate com- 

 pression. Indeed, it may in some degree be regarded 

 as the optic nerve passing into the filamentary ar- 

 rangement observed a little farther from the centre. 



" Such are the anatomical details exhibited in the 

 eye of the grey Libellula and of other insects, with 

 some modifications to be hereafter noticed. In exa- 

 mining each of these parts, we may, to a certain 

 extent, refer them hypothetically to the structures 

 forming the simple eye of the vertebrated animals. 

 In fact, we find in these compound eyes a nervous 

 filament attached to the extremity of a transparent 

 body representing the vitreous humour and crystal- 

 line lens ; a transparent cornea covering externally 

 this apparatus ; and a choroid membrane, represented 

 here by a coloured pigment, which surrounds, as in 

 the vertebrated animals, these minute organs of 

 refraction and sensation. We may still further 

 remark that the pigment, continuous in all parts, 

 although varying in thickness, forms between the 

 cornea and the transparent or crystalline cylinder, an 



N3 



