Visual Organs of Insects and Crustacea, 



2^23- 



equal in number to the facets of the cornea. The orange 

 and black tints already mentioned are owing to the coloured 

 pigment which extends between these crystalUne cyUnders, 

 surrounding and insulating them throughout their whole 

 length {Jig, 36. fj Jig. 37. d). Besides the general difference 

 in size just mentioned, the cyHnders are found to be much 

 longer at the back than in the front of the eye ; all are per- 

 pendicular to the surface of the cornea, and they converge 

 regularly towards the centre of the eye. When examined 

 individually [Jig- 36./), they are seen to be exactly rectilinear 

 and parallel to each other, except, of course, the slight diver- 

 gence consequent upon their radiated arrangement. They 

 are cylindrical in the greater part of their length, and from 

 ten to tw^enty times longer than they are broad. This great 

 length of these diaphanous bodies is 

 one of the peculiarities of the eyes of 

 the Libellulae : it is much less in most 

 other insects, in which also they are 

 conical. Their perfect transparency 

 has caused them to be mistaken for 

 bundles of tracheae mixed with nervous 

 filaments ; but the absence of all lines, 

 whether spiral or otherwise, in their 

 structure, ought to have prevented this 

 error. They refract light in the same 

 manner as it is done by glass cylin- 

 ders. When torn and emptied, they 

 appear as membranous sheaths, which, 

 in the perfect state, contain a viscid 

 humour, requiring some pressure for 

 its expulsion. The contained humour 

 is coagulated by alcohol ; is of greater 

 density than water, in which it sinks; 

 and the perfect cylinders themselves 

 very evidently refract light when they 

 are immersed in water. The ex- 

 tremity of each cylinder, towards the 

 cornea, terminates in an obtuse point 

 {Jigs, 37, 38.y), which is inserted in 

 the perforations of the superficial pig- 

 ment already noticed. At their opposite 

 extremity, these bodies become sud- 

 denly very slender, and are then continuous with the nervous 

 filaments {Jig. 36. ^), which constitute part of the deeper zone 

 mentioned in the general description of the parts seen in a 

 section of the eye of the Libellula. ^. . - , 



