136 INSECT MISCELLANIES. 



the centre of the eye. When examined individually, 

 they are seen to be exactly rectilinear and parallel 

 to each other, except, of course, the slight divergence 

 consequent upon their radiated arrangement. They 

 are cylindrical in the greater part of their length, 

 and from ten to twenty 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 trachae 

 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 extremity of each cylinder, towards the 

 cornea, terminates in an obtuse point (/), vvhich is 

 inserted in the perforations of the superficial pigment 

 already noticed. At their opposite extremity, these 

 bodies become suddenly very slender, and are then 

 continuous with the nervous filaments which constitute 

 part of the deeper zone already mentioned. 



^ This zone (g), of a deeper black colour than the 

 preceding, and of greater thickness at the front than 

 at the back part of the eye, contains the nervous fila- 

 ments, which, arising from the bulb or ganglion of 

 the optic nerve, terminate in the transparent cylin- 

 ders already described. Like these latter, the fila- 

 ments converge from the circumference towards the 

 centre, being linear, straight, and as nearly parallel 



