and smooth or simple Eyes of Insects. 291 



fropi that of the eyes of the other apterce. The cornea is all of 

 a piece, as in the smooth eves : it seems externally of a brilliant 

 black; but this colour does not belong to its texture; it is pro- 

 duced by the tunic of that membrane Vihich presents this shade. 

 This cornea is very convex, rounded, and polished. Placed in a 

 projection of the head, the eyes of the spider are very close to 

 each other, being only separated by a deep sinus. The eyes are 

 placed a little on one .side of the projection which forms the co- 

 riaceous envelope, so that they ought not to be able to distin- 

 guish objects except above tliem and laterally. As they are im- 

 moveable -as well as the parts on which they are situated, it must 

 be difficult for the phalaiighim to perceive objects placed in a 

 straight line with the body. This position of the eyes is the 

 more remarkable, as it is the only one of this kind which exists 

 in insects. It is not found in the nrachno'ideSy and is only seen in 

 certain species of criislacece, and still under many modifications. 



The eyes of the phalungium are protected by an interorbitary 

 arcade, and this forms above them a projection for defend- 

 ing them against the impression of external objects. These eyes, 

 so far as their diminutive size has permitted me to judge, are 

 formed by a cornea of one hard piece, thick, convex, and trans- 

 parent. It seems, however, to be black when viewed externally; 

 but this is owinv, as we have already said, to the colouring of the 

 tunic placed under this membrane. This timic is besides very 

 thick. We see it traversped by a nerve which corresponds with 

 the concave surface of the cornea. The choroid and its varnish 

 are completely black : probably, the texture of this membrane is 

 formed by tracheae, nevertheless it is not surrounded by a circular 

 trachea. There proceed towards this part numerous tracl^eoe, 

 but they arc there ramiiicd, instead of forming a principal tra- 

 chea. This is nearly all that I could ascertain respecting the 

 structure of the eyes of this kind of spider, so similar to the arach- 

 noidei in external character, and yet so different in the organs of 

 respiration and circulation. 



Having been able quite recently to ascertain the structure of 

 the compound eyes of the crepuscular insects, such as the 

 sphijrix and rhe noc.tu<e, I ought to state that nothing which 

 I have said on the vision of insects in general is applicable to 

 them. The latter liave, in fact, an organization so particular 

 in the structure of their eyes, that there is necessarily a great 

 difference in tlie way in v/iiich vision is effected. I thought I re- 

 cognised in these eves humours of different densities, and even 

 a species of crystalline ; we know that there exists nothing si- 

 milar in the other insects. 



The^e are tlie diflvrent-es which exist between order and order, 

 T2 and 



