and smooth Eyes of Insects, 111 



them. In a word, the whole cornea is a true hexagonal net- 

 work, the internal surface of which is divided into as many hex- 

 agonal facets as there are at the external surface. 



Swammerdani thought that the cornea received some tra- 

 cheae, and that these constituted the hexagonal mes^hes of 

 the cornea. He seeihed to think also, that these tracheae 

 could in the eyes of the nympkce serve for their expansion and 

 unravelling. As to the first of these opinions, it docs not seem ' 

 to agree with tiie organization of eyes in general ; for it is very 

 rare that the tracheae penetrate through the choroid and reach 

 the cornea ; and I only know one example, and that is the Li- 

 bcliula viilQaris. The second fact, heing only a consequence of 

 the first, cannot be admitted, ii' it be true that the trachea 

 rarely ever reach tVie cornea. 



The form of the cornea necessarily determines that of the 

 eve, and seems to have certain relations with the manner of 

 living of the species. Sometimes this form varies in its propor- 

 tions, and that in the species of one and the same genus ; but it. 

 seems as if the sfj/iericitj/, or the more or less angiilar form, of 

 this same coinea is little subject to vary. In general, the cornea 

 is the more spherical and the more projecting in proportion as 

 the animal is carnivorous, or when the eye is concealed under a 

 f5ap of the eyelid, as we see in the lampyrce and others, where 

 the sphericity of the eyes is so considerable that it comprehends 

 almost the whole head. We may also observe that the smaller 

 the compound eyes are, the greater is the convexity of the 

 cornea. It is not :he same with simple eyes, which in general 

 vary little in point of size. 



In order to give a precise idea of these variations, we have 

 described them in a certain number of families, in order that 

 tiieir importance may be known. 1 he cornea is in general 

 cased in a cavity in the hard parts of the head. This union is 

 so complete that it is frequently impossible to separate them : 

 we might even doubt that any separation was possible, were it' 

 not for a small ring externally which marks the line of adhesion. 

 When the conea is entirely separated from all the parts situated 

 below its internal face, as if troiii its covering, it seems white 

 and brilliant, whicli renders it similar to horn. This horn is 

 very thick in certain species. It is transparent if we exaniine 

 it externally; but when it is not freed from the other parts si- 

 tuated below, it presents either bands or stripes of various co- 

 lours, or a marbled appearance. It seems even completely 

 black in a great number of species : most of the colcopttrcVy 

 hyvtenopterce, and lepnlopUrw, present this anangement. 



To conclude : this black colour and marbled tinge do not by 

 »ny means belong to its textuic, but depend, as may be easily 



ascertained. 



