8 CLASS INSECTA. 



titude of small facets, by so much the more convex as the 

 insect is more carnassial, clothed at its external face with a 

 substance not very fluid, opake, variously coloured, but in 

 general black, or of a sombre violet ; secondly, of a choroid, 

 fixed both in its extent and by its edges to the cornea, co- 

 vered with a black varnish, exhibiting a multitude of air- 

 vessels proceeding from tolerably thick trunks of tracheae si- 

 tuated in the head, and whose branches form around the eye 

 a circular trachea. This is wanting, as well as the choroid, 

 in divers lucifugous insects. Thirdly, of nerves, which ori- 

 ginate from a thick trunk, proceeding immediately from the 

 brain, expanding afterwards into the form of an inverted 

 cone, the base of which is on the side of the cornea, while its 

 radii or threads, traversing the choroid and the covering of 

 the cornea, end each of them at one of its facets. There is 

 neither crystalline nor vitreous humour. 



Many insects besides the compound, or complicated eyes, 

 have simple ones, or such as have the cornea altogether 

 of a piece. These are usually three in number, and disposed 

 in a triangular form on the top of the head. In most of the 

 apterous insects, and of the larva of those which are winged, 

 they supply the place of other eyes, and are often united in a 

 group. If we may judge by those of the arachnida, they 

 must serve the purposes of vision. 



The mouth of insects with six feet is in general composed 

 of six principal pieces, four of which are lateral, disposed in 

 pairs, and move transversely. The other two, opposite to each 

 other, in a contrary direction to that of the preceding, fill 

 the vacancies comprised between them. One is situated 

 above the upper pair, and the other below the lower. In 

 the grinding insects, or such as feed on solid substances, the 

 four lateral pieces perform the office of jaws, and the other 

 two are considered as lips : but, as we have already observed, 

 the two upper jaws have been distinguished by the peculiar 



