Visual Organs of Insects and C?'ustdcea. 22 Y 



cornea are hexagonal ; but in some of the lower tribes the 

 cornea is quite smooth and without facets : this latter cir- 

 cumstance, however, will be noticed again hereafter. 



I shall now point out such diversities of form, &c., in these 

 several parts of the compound eyes, as seem to be most 

 deserving of notice. 



The Cornea, In insects, the facets of the cornea are 

 hexagonal ; in the Crustacea, they are sometimes hexagonal, 

 sometimes quadrangular. The number of facets, according 

 to the observations of Swammerdam, Leuwenhoek, and others, 

 is here given ; but, of course, these numbers ought to be re- 

 garded, for the most part, merely as approximations. 



The size of the facets, as it might from the above be expected, 

 varies extremely in the different animals ; and in the Libellula 

 vulgaris, as already noticed, those of the upper segment of 

 the same eye are much larger than those of the other parts. 



In a few insects, hairs are seen in the interstices of the 

 facets ; this is observed in the orders Hymenoptera and 

 Diptera. When these hairs are met with, they are most 

 numerous at the lower part of the circumference of the eye. 

 The structure of the cornea, and of its facets, varies much in 

 different insects ; in many, each facet is a double convex lens, 

 as may be seen by examining the section of the cornea under 

 a microscope. In the Sphinges, the axis of the facet is to its 

 diameter as 1 to 2. In many others, and apparently in all those 

 which several times renew their cornea during their incom- 

 plete metamorphosis, as in the orders Orthoptera and He- 

 miptera, the facets are much less convex, and generally flat 

 on the internal surface. In the order Orthoptera the thickness 

 of the cornea, in relation to the diameter of the facets, is 

 commonly very great : thus, the diameter of each facet, in 

 the Gryllus hieroglyphicus, is to its thickness as 1 to 7. The 

 quadrangular facets in oysters are flat at their internal 

 surface. 



The Crystalline Bodies or Cones, These bodies probably 

 belong to the compound eyes with facets of all insects and 

 crustaceous animals. If these eyes are examined in a fresh 

 state, the cones, although completely separated from each 

 other by being individually invested with a layer of choroid 

 pigment, are yet so soft and gelatinous that it is difficult to 

 make a section of the parts without mingling and confounding 

 together the greater portion of the substance of the cones 



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