and smooth cr simple Eyes of Insects. 187 



compound eyes, is that of the d'wpsis ichnetimonea. This insect 

 exhibits its eyes situated nearly hke those of the mollnsci: thus 

 thev are placed at the extremity of a long pedicle, or a kind of 

 contractile tentacular point, but which can at least follow all 

 the motions of the head. We may say, on observing this sin- 

 gular organization, that this insect has its eyes at the extremity 

 of a telescope ; but as we have not seen it alive, nor dissected 

 it, it would be difficidt to decide how far this idea is correct. 



As to the descriptions which we shall presently give of the re- 

 lation which exists between the size of the bodies and that of 

 the compound eyes, they prove that the d/pterce, the liymeno- 

 pterce, lepidopterce, and neuropterce have very large eyes : it 

 even seems that there are few animals equally favoured. As a 

 great number of colenpierca and liemipterce have also very large 

 eyes in comparison to their bodies, we may say that this size is 

 very general in all insects. The two solitary examples of the 

 phannce and scolopendrcs ought not to prevent us from con- 

 cluding that, of all the animals, insects have the largest eyes in 

 comparison with the size of their bodies. 



§ II. Of the simpk or smooth Eyes. The number of simple 

 eyes is far from being as constant as that of the compound eyes. . 

 Certain species present two ; others four, six, and eight ; but in 

 general we observe three. I'his number is even pretty constant 

 in families which have at once compound and smooth eyes. 

 Werarelv see exceptions to this arrangement; and the Hat la, as 

 well as certain acheta, are perhaps the only kinds wliich, having 

 compound eyes, have only two simple eyes situated on the upper 

 part of the head. When there are three simple eyes, they are 

 always arranged in the form of a triangle, so that there are two 

 lateral, and one in the middle : their situation is also constantly 

 on the summit of the head. Sometimes, however, the simple 

 eyes are arranged two on the summit, and one in the middle of the 

 front of tlie head. This arrangement holds in the gryllus, the 

 truxalis, the acrydium, the Incusta, and the gryUotalpa. 



The empi/sa, which have upon the sunmiit of the head a small 

 triangular elongation, cannot on account of this organisation 

 see objects with their eye in the middle. 



The general form of the simple eyes is very variable : it seems, 

 however, that in general that of the lateral eyes is elongated 

 and elliptical, whilst that of the middle one is round : there are 

 also many exceptions in this respect, since the acheta and the 

 gryUotalpa present tliat in the middle of a very much elongated 

 oval of which the greatest diameter is transversal. 



The smooth or simple c^■e is formed of an external hard 

 transparent membrane, convex externally, and concave internally. 

 This membrane enveloping the eye necessarily determines its 



form : 



