370 Discoveries of Midler and others in the 



grasshoppers, accord with this organisation. Eyes of greater 

 convexity are met with in the carnivorous tribes, as in the 

 genus Mantis, and still more so in the genera ^cheta and 

 Gryllotalpa, 



Instances of eyes placed in the under part of the head are 

 rare ; the species of Onitis, however, are examples. The 

 situation of the eyes is also very low in some of the Cole- 

 pptera, as in some species of Hydrophilus ; but, in many 

 insects with lateral eyes, the lower part of these organs is 

 alone exposed to the light, the superior portion being alto- 

 gether covered by a projection of the corselet, as in the genera 

 jLampyris and ^latta, especially in the B. gigantea and B, 

 coloss^a. Very frequently, on the other hand, the eyes are 

 placed exclusively on the top of the head ; such is the case 

 with the species of Forficula, which, from the subterranean 

 mode of their life, have especial need of the organs of vision 

 being situated in that position. 



As the complete insulation of the rays of light emitted or 

 reflected from all points of the external object is the chief 

 condition required for clearness or distinctness of the visual 

 image or impression, it must follow that this distinctness 

 will bear a direct relation to the number of facets in equal 

 surfaces of different eyes. A very small and a large eye, 

 each having the same number of facets on an equal ex- 

 tent of surface, will see with equal distinctness any object 

 placed within the limits of the visual field of such sur- 

 face. The distinctness of the image will, as already stated, 

 increase with the length of the crystalline or vitreous cones ; 

 and, indeed, it may be laid down in general terms, that the 

 clearness of the visual impression will be greater in propor- 

 tion to the size of the spheres of which the eyes form seg- 

 jments, to the convexity of the surface, to the nearness of the 

 object viewed, to the number and minuteness of the facets, to 

 the length of the cones, and (consequent upon this latter cir- 

 cumstance) to the more accurate exclusion from the nervous 

 structure of all those rays entering the eye obliquely and out 

 of the direction of the long axis of the cones. 



The laws of refraction not being applicable to the com- 

 pound eyes of the articulated animals, it follows that there 

 can be no possibility of adapting these organs to see clearly 

 at different distances. 



To understand how the crossing of the visual fields of the 

 two eyes is prevented, the reader must bear in recollection 

 what was stated respecting the immobility of the compound 

 eyes. As the cones, when illuminated throughout, convey to 

 the optic nerve impressions of those objects alone which are 



