276 Memoir upon the compound 



ments destined to move them. As to the applications in which 

 M. Pievo&t inclii'ges, and which make him conclude that insects 

 oiiglit to have a distinct view within very narrow limits, they 

 cannot be just, since tliey rest upon facts vdiic'a are not exact. 

 Thu"? the same naturalist says, that th.e eyes resembling in shape 

 those oi bees cannot distin;;aish o!)jects further olf than five centi- 

 metres and one niillimetre ; and linally, those of the ant species 

 further than five millimetres and one-tenth millieme of a metre *. 

 But it is evident from what has iieen said, that it is inipossil)le 

 to brir.g any proof in favour of this assertion. Besides, in a 

 problem of this kind, v/e must pay attention not only to the ab- 

 solutely diminutive size of the eves, but also to their relative 

 dianicter, as well as to the greater or less convexity ; for an eye 

 the diinneter cf which would l}e greater than that of another 

 miglit nevertheless take in less distinctly objects at a certain 

 distance, if at the same time its convexity were more consider- 

 able. This is also what observation teaches us in a direct 

 manner, by proving to ns that natiuc has always combined in 

 the kinds of insects which have the most extensive eye-sight, the 

 sphericity of the cornea with the size of the eye. The libeLtulfP, 

 the mantes, the cicindclce, ai well as most of the IcpidoptercPy 

 the hymenopterce, and the diptercpj furnish striking examples ; 

 and if the size of their eyes is favouraljle to make them per- 

 ceive objects at a greater distance, the sphericity of tlieir cornea 

 is not less advantageous for rendering distinct the impression of 

 external objects, by uniting a greater number of rays, and lun- 

 dering them from removing from the axis of vision. , There is 

 no person, in siiort, who, in observing the motions of insects, has 

 not reuiarked, wlien ycu wii-b to catch them, hoAV acutely they 

 see you even at a considerable dista,nce: there are even whole 

 species which cannot be cavicht until vou keep out of their line 

 of vision. It cannot be said, however, that the insects are then 

 guidtc! by their sense of smeliiug, for tliis sense serves them 

 probably only for better getting at their prey when they are 

 still far irom it. 



We differ uith M. Prevost also when he says that, in order 

 that vision should be uscnd to insects, ihev must ordy see tliat 

 clearly which passes close to them, probably even at the distance 

 of their feelci.s and tiieir mouths, a distance at which the hu- 

 man eye sees with sucli indistinctness, that it may well be said 

 it does not sec at all. In the first place, there is no similitude 

 whatever between the human eye and that of insects, and con- 



* Those who have studied the liabits of insects must have rcmnrktd, 

 thnt tiierc are certain lefiidnpterce, some iliptcra, and even orthoptera and 

 hipntnopterw, who flv when \vc cppioach widiiii tlie diitaiice of ahixit six 

 metres. scquently 



