THE THEORY OF ARTHROPOD VISION. 567 



of the normals to the corneal surface are taken as the measure 

 of the visual field, this must be very large ; it varies in different 

 insects from over 180 in each eye in one or more meridians, 

 and is seldom less than 120 in any direction ; but the sharp- 

 ness of vision clearly varies greatly in different parts of the 

 visual field, just as it does in the vertebrate eye. 



Sharpness of Vision. The sharpness of vision in insects is a 

 subject which has led to numerous discussions. Some (Marcel 

 de Serres, Claparede, Dor, and Patten) hold, as I do, that 

 insects see with great distinctness both near and distant 

 objects. Evidence of this is abundant, and numerous facts 

 will occur to the practical entomologist which are inexplicable 

 under the view that insects see indistinctly, the hawking of 

 Flies and Dragon-flies, the close mimicry of the Volucellas, the 

 manner in which the Hymenoptera find their nests or their 

 favourite flowers, and the fact that Bees will visit artificial 

 flowers, are all indications of sharp vision. 



Other authors, including Lamarck, Treviranus, J. Miiller, 

 Grenacher, Notthaft, Hickson, Carriere and Forel, have arrived 

 at the conclusion that the compound eye is a very defective 

 organ of vision, and that insects see little more than moving 

 shadows. 



Plateau [243] cites numerous observations and experiments, 

 from which he concludes that vision in insects is very defective. 

 These experiments and observations may be divided into three 

 groups : 



1. Experiments made on gratings and free openings of exit 

 from a chamber, or gratings, the bars of cages, etc., in the 

 open air. He found that insects usually fly to a grating rather 

 than to a free aperture when the former is larger and admits 

 more light, even when the grating is an absolute obstacle to 

 escape. He subsequently found, however, that birds do exactly 

 the same thing ; therefore either his experiments do not show 

 defective vision, or birds have an equally defective sense of sight. 



2. Experiments made with cardboard labyrinths, in the 

 centre of which cursorial insects were liberated. In these 

 experiments he found that the insects ran up to the obstacles, 



