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, Claparéde, 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, Carriére 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, 
