THE SIMPLE AND COMPOUND EYES OF INSECTS. 
597 
measured by the denominator of fraction for man, and by the numerator for the insect. 
Thus a Dragon-fly would see an object 20 feet from its eye in the same detail that a 
man would perceive if it were seen at a distance of 160 feet. 
Least angle. 
Greatest 
angle. 
Diameter of 
facets. 
Greatest 
sharpness of 
vision. 
Least 
sharpness of 
vision. 
8' 
1 
O 
CO 
1 
y 5 o 
2 0 
16 0 
2 0 
6 0 0 
8' 
85' 
2 0 0 0 
2 0 
16 0 
2 0 
17 0 0 
8' 
85' 
1 
2 0 0 0 
2 0 
16 0 
2 o 
17 0 0 
8' 
30' 
1 
i oTfo 
2 0 
i7To 
2 0 
6 0 0 
18' 
12° 
i 
10 0 0 
2 0 
alTo 
2 0 
14400 
1° 
4° 
1 
10 0 0 
2 0 
12 0 0 
2 0 
4 8 0 0 
1° 
6° 
1 
10 0 0 
2 0 
12 0 0 
2 0 
7 2 0 0 
1° 
2° 
1 
I 0 0 0 
2 0 
12 0 0 
2 0 
2 4 0 0 
2° 
12° 
1 
10 0 0 
1° 
op 
«l<fl 
2 0 
1 4 4 0 0 
4° 
5° 
1 
10 0 0 
2 0 
4800 
2 0 
6 0 0 0 
rEschna grandis 
Vespa rufa , worker 
„ vulgaris, worker 
Bombus muscorum, female 
Tabanus bovinus, male 
Syr pirns, sp. 
Musca vomitoria 
Colias edusa . . 
Noclua, sp. . . 
Tipida oleracea 
The region of the most distinct vision extends from the visual line or the perpen- 
dicular to the centre of the least curved portion of the cornea, to a distance of from 
twelve to fifteen degrees in the horizontal, and from twenty to thirty degrees in the 
vertical meridian ; so that the region of the most distinct vision for each eye is 
approximately half an ellipse, with its long axis vertical in front. But when the 
two eyes are taken it is approximately a circle in front of the insect ; the two fields 
do not overlap in this direction. 
It will be seen that the acuity of vision, according to Muller’s theory, must vary 
directly with the radius of curvature of the surface of the cornea, and inversely as the 
diameter of the corneal facets. In many of the Diptera, as in Tabanus, the facets of 
the peripheral region of the cornea are three times the diameter of those in its centre.'” 
The size of the corneal facets varies in different insects from y-4-gth to yuVo"th of an 
inch. They seem to bear a relation to the size of the insect, as the largest are found in 
the largest and the smallest in the smallest insects ; lint I have found none less than 
koijcjth of an inch, although I have examined the eyes of many Diptera, of a fine or 
less in length. As the radii of curvature in very small insects are also very short, 
the vision of such insects is less distinct than that of larger insects ; at least, the 
distance at which objects can be seen distinctly must be very small. 
J. Muller has pointed out that the flight of insects depends on their power of 
vision, and this will account for the distances which large insects sweep through when 
In tlie genus JEschna the facets of the upper third of the compound cornea are twice the diameter of 
those of the lower two-thirds ; there is apparently no difference in the other part of the eye, except a 
proportionate increase in size. 
