4.94 EXTERNAL ANATOMY OF INSECTS. 
As to their structure,—when seen under the microscope 
they appear to consist usually of an infinite number of con- 
vex hexagonal pieces. If you examine with a good glass 
the eye of any fly, you will find it traversed by numberless 
parallel lines, with others equally numerous cutting them 
at right angles, so as apparently to form myriads of little 
squares, with each a lens of the above figure set in it. 
The same structure, though often not so easily seen, ob- 
tains in the eyes of Coleoptera and other insects. When the 
eye is separated and made clean, these hexagons are as 
clear as crystal. Reaumur fitted one eye to a lens, and 
could see through it well, but objects were greatly mul- 
tiplied *. In Coleopterous insects they are of a hard and 
horny substance; but in Diptera, &c. more soft and 
membranous. ‘The number of lenses in an eye varies 
in different insects. Hooke computed those in the eye 
of a horse-fly to amount to nearly 7000°; Leeuwen- 
hoeck found more than 12,000 in that of a dragon-fly «; 
and 17,325 have been counted in that of a butterfly. 
But of all insects they seem to be most numerous in the 
beetles of Mr. W. S. MacLeay’s genus Dynastes. In 
the eyes of these the lenses are so small as not to be 
easily discoverable even under a pocket microscope, ex- 
cept the eye has turned white‘: it is not, therefore, 
wonderful, that Fabricius should call these eyes szmple *. 
In some insects, however, as in the Sérepszptera, the 
lenses are not numerous: in Xenos they do not exceed 
* Reaum. iv. 245. > Microgr. 176. 
© Epist. Mar. 6. 1717. * Ameen. Academ. vii. 141. 
e I possess a specimen in which the eye is partly black and partly 
white: the lenses are invisible in the 4/ack part, but very visible in 
the white. f Philos, Entomolog. 19. 
