INSECTS. 
11 
ill some insects, as in Spiders, they are six 
or eight in number. In the major part of 
the Insect tribe the eyes may be considered as 
compound; at least with respect to the exterior 
coat or cornea, which, when viewed with a mi- 
croscope, presents the appearance of an infinite 
number of separate convexities, which are of a 
shape exactly hexagonal, and appear to be so 
many real convex lenses or glasses; but the exact 
manner in which vision is performed in Insects is 
perhaps not yet ascertained. Some have supposed 
each of the hexagonal lenses to act as a real and 
separate eye, and that the optic nerves are ex- 
panded in separate branches at the bottom of 
each as a retina; or that one universal retina is 
expanded under all, which probably, is the real 
structure. Yet it still remains difficult to account 
for this prodigious multitude of eyes on the head 
of one single animal. The head of the common 
Libellula or Dragon-Fly is perhaps furnished with 
not less than twenty-five thousand of these little 
lenses. Whatever be their use, this particular 
structure cannot be contemplated without the 
highest admiration, and constitutes one of the 
most curious particulars in the comparative ana- 
tomy of Insects. That they are real convex lenses 
seems demonstrated by their exhibiting every 
phenomenon of such; inverting any object viewed 
through them when magnified, as the flame of a 
candle, the chimney of a house, or any other 
object towards which they are directed; and that 
they are double-convex lenses has been generally 
