would show that the surface of the eye was formed by a
transparent body like a lens, and that behind this lay a complicated
arrangement of rods passing gradually into the nerves of sight. Only
_ocelli_, as these eyes are called, are found in the spider and his
kind. But in true insects, like the dragon-fly, or the butterfly, we
meet with eyes of another kind, in addition to ocelli. These are known
as compound eyes. Where compound eyes are found, the ocelli never exceed
three in number, and are arranged in the form of a triangle, and placed
in the middle of the head (figs. 2, 3, and 4).

The compound eyes vary greatly in their size. In some insects they are
placed one in each side of the head (fig. 1); in others, as in the drone
bee, they meet one another at the top of the head (fig. 3, spot marked
O) and extend downwards to the mouth. In others, yet again, they may
attain a huge size, and occupy even the whole front of the head,
crowding over the ocelli to form a little group at the top, as in the
head of a species of fly known as the Bibio (fig. 4).

The compound eye is so delicate and wonderful, that great knowledge of
anatomy or the science of optics is necessary before it can be really
appreciated. Briefly, it is made up of a cluster of simple eyes, in each
of which there are several parts. Beginning at the surface we have what
is known as the facet, or cornea, which roughly corresponds to the
surface of our own eyes. Next we meet with a clear, glassy rod, and this
passes downwards into the nerve of sight. Around these rods is a sheath
of black colouring matter, so that each eye is cut off from its
neighbour. Thus the whole eye may be likened roughly to a bundle of
telescopes.

Of what use, it may be asked, are the three little eyes in the middle of
the head of insects which have these wonderfully complex eyes? Well, the
large compound eyes are used to watch the movements of other animals;
thus they are enabled to escape their enemies. Many of you doubtless
have tried to catch butterflies, and if so you will know how suddenly
and quickly they avoid the master-stroke that is to land them in the
net. But the use of the three little eyes seems to be to enable their
possessor to see in the dark. By their means the bee (figs. 2 and 3) can
distinguish objects even in the darkest parts of the hive; so too the
ant can find his way about the galleries of his underground home.
Night-flying moths all have these little eyes,