Animals with more than Two Eyes. 209 
are instances in point. We shall find the drone-fly, known as 
Eristalis tenax, hovering over or alighted on a head of flowers in 
full bloom. He is sucking the juices from the petals or eating 
the pollen from the anthers. He is a stout, pitchy-black, hairy 
fly, more than half an inch in length. Notice the tawny spots 
on the abdomen, and the triangular spots of the same colour on 
the side, and you will remember him. 
The two compound eyes, projecting on each side of the head, 
are easily seen; half globular in shape, they are relatively im- 
mensely larger than the eyes of the higher animals. I take a 
dead specimen, and tenderly remove the front membrane of one 
of these compound eyes. I carefully remove the. dark colour- 
ing matter at the back, using a soft camel’s hair brush for the 
purpose; and, after washing the membrane in spirit, I put it on 
_ a thin slip of glass, and then look at it, or, rather, through it, 
with a hand-lens. 
What do I see? The cornea proves to be a beautifully 
transparent lattice, fitted with thousands of six-sided window- 
panes. Is any cathedral window, however vast, half so 
wonderful? I can count the number of these separate window 
panes, each of which, again, is a complete eye. There are more 
than four thousand of them. But, as I trace them downward, I 
notice a curious change in their shape. 
They gradually pass from hexagons into squares—from six- 
sided panes into four-sided panes. The upper half of the 
window, as I have called the compound eye, 1s filled with panes of 
one pattern, and the lower half with panes of another pattern. 
This is a very remarkable occurrence. As far as I know,—and 
I have examined some scores of insects’ eyes of different species,— 
it is confined to the drone-fly. 
The “portcullis eye” of the house-cricket is an example of 
the square-shaped eye-facet, in which the lens is framed ; but in 
this case, all are squares, and none of them hexagons. If you 
examine the cricket’s eye, you will find hundreds of the eye-facets 
arranged in rows. Each facet is barred off from its neighbour by 
a thick, horny partition, giving the whole the appearance of the 
heavily-timbered framework, which used to be let down before the 
entrance of old castle gateways. Hence the name “ portcullis ” 
eye. 
: We have next to deal with much larger kinds of animals than 
those hitherto mentioned. The discovery that lizards have a 
third eye, now in most cases buried beneath the skin, but formerly 
situated at the top of the head, is one of the very newest and 
most startling achievements of zoological investigation. In some 
of the smooth-skinned lizards, this third eye, though no longer in 
use, is still visible on the top of the scaly head, being placed just 
Vol. III. II—II 
