nces to eat its way upwards, forming
at first a very small tunnel, but gradually increasing it, as the
caterpillar grows larger, into a passage of about half an inch in
diameter. In such a position, surrounded as it is by solid wood, the
thickness of which would probably not be less than one and a half or
two inches, we might suppose that the caterpillar would be safe from
its enemies, but it is not: there is a large Ichneumon Fly which
cannot propagate its species unless it can lay its eggs in the body of
this particular caterpillar. This Ichneumon Fly can, from outside, not
only tell that inside the stem of that tree there is a caterpillar,
but can locate the exact spot, and, still more wonderful, is able to
determine whether or not that caterpillar is the particular species it
is in search of. There are numerous other species of moths whose
caterpillars feed in the centre of trees, and yet this female
Ichneumon is able to mark down as her prey, although far out of reach
of any sense known to us, that one species which alone can serve her
purpose. As soon as she has located the exact position of the
caterpillar, she unsheathes a long delicate ovipositor, with which
she is provided, and drills it right through the intervening solid
wood until it pierces the body of the caterpillar; she then lays an
egg down that long tube into its body and repeats the process two or
three times. The caterpillar itself does not appear to feel any
inconvenience from this process and continues to feed and grow larger;
but it has the seeds of death within itself, and the two or three
little caterpillars, which hatch out of the eggs of the Ichneumon, are
also growing rapidly inside it. At last, when the time comes that the
large caterpillar should have been full fed, and it has eaten its way
outwards until it rests close under the bark, preparatory to turning
into a chrysalis, its enemies finish their destructive work, and, if
the tree is then opened, the empty skin and cartilage skeleton of the
large caterpillar is found, together with two or three large cocoons.
These cocoons, if kept, will produce in due time specimens of the
Ichneumon Fly, and these will in their turn go about their murderous
work as soon as their proper hunting season comes round again.

This is only an isolated case out of thousands of similar occurrences
in every locality; in fact, if you walk along any palings in the
country in the early summer, you will see at ever