tside the brain of the
knowledge of past generations, thus serving his plastic activity, at
once the adapter and combiner of what the past has bequeathed to it.

According to the families, _genera_, and species of insects, the
development of different senses varies extremely. We meet with most
striking contrasts, and contrasts which have not been sufficiently
noticed. Certain insects, dragon-flies, for instance, live almost
entirely by means of sight. Others are blind, or almost blind, and
subsist exclusively by smell and taste (insects inhabiting caves, most
working ants). Hearing is well developed in certain forms (crickets,
locusts), but most insects appear not to hear, or to hear with
difficulty. Despite their thick, chitinous skeleton, almost all insects
have extremely sensitive touch, especially in the antennae, but not
confined thereto.

It is absolutely necessary to bear in mind the mental faculties of
insects in order to judge with a fair degree of accuracy how they use
their senses. We shall return to that point when summing up.

_II.--The Vision of Insects_

In vision we are dealing with a certain definite stimulus--light, with
its two modifications, colour and motion. Insects have two sets of
organs for vision, the faceted eye and the so-called simple eye, or
ocellus. These have been historically derived from one and the same
organ. In order to exercise the function of sight the facets need a
greater pencil of light rays by night than by day. To obtain the same
result we dilate the pupil. But nocturnal insects are dazzled by the
light of day, and diurnal insects cannot see by night, for neither
possess the faculty of accommodation. Insects are specially able to
perceive motion, but there are only very few insects that can see
distinctly.

For example, I watched one day a wasp chasing a fly on the wall of a
veranda, as is the habit of this insect at the end of summer and in the
autumn. She dashed violently in flight at the flies sitting on the wall,
which mostly escaped. She continued her pursuit with remarkable
pertinacity, and succeeded on several occasions in catching a fly, which
she killed, mutilated, and bore away to her nest. Each time she quickly
returned to continue the hunt.

In one spot of the wall was stuck a black nail, which was just the size
of a fly, and I saw the wasp very frequently deceived by this nail, upon
which she sprang, leaving it as soon as she perceived her error on
touch