spider,
the one mimicked and the other mimicking, are always found together.
So far as I can discover, however, the ant-like spiders of North
America are not found in company with any species of ant which they
resemble. This may be because they do not mimic any particular
species, but only the general ant-like form; or, considering that the
genera which contain their nearest relatives are much more abundant in
Central and South America, it may be that these forms were originally
tropical, mimicking some tropical species of ants, and that after the
Glacial Epoch they migrated northward, leaving the ants behind them.
However this may be, their peculiar form has served them well, since
they have maintained themselves as fairly abundant species with a
lower fecundity than is found in any other group of spiders.

The cases in which one species mimics another may be divided,
according to the kind of benefit derived, into four classes: Class 1.
As a rule, where we find one species mimicking another, the mimicked
species possesses some special means of defence against the enemies of
both. This defence may consist of a disagreeable taste or odor, as in
the Heliconidae, which are mimicked by other butterflies; of some
special weapon of offence, as where wasps and bees are mimicked by
flies and moths, or poisonous vipers by harmless caterpillars; or of a
hard shell, as where the coriaceous beetles are mimicked by those that
are soft-bodied.

Instances of this rule are exceedingly numerous; indeed, Wallace says
that specially protected forms are always mimicked; still we have
nothing mimicking our Gasteracanthidae.

Class 2. The mimetic may prey upon the mimicked species, its disguise
enabling it to gain a near approach to its victims; as the mantis,
mentioned by Bates as exactly resembling the white ants upon which it
feeds; and the flies which mimic bees, upon which they are parasitic,
and are thus able to enter the nests of the bees and lay eggs on the
larvae.

Class 3. The mimetic species may, by its imitation, be protected from
the attacks of the creature it mimics, as is the case with the
crickets and grasshoppers which mimic their deadly foe, the hunter
wasp.

Class 4. The mimetic species may prey upon some creature which is
found commonly with, and is not eaten by, the mimicked species.

No two of these classes are mutually destructive so that in any case
of mimicry a double advantage may be gained.

Let us see w