As with the varieties of the stock, so with social
insects, selection has been applied to the family, and not to the
individual, for the sake of gaining a serviceable end. Hence, we may
conclude that slight modifications of structure or of instinct, correlated
with the sterile condition of certain members of the community, have proved
advantageous; consequently the fertile males and females have flourished,
and transmitted to their fertile offspring a tendency to produce sterile
members with the same modifications. This process must have been repeated
many times, until that prodigious amount of difference between the fertile
and sterile females of the same species has been produced which we see in
many social insects.
But we have not as yet touched on the acme of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile
females and males, but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but are perfectly well
defined; being as distinct from each other as are any two species of the
same genus, or rather as any two genera of the same family. Thus, in
Eciton, there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they have
an enormously developed abdomen which secretes a sort of honey, supplying
the place of that excreted by the aphides, or the domestic cattle as they
may be called, which our European ants guard and imprison.
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