Some species,
likewise, of Sphegidae (wasp-like insects) are parasitic on other
species; and M. Fabre has lately shown good reason for believing that
although the Tachytes nigra generally makes its own burrow and stores
it with paralysed prey for its own larvae to feed on, yet that when
this insect finds a burrow already made and stored by another sphex,
it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I
can see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose
nest and stored food are thus feloniously appropriated, be not thus
exterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica
(Polyerges) rufescens by Pierre Huber, a better observer even than his
celebrated father. This ant is absolutely dependent on its slaves;
without their aid, the species would certainly become extinct in a
single year. The males and fertile females do no work. The workers or
sterile females, though most energetic and courageous in capturing
slaves, do no other work. They are incapable of making their own
nests, or of feeding their own larvae. When the old nest is found
inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvae and pupae to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger.
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