Many bees are parasitic, and regularly lay their eggs in the nests of other
kinds of bees. This case is more remarkable than that of the cuckoo; for
these bees have not only had their instincts but their structure modified
in accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would have been indispensable if they had
stored up food for their own young. Some species of Sphegidae (wasp-like
insects) are likewise parasitic; 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, 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 that of the Molothrus or 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
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 of any kind, and 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