SLAVE-MAKING mSTINGT. 355 



with the cuckoo, at intervals of two or three days. The 

 instinct, however, of the American ostrich, as in the case 

 of the Molotlirns bonariensis, has not as yet been per- 

 fected; for a surprising number of eggs lie strewed over 

 the plains, so that in one day's hunting I picked up no less 

 than twenty lost and wasted eggs. 



Many bees are parasitic, and regularly lay their eggs in 

 the nests of other kinds of bees. This case is more re- 

 markable 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 

 possessess 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 gen- 

 erally makes it own burrow and stores it with paralyzed 

 prey for its own larvee, 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 par- 

 asitic. 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 For- 

 mica (Polyerges) rufescens by Pierre Iluber, a better 

 observer even than his celebrated father. This ant is ab- 

 solutely 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 larvse. When the old nest is found incon- 

 venient, 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 



