216 SPECIAL INSTINCTS. [Chap. Till. 



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

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

 othrus bonariensis, has not as yet been perfected ; 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 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 Sphegida? (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 larva?, 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 exter- 

 minated. 



Slave-making instinct. — This remarkable instinct was first dis- 

 covered in the Formica (Polyerges) rufescens by Pierre Huber, a 

 better observer even than his celebrated father. This ant is abso- 

 lutely 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 nc 

 other work. They are incapable of making their own nests, or of 

 feeding their own larva?. 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 own larva? and pupa? to stimulate them to work, they 

 did nothing; they could not even feed themselves, and many perished 

 of hunger. Huber then introduced a single slave (F. fusca), and she 

 instantly set to work, fed and saved the survivors ; made some cells 

 and tended the larva?, and put all to rights. What can be more extra- 

 ordinary than these well-ascertained facts ? If we had not known of 

 any other slave-making ant, it would have been hopeless to speculate 

 how so wonderful an instinct could have been perfected. 



