64 
ANIMAL INTELLIGENCE. 
workers returning to the nest from the tree on which the milk- 
ing was going on, a far smaller number had distended abdomens 
than among those descending the tree itself. A closer investi- 
gation showed that at the roots of the trees, at the outlets of 
the subterranean galleries, a number of ants were assembled, 
which were fed by the returning ants after the fashion already 
described in feeding the larvae, and which were distinguished 
by the observer as 6 pensioners/ MacCook often observed the 
same fact later, among, with others, the already described 
Pennsylvanian wood-ant. Distinguished individuals in the 
body-guard of the queen were fed in like fashion. MacCook is 
inclined to think that the reason of this proceeding is to be 
found in the 4 division of labour ’ so general in the ant repub- 
lic, and that the members of the community which are em- 
ployed in building and working within the nest, leave to the 
others the care of providing food for themselves as well as for 
the younger and helpless members ; they thus have a claim to 
receive from time to time a reciprocal toll of gratitude, and 
take it, as is shown very clearly, in a way demanded by the 
welfare of the community. 1 
Aphides are not the only insects which ants employ as 
cows, several other insects which yield sweet secretions 
being similarly utilised in various parts of the world. 
Thus, gall insects and cocci are kept in just the same way 
as aphides ; but MacCook observed that wdiere aphides 
and cocci are kept by the same ants, they are kept in 
separate chambers, or stalls. The same observer saw 
caterpillars of the genus Lycoena kept by ants for the sake 
of a sweet secretion which they supply. 
Habit of making Slaves .— This habit, or instinct, 
obtains among at least three species of ant, viz., Formica 
rnfescens , F . sanguinea , and strongylognathus . It was 
originally observed by P. Huber in the first-named species. 
Here the species enslaved is F.fusca , which is appropriately 
coloured black. The slave-making ants attack a nest of 
F. fusca in a body ; there is a great fight with much 
slaughter, and, if victorious, the slave-makers carry off the 
pupae of the vanquished nest in order to hatch them out 
as slaves. Mr. Darwin gives an account of a battle which 
he himself observed . 2 
1 Log. cit. p. 123. 
2 Origin of Species , 6th ed. p. 218, 
