top, and that in this way many of the lower leaves had been planted with colonies of aphides. After some weeks the evil was as great as ever. The tree stood alone on the grass plot, and offered the only situation for an aphides' colony for the countless ants there present. I had destroyed this colony; but the ants replanted it by bringing new colonists from distant branches, and setting them on the young leaves.[27] Again-- MacCook noticed, of the mound-making ants, that of the workers returning to the nest from the tree on which the milking was going on, a far smaller number had distended abdomens than among those descending the tree itself. A closer investigation 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 larvæ, and which were distinguished by the observer as '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 'division of labour' so general in the ant republic, and that the members of the community which are employed 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.[28] 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 where 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 rufescens_, _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 pupæ of the vanquished nest in order to hatch them out as slaves. Mr. Darwin gives an account of a battle which he himself observed.[29]