216 SOCIAL BEHAVIOUR 
circumstances they remained in perfect health, while, but for 
the slaves, they would have perished in two or three days.” * 
In this matter, we have in different species successive stages 
in the development of the instinctive behaviour which is thus 
carried so far in Formica rufescens. Our English ants, of the 
species Formica sanguinea, have fewer slaves and are less 
dependent on them ; they can feed and forage for themselves, 
and during migration carry their slaves—which are of the 
same species as in the other case—instead of being carried by 
them. In the nests of the common wood ant or horse ant 
(Formica rufa) there are occasionally a few slaves. Lord Ave- 
bury thinks it likely that they are developed from larve or 
pup, originally taken for food, which have by chance come to 
maturity in the nest of their captors. 
But one more incident in the social life of ants can here be 
noticed—though many others could be given did space permit. 
The leaf-cutting ants of America form paths from their nests 
to suitable trees, from which to obtain the small coin-like leaf 
fragments, which they carry in the mandibles, and hence have 
gained the name of umbrella or parasol ants. These paths are 
sometimes underground ; and Mr. McCook measured one which 
ran at a depth of some 18 inches beneath the surface for 
448 feet, and was then continued for another 185 feet to’ the 
tree which the ants were stripping. The whole path was in an 
almost perfect straight line from nest to tree. The leaf 
fragments are stored in large quantities in the nest, and it was 
long a matter of uncertainty for what purpose they were 
collected. The problem was solved by Alfred Mdller, who 
found that the leaves, which are subdivided and masticated by 
a special set of workers within the nest, form the appropriate 
material in which the threads of a fungus ramify and flourish. 
This fungus is tended by the ants with great care, and is made 
to produce a specially modified form of growth, not found 
under other circumstances, in the form of white ageregations, 
termed by Moller “Kohlrabi clumps.” These form the 
principal food of the ants; and the spongy mass of earth and 
* Lord Avebury (Sir John Lubbock), “ Scientific Lectures,” pp. 78, 79. 
