142 



THE FACTORS 



[Part I 



ants, or rather they would be so if they were not kept at a distance 

 by their relatives which form the defensive army. They show such 

 a preference for the foliage of Cecropia, that, at Blumenau, Fritz Miiller 

 and I found every one of the uninhabited trees, which are rare, had its 

 leaves bitten down to the midrib, whereas not a single tree with a 

 protective army of ants showed a trace of such injury. Only during 

 the lowest winter temperatures is an inhabited tree exposed to its enemies, 

 for the protective ants are much more sensitive to cold than are the 

 parasol-ants 1 . Other ani- 

 mals, as it appears, are not 

 kept off. Caterpillars occur 

 on the tree, though without 

 doing much damage, and 

 the sloth exhibits such a 

 preference for it, as to re- 

 ceive the same name (im- 

 bauba) in Brazil. None of its 

 enemies, however, can com- 

 pare with the parasol-ants 

 in destructiveness. 



A closer investigation 

 proves that the imbauba-tree 

 provides its guests with a 

 dwelling and food. The cen- 

 tre of the stem is traversed 

 by a transversely divided 

 cavity, which increases in 

 calibre from below upwards 

 like a funnel, corresponding 

 to the increasing thickness 

 of the growing apex, so that 

 the uppermost chambers in 

 the stem are larger than 

 those represented in our 

 illustration (Fig. 77). The 

 cavity, and therefore the 

 dwelling-place of the ants, in spite of its great utility, is not an adaptation 

 to the guests ; it represents rather a feature that is common to many 

 other plants, and may be explained on the mechanical principle of 

 construction as being the method of producing the greatest resistance 

 to bending with the least expenditure of material. The dwelling existed 

 before the symbiosis. It is otherwise with the entrances to it. Here 



1 Alf. Moller, op. cit, p. 82. 



Fig. 77. Cecropia adenopus. Portion of a young stem 

 split longitudinally. Central hollow with septa per- 

 forated by the ants, and structures made by them. Natural 

 size. 



