SOCIAL LIFE 221 



the Guiana forests. These are small, narrowly elongate 

 insects with clubbed feelers and short legs, which, after 

 entering the hollow stalks, live and feed along strands of 

 especially nutritive tissue ; after a time their excrement 

 accumulates in longitudinal ridges adjacent to the ** food- 

 grooves." Following the beetles, large numbers of small 

 mealy bugs (Pseudococcus) invade the hollow stalks and 

 begin also to feed along the nutrient strands ; then the 

 beetles and also their larvae go to these mealy-bugs for 

 nourishment, stroking with their feelers the little white 

 sucking insects and inciting them to discharge from their 

 intestines the sweet honey- dew. When two or more 

 beetles," writes Wheeler, '' or two or more larvae or a group 

 of beetles and larvae happen to be engaged in stroking the 

 same mealy-bug, they stand around it like so many pigs 

 around a trough, and the larger or stronger individual 

 keeps butting the others away with its head." From this 

 account it seems that the " self- regarding " instincts are 

 not wholly eliminated in the social life of these beetles of 

 the Tachigalia trees. Wheeler, in his account of these 

 insects, lays stress on the fact that they are found in the 

 leaf- stalks only so long as the trees are young enough to 

 form part of the forest undergrowth. '* The older trees . . . 

 have all their petioles inhabited by viciously biting or stinging 

 ants." These latter invade the leaf- stalks as the tree grows, 

 driving out the beetles, but preserving the mealy-bugs 

 and adapting these '' cattle " to their own use. It is well 

 known that many societies of insects, and especially ants, 

 harbour a miscellaneous assemblage of '* guests," some of 

 which are clearly of service to their *' hosts." It is of much 

 interest to trace in the succession of insect inhabitants of 

 the leaf- stalks of these tropical American trees the varying 

 relations between the plants, the mealy-bugs, and the two 

 strikingly diverse types of communities, first beetles and 

 then ants, which successively make use of other organisms 

 for obtaining shelter and food. 



We may now pass on to consider the social life of those 



