1^,1 



The Belt-Delpino hypothesis clearh' inlplit^ that unless pro- 

 tected by the Pseudomyrmas the acacias would be destroyed 

 either by the browsing cattle or by the leaf-cutting ants. Yet 

 it must be evident at once, to any one who sees these plants 

 growing in the >avannahs of Panama and Guatemala, that their 

 thorns alone w^nild protect them from the attacks of horses and 

 ■cattle, for these thorns are not weak or ineffective, as some writers 

 seem to imagine, but are, as Rettig says, at least as formidable 

 as the thorns of many species of hawthorn {Cratœgus) . I should 

 say that they are even more formidable than an^- of the thorns 

 I have seen on the numerous hawthorns growing in the Arnold 

 Arboretum. 



Most authors, however, dwell more on the leaf-cutting ants 

 of the genus Atta as the principal enemies of the acacias. Belt's 

 seedlings were destroyed by these pests, and I willingly admit 

 that a similar fate may occasionally overtake these plants in 

 Guatemala, although I never saw one attacked, even in Quirigua, 

 where both seedlings and older bushes grow in the clearings near 

 the large Atta formicaries, and almost in the beaten paths of the 

 ants. In other locahties, such as Zacapa and Patulul, I was often 

 unable to find leaf-cutters in any part of the extensive areas occu- 

 pied by the acacias. From my observations on these ants in 

 Panama, Costa Rica, Guatemala, Mexico, and Texas I am con- 

 vinced that their rôle as destroyers of plant-life has been grossly 

 exaggerated, and von Ihering and Fiebrig have acquired 

 the same conviction from their observations in Brazil and Para- 

 guay. The Attas do, indeed, occasional!}- defoliate trees or 

 shrubs, but they are not sufficiently numerous to do this over 

 any considerable area, nor so thorough h- and repeatedh' as to 

 endanger the existence of any native plant species. That they 

 often destroy introduced or cultivated plants, such as rose 

 bushes, is true, but these are grown in small, compact cultures, 

 and are not scattered over immense stretches of countr}-, much 

 of which is always quite free from Atta colonies. I conclude, 

 therefore, that the existence of the acacias as species is very far 

 from being endangered by the leaf-cutters. What, then, are the 

 great destroyers, against which such a bod}--guard of stinging 

 ants has to be levied, garrisoned, and fed by the acacias ? As 

 nobody is able to tell us, are we not justified in casting the onus 



