heavy showers as much as from marauders. Sting- 

 ing ants inhabit the hollow stems of sapling cecropia 

 trees and the swollen bases of leaf petioles of the 

 leguminous tachigalia tree, and for this protection 

 are supposed to defend the trees against the attacks 

 of the leaf-cutting ants. 



Many other interesting coactions exist. Collared 

 peccaries on Barro Colorado Island in the Canal 

 Zone make narrow trails through the bushy tangles 

 and dense undergrowth and proceed single-file from 

 one place to another. These well-defined paths are 

 utilized also by the coati, octodonts, and several kinds 

 Mt marsupials which probably could not otherwise 

 penetrate these areas (Enders 1935). The tapir is a 

 trail-maker in the South American jungle, and the 

 elephant in Africa ploughs its way through the forest 

 by sheer strength. The trails it breaks are followed 

 at later times by the hippopotamus, rhinoceros, 

 buffalo, lion, leopard, hyena, pig, and baboon, which 

 in turn make the trail more passable for lesser forms 

 (Hesse, Allee, and Schmidt 1951). 



In contrast with the sporadic occurrences of most 

 species, ants and termites are abundant in the Amer- 

 ican tropics. The leaf-cutting ants and the insect- 



eating army ants are especially characteristic (Belt 

 1873). Termite nests occur in all strata, and the 

 wood-eating habits of these insects hasten the de- 

 struction of woody materials. The Hymenoptera, 

 Diptera, and Coleoptera are, in general, the best- 

 represented groups among the insects (Briscoe 

 1952). 



With temperatures high and uniform throughout 

 the year, the developmental period of cold-blooded 

 animals is shortened, and there is a general speeding 

 up of the life-cycle. Insects possessing only a single 

 generation per year in the north temperate zone 

 may complete their life cycle in 3-4 weeks in the 

 Tropics, and may have several generations during 

 the year. On the other hand, high uniform temper- 

 atures are depressing for the metabolism and ac- 

 tivities of warm-blooded animals, and the pace of 

 their activities is comparatively slow. 



Cold-blooded animals, particularly reptiles and 

 arthropods, reach their largest adult sizes in the 

 tropics. In the Amazon forest there is a spider large 

 enough to catch and feed on small birds that are 

 caught in its web (Bates 1864). Some moths have a 

 wing-spread of 30 cm : a millipede reaches a length 



Tropical biomes 347 



