384 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



reaction to the lower temperatures of the forest at that time, the 

 bivouac walls become tighter and better conserve heat produced in- 

 ternally (by the brood in particular) . Conversely, after dawn, when 

 increasing light excites growing numbers of ants to leave the bivouac 

 walls, as the raid grows, this wall thins out, usually develops small 

 apertures, and is undercut at the bottom. The effect is to increase 

 the internal air circulation as well as to cool the atmosphere in the 

 interior through evaporation so that the internal temperature of the 

 bivouac does not rise to the height reached by midday in the environs. 

 The incubation properties of the bivouac represent an important 

 factor in Eciton life, for with less regular atmospheric conditions in 

 the nest, the stages of brood development could not have their typical 

 regularity in timing. More than a limited irregularity in these bio- 

 logical processes would surely have important distorting effects in 

 the Eciton activity cycle. 



An Eciton colony is a complex organization that contains in its 

 regular membership not only a fertile queen and a great horde of 

 neuter females or workers, but also an immense brood of developing 

 individuals (usually all workers), and in addition many non-insect 

 guests or "ecitophiles" (beetles and others) ranging to parasitic forms. 

 The Eciton workers, which carry on the raiding and other labors of the 

 colony, make up most of the social unit and may number into some 

 hundreds of thousands as in the case of Eciton hurchelli. Colonies of 

 Eciton hamatmn usually are much smaller than those of hwchelli^ 

 and probably average not much more than 150,000 workers. In 

 strictly subterranean species, such as Eciton praedator and Eciton 

 crassicome, the populations seem to be larger on the whole, judging 

 especially from the durations of the emigrations. 



A population component of great importance in Eciton life is the 

 brood, which is always present at some stage of development in any 

 functional colony. Without their broods, these colonies cannot exist 

 very long. Throughout the year, at fairly regular intervals, the 

 population of the colony is increased by the emergence of successive 

 great new broods, which virtually always consist entirely of workers. 

 In the case of Eciton hrnnatimi we have counted more than 40,000 

 individuals in a single all- worker brood, probably the typical brood 

 size for that species. Single broods of Eciton hurchelli are much 

 larger, approaching twice the size of those of hamatum. 



Of course the colonies do not continue to grow indefinitely by the 

 addition of new broods. There are at least two measures of popula- 

 tion control exerted in the normal environment: one, losses involved 

 in the daily raiding forays, in which great numbers of workers are 

 lost in combat with insects whose nests are pillaged; the other, nat- 

 ural hazards such as exposure to environmental extremes, particularly 



