ARMY ANTS — SCHNEIRLA 385 



in the dry season. The result is that on the Avhole, during most of the 

 year, the majority of colonies do not change greatly in size, notwith- 

 standing the regular addition of large worker increments from new 

 broods. 



There is one major departure from the annual series of all-worker 

 broods. In the last months of the rainy season, evidently a time of 

 exceptional plenty for most colonies of army ants, many Eciton col- 

 onies seem to increase their colony populations very appreciably. 

 There often follows a further exceptional occurrence, namely, the 

 production of a bisexual brood consisting of a great many males and 

 a very few queen individuals. Such broods occur most frequently in 

 the first third of the dry season. Through an intricate process which 

 we have studied in some detail, production of the sexual brood leads 

 to a two-way division of the colony, another process of importance in 

 population control. 



Figure 1. — Representative workers from the population series of an Eciton hamatum colony. 

 Differences in size and structure through the population are represented here by individuals 

 arbitrarily selected from the minim and major extremes and the intervening 20-percentile 

 points. Body lengths: minim worker, 2.9 mm.; major worker, 9.9 mm. 



The ecitons exhibit in an outstandingly regular way that important 

 social characteristic, polymorphism — the occurrence of morphological 

 differences through a population of one sex, in this case the female 

 sex. The workers present differences in both size and form. The 

 main differences are shown in figure 1. The worker series from the 

 largest through the intermediate to the smallest size is not a broken 

 series of radically different types but a smooth and gradual one on the 

 whole. The five worker types of Eciton hamatum shown in the figure 

 represent arbitrary steps in a regular series ranging from the smallest 

 or minim workers (about 2.9 mm. in length) to the largest or major 

 workers (about 12 mm. in length). The largest workers have dispro- 

 portionately large heads and anterior bodies, an anatomical peculi- 

 arity which ends, as in a burst of glory, with the enormous head and 

 mandibles of the major workers. These individuals are character- 

 ized by their huge sickle-shaped mandibles and by the great strength 

 which can be exerted in closing and holding them shut. The jaws, 

 together with the stings and poison glands, make the major workers 

 formidable foes of any intruders into bivouacs or raids. 



