Caste and division of labor in leaf-cutter ants (Hymenoptera: Formicidae: Atta)

SummaryThe size-frequency distributions of workers were followed in A. cephalotes colonies from the beginning brood (collected in Costa Rica) through 4 years of growth in the laboratory to a worker population of up to 64,530. Workers in the founding groups had a nearly uniform distribution, with a range in head width of 0.8–1.6 mm (Figs. 1–3, Table 1). This range is the minimum required both to harvest fresh vegetation and cultivate the symbiotic fungus. Thus in founding the colony the queen produces close to the maximum mumber of individual workers that can collectively perform all of the essential tasks.By the time the worker population reaches approximately 500, the size-frequency distribution has begun to shift to the distinctively “adult” form seen in much larger colonies. Yet over a span of 3–4 years, encompassing the growth of the incipient colonies into the largest studied, the pattern of energy investment remains relatively little changed, with the maximum investment placed in the large minor to small media size classes of head width 1.0–1.6 mm, in other words the minimally essential size range (Figs. 2, 3).In spite of the large shifts in size-frequency distribution during the earliest stages of colony growth, only relatively minor shifts occur in division of labor (Table 2).The question was posed: which is more important in the ontogeny of the caste system, the size of the colony or its age? In order to provide an answer, I selected four colonies 3–4 years old and with about 10,000 workers and reduced the population of each to 236 workers, giving them a size-frequency distribution characteristic of natural young colonies of the same size. The worker pupae produced at the end of the first brood cycle possessed a size-frequency distribution like that of small, young colonies rather than larger, older ones. Thus colony size is more important than age.