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

Summary

Atta sexdens was selected for close study because its caste and polyethism systems, which are among the most complex found in all ants, can be expected to provide an estimate of the upper limits of colonial organization in these insects.

A total of 29 tasks were identified. These are performed by an estimated four physical castes, of which three are further subdivided into temporal castes to make a total of at least seven castes overall (Figs. 1–11 and Table 1).

The physical castes of A. sexdens were defined on the basis of ‘role clusters,’ which are more or less segregated or distinctively shaped sets of polyethism curves. The four castes can be characterized broadly according to role as gardener-nurses, withinnest generalists, forager-excavators, and defenders, respectively (Fig. 12). This discretization is only partial, and finer, arbitrary subdivisions at the lower end of the size scale are possible.

The physical castes have been generated by a large evolutionary increase in size variation accompanied by the adoption of relatively elementary rules in allometry and alloethism. The increase in caste diversity beyond that possessed by monomorphic attines serves the relatively narrow primary function of cutting and treating fresh vegetation for use as the fungal substrate. The material is processed and the fungus cultured in an assembly-line fashion, with the succession of tasks (gathering, degrading, and implanting of substrate, followed by implanting and care for the fungal hyphae) being performed by ever smaller workers (Fig. 13).

The rules of allometry and alloethism create far less caste diversity than would be possible by a few additional minor evolutionary adjustments in larval growth and allometry during adult development (Fig. 17). Evidence is presented for the existence of secondary roles possessed by various size ensembles in addition to the primary roles, and for the suboptimal performance of a few size classes. These two phenomena are considered to result from the simplicity and hence restrictiveness of the allometry and alloethism rules (see Discussion).

Polyethism has evolved further than polymorphism: the alloethic curves rise and descend more steeply than the size-frequency distributions and they are generally steeper than the allometric curves drawn for any selected pair of physical measurements.

A technique is introduced by which the functions of previously unstudided or poorly understood organs can be inferred. It consists of noting in which size classes the organs are maximally developed, then determinig the labor specializations of the size classes. The method is shown to work in the cases of pronotal spination, poison sac, postpharyngeal gland, metapleural gland, and mandíbular gland, and it is extended in a speculation concerning the functions of other exocrine glands (Fig. 16).

A distinction is made between fixed tasks, which are attended by relatively limited age-size ensembles of workers, and flexible asks, which are attended by ensembles that vary in size (and perhaps age) according to the objects treated during the performance of the task (Fig. 15).

The concept of the primitive caste is introduced; it is the size class that most resembles related monomorphic species and performs the most generalized services for the colony. Evidence is presented that in A. sexdens this class is composed of workers with head widths around 1.4–1.6 mm.