Division of labor in fire ants based on physical castes (Hymenoptera: Formicidae: Solenopsis).

Abstract

?Division of labor in two species of fire ants, Solenopsis invicta and S. geminata, was studied by a cross-indexing method that employed (1) the description of the total behavioral repertory of each principal size class and (2) the plotting of the frequency distributions (polyethism curves) of the size classes engaged in each role. The change in behavior as a function of size was further analyzed with the aid of alloethism curves, which were based on the data on polyethism. The various roles of the were elucidated by field and laboratory experiments that involved several kinds of stress and foraging opportunities. S. geminata, which is the more advanced species with reference to the anatomy of its larger size classes, is also more advanced in the overall pat tern of division of labor. The larger medias and majors are specialized for the milling of seeds, the majors so much so that they have the smallest behavioral repertory (two categories only) thus far recorded for a social insect. The trend toward the specialization of the larger size classes is already apparent in S. invicta but has been carried to extreme lengths in S. geminata. The difference is associated with a greater readiness on the part of S. geminata to feed on seeds both in the field and under labora tory conditions. Through all size classes of S. invicta and the minors and small medias of S. geminata, the substantial size variation is correlated with the size of particles carried by individual workers during nest excavation and forag ing. Finally, the smallest minor workers of both species concentrate more on brood care than do the larger size classes. One of the least explored phenomena in the organization of insect societies is also the most central: polyethism, or the division of labor. A comprehensive theory based on evolutionary optimization has only recently been completed (Oster and Wilson, 1978). In the case of and termites, which have the most advanced systems, entomologists have only begun to explore such subjects as the differences in behavior among size classes of continuously size-variable worker populations and the changes of role by single individuals with aging. The present article, a close analysis of the first of these two aspects, is designed to contribute to the understanding of the relationship of division of labor to economic ef ficiency. This content downloaded from 157.55.39.67 on Wed, 27 Apr 2016 04:50:24 UTC All use subject to http://about.jstor.org/terms 616 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY The insects chosen were two species of fire ants: Solenopsis (S.) gemi nata (Fabricius), the fire of the extreme southern United States (a species that also ranges throughout the American tropics and has been introduced by man into Africa, Australia, and other Old World tropical localities), and Solenopsis (S.) invicta Buren, the red imported fire ant, which is native to southern Brazil but has been introduced into the southern United States. These and other species of fire comprise the subgenus Solenopsis of the genus Solenopsis, distinguished from the thief ants of the subgenus Diplorhoptrum and other minor groups, the subgenera Dia gyne, Euophthalma, and Oedaleocerus. All fire are native to the New World, with the greatest concentration of species occurring in tropical and subtropical South America (Creighton, 1930; Allen et al., 1974). They are distinguished from other members of the genus by their larger size and by polymorphism in the worker caste, a trait shared with only a few species of Diplorhoptrum. Solenopsis invicta and S. geminata were chosen for detailed comparison because they clearly represent two stages in the evolution of the worker caste. S. invicta, in common with all other fire ant species except S. gemi nata, exhibits weak polymorphism: there is substantial size variation, but allometry is weak and the size-frequency distribution of workers is con tinuous and either skewed to the larger size classes or at most weakly bimodal with larger workers being less numerous (for a discussion of these variables and the evolution of polymorphism in generally, see Wilson, 1971). S. geminata has a much more advanced form of caste differentia tion. Although the size-frequency distribution of its workers is continuous and at most weakly bimodal, the allometry of the head is very strong. S. geminata majors are peculiar in form, possessing massive, quadrangular heads with strongly incurved, toothless mandibles. The essential features of polymorphism in the two species are illustrated in Fig. 1. My research was initiated with two key questions in mind. First, what is the form of polyethism (if any exists) associated with elementary physical systems of the Solenopsis invicta type? And second, why has S. geminata made such a radical departure from the baseline allometry of its congeners? These two fire ant species span a considerable portion of the total variation in physical caste systems found in as a whole. Discovering the basis for the difference might shed light on the evolution of worker sub castes in general. In order to examine polyethism more closely, three descriptive pro cedures were utilized: (1) An estimation of repertory distributions, based on a catalog of behavioral categories observed in each caste and the fre quency distribution of acts within each caste; (2) an estimation of poly ethism distributions, consisting of the frequency of various castes engaged in each discrete task performed in the service of the colony; and (3) an esti This content downloaded from 157.55.39.67 on Wed, 27 Apr 2016 04:50:24 UTC All use subject to http://about.jstor.org/terms VOLUME 51, NUMBER 4 617