Caste-biased Polyethism in a Mound-building Ant Species

Abstract

Formica obscuripes Forel, the western thatching ant, a mound-building species found at Illinois Beach State Nature Preserve, has three morphological castes (major, media and minor, in order of decreasing size). Observations on the activities of each caste were made from May to September 1975, June 1976, and September 1976. Behaviors were categorized and frequencies of the three castes exhibiting each behavior type were calculated. Caste strongly influenced occupational repertoires of the ants. Repairing the nest, carrying dead conspecifics and bringing in prey were performed primarily by majors, whereas medias carried brood and foraged on vegetation for honeydew from aphids or plant sap. Minors were seen outside the nest too infrequently to allow a detailed analysis of their activities. I NTRODUCTION Colonies of social insect populations are intriguing for behaviorists because of their high degree of organization. The performance of numerous tasks is required for successful colony maintenance, and division of duties among individuals has been demonstrated for bees (Brian, 1952; Free, 1965), wasps (Akre et al., 1976; Potter, 1964) and ants (Weir, 1958; Wallis, 1965). Oftentimes, workers assume certain duties as a result of morphological specialization. For example, army ant soldiers with huge heads and jaws serve primarily for defense while smaller individuals assume the domestic chores of brood rearing, nest cleaning and so forth (Schneirla, 1971). However, a generalized relationship between polymorphism and the division of labor, or polyethism, cannot be defined for those groups displaying continuous size-frequency distributions (Wilson, 1971). This paper reports a morphology-behavior association for the western thatching ant (Formica obscuripes Forel) at Illinois Beach State Nature Preserve in Lake County, Illinois. METHODS Workers collected from colonies at Illinois Beach were examined in the laboratory. Morphological measurements were made with the use of an ocular micrometer mounted in a dissecting microscope. Head width across the eyes, head length from back to mandible base, and alitrunk length from head to petiole were recorded in micrometer units and subsequently converted to millimeters. A total of 241 individuals were measured and then assigned to castes according to a decision function using both size and coloration: majors, the largest-sized group, have red heads and thoraxes; the smaller medias have reddish streaks on the heads and rarely on the thoraxes; minors, the smallest worker group, are uniformly black. Data on division of labor were collected in the field during 2 successive summers. Data from 1975 were compiled from incidental observations throughout the season. Over 2-7 days in both June and September 1976 each of the nests was watched for 5-10 min during mornings and late afternoons when overall activity was high. Workers were assigned to a caste as outlined above. Records of nest location, caste designation and behavioral activities were tallied throughout the observation periods. CASTE MORPHOLOGY Workers of Formica obscuripes exhibit continuous size variation; nonetheless, 'Present address: Department of Biological Sciences, Stanford University, Stanford, California 94305.