SOCIOMETRY AND SOCIOGENESIS OF COLONIES OF THE FIRE ANT SOLENOPSIS-INVICTA DURING ONE ANNUAL CYCLE

In social insects, colonies as well as individuals have evolving life histories. Identification of the life history tactics of a social inse ct requires data on colony attributes and their development. To this e nd a full range of fire ant (Solenopsis invicta) colony sizes was samp led and censused on seven dates throughout 1 yr. Data included: mound volume; the number, dry masses, and fat contents of sexual and worker adults and immatures; stratified nest temperatures; worker distributio n within the nest throughout the year; duration of the pupal stages; a nd respiration rates. Analysis showed: 1. Colonies reached their annua l maximum population size in midwinter and their maximum biomass in sp ring. During the spring sexual production period they declined to a mi dsummer minimum. Calculations showed that the magnitude of this declin e increased with colony size. During January to July, worker mortality exceeded natality, causing colony decline, while from July to Decembe r, natality predominated, causing growth. 2. Mound volume was closely related to the total mass of ants in the colony, and varied with seaso n paralleling the mass of ants. 3. The mean size and variability of wo rkers, and the percent major workers, increased with colony size and c hanged over the year. 4. The fat content (percent fat) of workers incr eased with worker size and colony size. Worker percent fat was lowest in summer after sexual production, climbed immediately to the annual m aximum and then declined gradually through winter and spring. 5. Altho ugh sexual male and female pupae were close in mean dry mass (2.55 mg and 3. 1 0 mg, respectively), males gained only 6% during adult matura tion while females gained 290%. Females gained fat more rapidly than l ean tissue causing their percent fat to increase from 31% to 49%. Mean mass of male and female sexual adults did not change with colony size . 6. The cost of worker maintenance declined from nearly 100% of total colony cost in winter to 46% in late spring when brood production pea ked. 7. Production rates peaked in spring, with colonies investing 50% of their daily production in sexuals. This peak production was not su stained through the summer, and was probably fueled by stored worker f at. Worker production dominated in the latter part of the summer. All measures of production rate as well as total annual production increas ed with colony size, but most did so less rapidly than colony size, re sulting in a declining efficiency of production and a declining natali ty rate. 8. The percent of annual production invested in sexuals incre ased sharply in colonies of between 20 000 and 50 000 workers, then re mained at almost-equal-to 33% for the remainder of colony growth, show ing that the transition from the ergonomic to the reproductive stages is sharp, and that colonies must grow in order to produce more sexuals . 9. Many quantitative colony attributes were related to one another b y differential growth, and can thus be seen as isometric or allometric measures. Rules of relative growth may thus constrain the possible co mbinations of attributes and their evolution. The methods of morphomet ric size and shape analysis are discussed as tools for understanding s uites of colony attributes, and comparing them among species. 10. The sociometric/sociogenic method is discussed as a way to compile, analyz e and compare data on social insect colony attributes and their growth and development.