Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




Two species of thatching ants (Formica obscuripes and F. oreas comptula) are common to the ecotone and woodland habitats of the Forest River Biology Station near Inkster, Grand Forks County, North Dakota. The general ecology for each species was determined with emphasis on mound morphogenesis, colony-founding and population characteristics. For each species the following samples and data were collected: random samples of each population, thatch sample, external and internal mound measurements, and field maps of distribution patterns.

The morphogenesis of F. obscurlpes mounds was found to follow a predictable pattern with three well defined but transitional nest forms: (1) Stage I, characterized by a thatched dome without excavation, (2) Stage IT, characterized by a raised, thatched dome, earthen embankment and an excavated pocket containing thatch, and (3) Stage III, characterized by a depressed dome of deteriorating thatch, producing a crater-like effect.

Random samples of workers taken from selected mounds in various stages of growth indicated that the ratio of minors to majors increased with growth and aging of colonies. Minors made up a larger percentage of the sample taken from Stage III mounds as compared to Stages I and II.

Annual measurements of mounds over a the growth of mounds and population seven-year period showed followed a predictable growth curve. Stage I mounds were typically geometric In their rate of growth whereas Stage II mounds had an arithmetic growth rate. Stage III mounds showed a negative growth rate.

The distribution patterns of colonies were found to be isolated and random (prairie and woodland habitats), uniform (prairie habitat), and clumped (woodland habitat).

The means by which new colonies were established varied with the habitat. Indirect evidence indicates that isolated prairie and wood land colonies are probably founded by solitary queens following the marriage flight, while mounds within clusters were founded by the process of colony division. A relatively rare process, colony translocation, resulted in the abandonment of one nest in favor of another of recent origin.

Individual F. oreas comptula nests were, for the most part, asymmetrical in shape with little change in morphology (except size) between Stage I and Stage III nests. No relationship was noted between major/minor ratios and nest size. The sequence of events leading to colony senescence as observed in F. obscuripes was net found to apply to this species.

Clusters of F. oreas comptula nests, usually associated with dense stands of Kentucky blue grass, were not found to be composed of discrete colonies but were parts of a complex of interconnected nests. The movement of tagged workers and the transfer of brood between nests indicated that such clusters were polydomous. Isolated and randomly dispersed colonies were rare.

The establishment of new colonies was through the process of colony budding. Spatial separation occasionally resulted in the establishment of secondary polydomous clusters.