1988] O'Neill — Trail patterns of Formica obscuripes 1 1 
absence of trails in F. opaciventris (Scherba, 1964) and at times 
when apparently temporary trails are formed in F. ulkei (Talbot, 
1961). When trails do exist in polydomous colonies they seem to 
function both as pathways for worker movement among nests and 
to foraging areas. Trail patterns may also be correlated with the 
shape of a colony’s territory both in Formica (Mabelis, 1979b; 
Skinner, 1980) and other ants (Holldobler, 1974, 1976) and, thus, be 
a product of both resource distribution and interactions among 
neighboring colonies. Habitat heterogeneity influences trail pat- 
tern, when the workers construct trails to avoid or cross unused 
areas of habitat (Reyes, 1986) and when features, such as roads or 
railway beds, define the pattern of usable habitat. The trail patterns 
observed in the present study suggest that habitat heterogeneity and 
the distribution of nests and Homoptera bearing plants influence 
trail location. 
Summary 
Workers of the ant Formica obscuripes Forel, at a site in south- 
western Montana, used a system of trails to travel between different 
nest mounds and between nests and foraging areas (primarily 
patches of plants bearing honevdew secreting Membracidae and 
Aphididae). Different groups of nests, served by non-overlapping 
systems of trails, apparently constituted polydomous colonies. 
Movement of workers among mounds was non-random both within 
and between nest groups. In a mark-recapture study, 97% of the 
workers recaptured on nests other than those on which they were 
marked were found on mounds within the same trail system. Exper- 
iments in which workers were transferred between mounds demon- 
strated that ants tolerated workers from mounds within their own 
nest group, but usually acted aggressively towards workers from 
other nest groups. The trail patterns remained stable during the 
three-month study and connected nests up to 135 m apart. The 
results are compared to those obtained in other studies of Formica. 
Acknowledgments 
This work was supported by the Montana Agricultural Experi- 
ment Station (grant MONB-155). Ronald Lang, Montana State 
University, determined the ant species and J. P. Kramer, Systematic 
Entomology Laboratory, U.S.D.A., Beltsville, Maryland deter- 
