Enikő CSATA 1 , Bálint MARKÓ 2 , Katalin ERŐS, Csilla GÁL, Anna-Mária SZÁSZ-LEN, Zsolt CZEKES Department of Taxonomy and Ecology, Babes-Bolyai University, Clinicilor 5-7 400006 Cluj-Napoca, Romania e-mails: 1 csataeniko@yahoo.com, 2 balintm@gmail.com (corresponding author) OUTSTATIONS AS STABLE MEETING POINTS FOR WORKERS FROM DIFFERENT NESTS IN A POLYDOMOUS NEST SYSTEM OF FORMICA EXSECTA NYL. (HYMENOPTERA: FORMICIDAE) POLISH JOURNAL OF ECOLOGY (Pol. J. Ecol.) 60 1 177–186 2012 Regular research paper ABSTRACT: The mound building narrow- headed ant Formica exsecta builds guarding out- stations, usually at the root of the aphid-housing plants. Colonies of this species can develop into huge polydomous systems, so-called supercolo- nies, made up of several hundred of related nests and outstations. Although the existence of these guarding structures is well-known, still there is little information available concerning their char- acteristics. In the frame of this study we analyze the characteristics and the integrative role of outstations within a large polydomous system of F. exsecta as a function of ant nest density. The study was carried out in Transylvania, Romania. Two areas with different ant nest densities were selected for the purpose of this study: high (HD) and low density (LD) sites. Parallel to the density of ant nests, the density of outstations was also higher at the HD site, while the size of outstations and the number of ants residing in them did not differ significantly between the two sites. Outsta- tions were usually maintained by a single ant nest at the LD site, thus the common exploitation of food resources was less frequent, whereas at the HD site outstations were used by more ant nests. This article provides the first evidence to shared use of outstations in F. exsecta. In the view of our results outstations, in addition to guarding aphid colonies, also function as permanent meeting points for workers from different nests, which contribute essentially to the integration of nests within a polydomous system. KEY WORDS: ants, aphids, foraging, Formi- ca exsecta, guarding, polydomy, supercolony 1. INTRODUCTION Ant hills are the most well-known above- ground ant-built structures. They serve as a nesting place for these social insects, contain- ing workers, gyne(s), eggs, larvae and pupae. Nevertheless, quite frequently, ants also build structures outside their nests, e.g. walled trenches, tunnels, bridges, protective pens, shelters, covers, byres, tents and outstations. These structures are distinguished from nests primarily by their role; they serve mostly to protect food sources or ensure the safety and economy of food retrieval (see Anderson and Mc S h e a 2001 for a comprehensive re- view). Some species, such as Dorylus nigri- cans, build walls around their trails leading to food sources (Anderson and McShea 2001), but such built-in trails or trenches are quite common in ants, including among the European species the Tetramorium caespitum or the Formica rufa species group (pers. obs.). In many cases ants build nest-like structures outside the nest to protect mutualistic homop- teran partners, e.g. Acanthomyrmex brunnea (Step 1924), Formica exectoides (Andrews journal 29.indb 177 journal 29.indb 177 2012-04-10 11:07:00 2012-04-10 11:07:00