Ins. Soc. 43:47-51 (1996) 1015-1621/96/010047-05 $1.50 + 0.20/0 9 1996 Birkh~iuser Verlag, Basel Research article Haplometrosis and pleometrosis in the ant Acromyrmex striatus (Hymenoptera: Formicidae) E. Diehl-Fleig 1 and A. M. de Aratijo 2 1 Laborat6rio de GenOtica, Departamento de Biologia, Universidade do Vale do Rio dos Sinos, 93.001-970 Sao Leopoldo, RS, Brasil 2 Departamento de Gendtica, Instituto de BiociOncias, Universidade Federal do Rio Grande do Sul, 90.001-970 Porto Alegre, RS, Brasil Key words: Colony foundation, haplometrosis, pleometrosis, Acromyrmex striatus, Attini. Summary Laboratory tests with mated females of the leaf-cutting ants Acromyrmex striatus (Myrmicinae, Attini) were conducted to determine if the colony foundation is a solitary or a mutualistic process. We have also tested the effect of foundresses density and number of available areas in this process. Three bioassays were performed: with single foundresses; with paired foundresses; and with groups of 8 to 18 foundresses. The results suggest that the foundation can be by haplometrosis or pleo- metrosis, possibly depending on physical distances between or densities of foundresses. Foraging activity was common in haplometrotic queens and in foundress pairs, but no foraging occurred in groups of foundresses. Introduction All ant species are eusocial and exhibit many different ways of starting their colo- nies. While apparently in many ant species method of colony foundation is fixed and haplometrotic, in many other species method of colony foundation is population- specific, e.g. haplometrotic and pleometrotic populations occur in Formica (Pamilo and Rosengren, 1984), Acromyrmex versicolor (Rissing et al., 1986, 1989; Rissing, pers. commun.) and Messor pergandei (Rissing and Pollock, 1991). Most ant colonies are started by a single female (haplometrosis). This process can be claustral when the foundress raised her brood with stored energy reserves or can be semi-claustral, with the foundress foraging (Buschinger, 1990). A number of species, however, groups of females start colonies (pleometrosis). This process