Abstract: Pleometrotic colony foundation of Lasius meridionalis (Bondroit, 1920) by Lasius paralienus Seifert, 1992, with comments On morphological and behavioural adaptions to socio-parasitic colony foundation (Hymenoptera: Formicidae). - Pleometrotic colony foundation in a temporary social parasite of the subgenus Chthonolasius is described for the first time. Eight dealate, freshly mated gynes of Lasius (Chthonolasius) meridionalis were observed in a nest of the host species Lasius paralienus at the Stilfser Joch in S Tirol / N Italy in August 1998. The observation represents the first direct evidence for L. paralienus to serve as host for L. meridionalis in addition to the main host L. psammophilus. Chorological data as well as structure and extension of some L. meridionalis colonies found in Central Europe suggest oligogynous-polydomous colonies for some of the sites. It is hypothesized that pleometrosis might increase the colony foundation success of L. meridionalis and could represent the initial step for the development of oligogyny instead of terminating the pleometrosis by lethal fighting of the queens. The deviating cephalic and mandibular morphology of all known Chthonolasius gynes, compared to the independently founding gynes of the subgenera Lasius s.str. and Cautolasius, is considered as synapomorphy that considerably increases the potency for lethal biting and disintegrating the host queen. A further adaption to socially parasitic colony foundation is the enlarged Dufour gland that is suspected as a main source for propaganda pheromones emitted to influence host workers.