Distribution of Arion hortensis S.S, and arion distinctus in Northern Switzerland

The distribution of the sibling slug species Arion hortensis s.s. and Arion distinctus in Northern Switzerland is described. In the sampled area, both species were almost equally abundant and their distributions overlapped. M ost of the sampled sites contained both species. Arion hortensis s.s. is mo re common in Switzerland than previously known. and its range extends more towards the east of Switzerland. Arion hortensis s.s. was more abundant in gardens than in the other habitats sampled. At sites where only A, distinct us occurred, the form with a bipartite oviduct prevailed. Ar sites where bo th species were present, the form of A. distinctus with a tripartite oviduc t was more common than the form with a bipartite oviduct. In captivity, one specimen of A. distinctus mated with one A. hortensis s.s., as indicated b y a spermatophore of A. distinctus that was found within the atrium of one A. hortensis s.s. A majority of the specimens could by assigned to the corr ect species by their colouration, but a safe distinction between A. hortens is s.s. and A. distinctus is only possible using characters of the reproduc tive system.