The yellow sac spider, Cheiracanthium punctorium (Villers, 1789), offers an interesting example of an expanding spider species. It is a comparably large, free ranging, and nocturnal hunter, living on fallows and meadows. Their historical European distribution was largely limited to warm climate regions in southern Europe with the southern Upper Rhine Plain marking the northern limit of its distributional range (Bellmann 2006). Beginning in the mid‐20th century, isolated occurrences have been reported from more northern latitudes in the eastern German plains and as far north as Sweden (Muster et al. 2008). These novel occurrences were geographically restricted to small, isolated sites. However, over the last decade, a marked expansion has been noted from these focal areas and the species is now found in very high abundances in most of northeastern Germany, southern Sweden, and the Baltic States. At the same time, large gaps are present between these sites, in which the spider is currently not found, for example, Poland, northwestern Germany, and Denmark (Muster et al. 2008; Jonsson 2005; Krehenwinkel, pers. observation, Talvi and Rutkowski pers. communication). The spread of C. punctorium is of special relevance, as it is one of very few Central European spider species whose venom can cause noteworthy effects on humans and may require medical treatment (Muster et al. 2008).