Arachnologische Mitteilungen 56

2 S. Toft by individual short-distance dispersal, induced by a surplus population being produced near the border of the original range. Alternatively, a species’ range may expand as a result of long-distance displacements performed by a few individuals that successfully settle and establish themselves at a place far outside the normal range limit. Such pioneer (bridgehead) subpopulations may eventually become incorporated in the species’ newly enlarged range by a combination of the two dispersal mechanisms: repeated jump dispersal events from the original range combined with local short-distance disper- sal by offspring of the original colonizers. No direct informa- tion exists on the mechanisms of migration of these harvest- men; in accordance with Noordijk et al. (2007) and Vestbo et al. (2018) it is assumed that transportation by human traffic (trucks and cars) is the most likely means of long-distance dispersal for these harvestmen. Below I will also discuss the suggestion by Noordijk et al. (2007) that wind dispersal may be involved. Material and methods As the immigrant harvestmen were first discovered in urban settings and the animals are most easily observed on house walls, churchyard walls and similar vertical structures, regis­ trations were made as surveys of cities and towns. Each regis- tration was conducted as a one hour walk through (part of ) the town, proceeding at a normal walking speed of ca. 3 km per hour, though some of the towns visited were too small to allow a full one-hour survey. Areas with plastered houses painted in light colours (white or yellow) were preferred, as the harvestmen are most easily discovered on such surfaces. In most cases, this means that older parts of the towns (from early 20 th century) were included if available. These quarters also had the advantage that the house walls often faced the pedestrian pavement without enclosed gardens in front. All harvestmen seen between ground level and 2 m height were collected in 70 % alcohol and later identified under the bino- cular microscope. At some localities these surveys were conducted repea- tedly between 2008 and 2017. In the two years 2010 (Toft & Hansen 2011) and 2017, they included a large number of towns covering most of the country (61 and 64 locations, respectively). All records have been submitted to the public database Naturbasen (2018). In the distribution maps, the data from these surveys are supplemented by observations re- ported by others to the database. The pattern emerging from my own data and from the combined data set are the same, except that the latter provides a more complete geographical coverage. An attempt, admittedly inaccurate, was made to estimate the rate of expansion of D. ramosus in northern Europe, using the locations of the first records of the species in the Nether- lands and Germany and the early finds from Denmark. The distance from Ede (Netherlands) and Bochum (Germany) to Årslev (Funen, Denmark) was divided by the number of years between the respective finds (14 and 4 years, respectively). Si- milar calculations were made between the Ede and Bochum finds and the most remote (i.e. furthest away from assumed places of origin) Danish finds (Copenhagen, easternmost point of Zealand, 2009; Skagen, northernmost tip of Jutland, 2010) (names of main Danish regions/island indicated on Fig. 1A). Dicranopalpus ramosus is an extremely characteris- tic species due to its unique position of the legs during rest (all four legs directed straight to the side). Therefore, it will draw the attention of active field naturalists; they can easi- ly recognize it and documentation by photos is unequivocal. In Denmark, the arrival of the species was anticipated (Toft 2004). It is therefore unlikely that the species had been pre- sent, widespread and abundant in Denmark before its first discovery. Similar arguments may refer to the situation in the Netherlands in the 1990s and to the records of O. spinosus . Furthermore, several locations in Denmark where D. ramosus later turned up were surveyed in 2003 without any trace of the species (Toft &Hansen 2011).Thus, in both countries the species may have first invaded a few years prior to discovery, but the time lag between arrival and discovery may be appro- ximately the same. Results Already in 2010, D. ramosus had arrived to many parts of Denmark, but it was also absent from large areas (Fig. 1A; Toft & Hansen 2011). This early distribution can best be characterized as a widely scattered (“patchy”) occurrence. At all locations where it was present the species was infrequent, Fig. 1: Known distribution in Denmark of ( A ) Dicranopalpus ramosus at the end of 2010 (from Toft & Hansen 2011), ( B ) D. ramosus at the end of 2017. Star indicates point of first Danish record (2007). Closed circles: D. ramosus present. Open circles: localities searched, but D. ramosus was not found