Hooiwagens op een Nijmeegse muur (Arachnida, Opiliones), II

During an inventory in 2009 of a 100 meter long wall bordering an old cemetery in Nijmegen, the Netherlands, all sighted Opilionids were recorded. The results can be compared with a similar research, carried out in 2005 (Wijnhoven 2006). From the beginning of June 2009 up to the end of December the site was visited once a week, 21 times in total. A total number of 339 harvestmen was found, belonging to six species: Phalangium opilio Linnaeus, 1761, Opilio canestrinii (Thorell, 1876), Oligolophus hanseni (Kraepelin, 1896), Paroligolophus agrestis (Meade, 1855), Odiellus spinosus (Bosc, 1792 and Dicranopalpus ramosus (Simon, 1909). Almost 80% of the records belonged to Dicranopalpus ramosus and Opilio canestrinii, two rather recently arrived species in the Netherlands. In 2005 this was 75%. It is suggested that the mentioned two species are in a process of „taking over‟ in man made habitats at the cost of other species such as Leiobunum rotundum, Oligolophus hanseni and Paroligolophus agrestis. Due to competition with O. canestrinii, Opilio parietinus, a species that is associated with man made environments, already has become extinct in the Netherlands, with last records of 2006. The first occurrence of longer periods of frost indicates the end of the remaining harvestman populations. In 2009 this was half December, in 2005 the end of February. Males of O. canestrinii averagely occupy higher levels on the wall than females, suggesting that males show territorial behaviour by monopolising the higher sites during the night.