Terrestrial isopods of the Ooijpolder: part 1. distribution (Crustacea: Isopoda: Oniscidea) The distributions and habitats of woodlice were investigated in ‘the Ooijpolder’, also named ‘the Ooij’. The area of approximately 41 square kilometers is located along the river Waal, east of Nijmegen, Province of Gelderland, The Netherlands (fig. 2). In total 1464 squares of 100 x 100 m (utm) (fig. 9) were sampled by hand from 1991 up to 1998 (Table 1). On a recording form (fig. 5) five first order habitats were distinguished (Table 2): 1. The river foreland is flooded on average for a few weeks once every winter or spring, and is bounded by a winterdyke. 2. The sandy river dunes in the flood plains are inundated only very occasionally. Three old brickstone factories are located here. 3. The Ooijpolder consists of sandy to heavy, young chalky river clays. Mainly rural, with some small villages and deciduous woods dominated by willow. 4. An area of very heavy, ill-drained clay soils (fig. 3), bounded by 5. 5. A lateral moraine deposited during the second last ice-age. Mainly mixed deciduous woodland dominated by oak and beech. On the recording form eight second order habitats were distinguished (see Table 2 for explanation). In the area 17 species of woodlice were found. Ligidium hypnorum was fairly common in wet habitats, especially wet ditches, canals and woodland, but it was absent on the lateral moraine. Surprisingly it proved unable to survive in the dynamic, periodically flooded foreland of the Waal (fig. 13). Haplophthalmus danicus was not widespread in the Ooijpolder, but at one particular site in a poplar woodland it occurred in extremely large numbers after a long period of heavy rains. Over a surface of 25 m2 several hundred thousands of animals were observed. Furthermore H. danicus has been found to show a clear preference for the clayey soils containing some amount of sand, as well as for the habitats richest in humus, e.g. dead wood and heaps of litter. Some records were from synanthropic sites such as compost heaps and gardens. However the habitats at the base of the lateral moraine seemed to be natural. Compared with H. mengii the species is more thermophilic. Haplophthalmus mengii is essentially a soil-dwelling species. The data on distribution and ecology show a striking resemblance with T. helveticus (fig. 20, 21). Their migration patterns were very similar (fig. 38). Of both species the females moved to the deeper levels of the soil when breeding. Haplophthalmus mengii had a long reproduction period, from the second half of april to the end of october. Interestingly the newly hatched juveniles of both species of Haplophthalmus had already developed the seventh body segment (fig. 22). The first population of Hyloniscus riparius for The Netherlands in natural habitats was found in the Ooijpolder in 1991 (Wijnhoven 1993). The distribution map (fig. 25) shows its strong association with the floodplains. Hyloniscus riparius proved to be able to survive submerged conditions for over eight weeks. It is suggested the species has arrived here rather recently by means of passive transportation by river debris or actively by expanding its range from Germany to the west. It is found in woodland litter and alongside ditches and canals, where it was often quite abundant. Until now Metatrichoniscoides leydigii is still primarily a ‘Dutch’ species. In the Ooijpolder it has been captured only a few times (fig. 30). It can be expected to be badly under-recorded because of its small size and soil-dwelling habit. It has been found mainly between 10 to 30 cm under the soil surface, often in soil types quite poor of humus. The pretty pink colored Trichoniscoides helveticus was my favorite species. It is essentially a soildwelling woodlouse. It was common in ditches and along canals. The animals’ migrations are illustrated in figure 38. Probably the heavy, ill-drained clay soils can not be colonised by T. helveticus because in cold winter periods the rising water level can reach the frozen surface layer (fig. 38). The breeding season started at the end of April up to the beginning of October. The low incidence of gravid females in populations, reported for several small trichoniscids, is undoubtedly a bias of collecting by hand. In the second part of this publication will be discussed in detail that most females migrate to deeper levels in the soil when breeding starts. These vertical migrations lead to a greater relative number of males near the surface levels of the soil. Trichoniscoides albidus is quite common in the western parts of The Netherlands. Towards the east the species progressively is less frequently recorded. In the Ooijpolder it seems to be introduced, because of its strong association with draining canals of fruit plantations. It prefers habitats with a high humidity, rich in humus. In the study area Trichoniscus pusillus pusillus was extremely abundant and widespread. Trichoniscus pusillus was acting as the most important host for IIV (Isopod Iridescent Virus) (Wijnhoven & Berg 1999). Only one male specimen was collected. Trichoniscus pygmaeus was one of the rarest species in the Ooij. It was captured once in a garden. The other sites, ditches and ponds, seemed to be natural. Yet T. pygmaeus is likely to be an introduced species. The distribution map of Oniscus asellus showes a preference for rural and wooded areas, whereas the domestic and urban sites were under-recorded. It was very common in association with dead wood and litter in a wide range of natural and synanthropic habitats. Philoscia muscorum, together with T. pusillus, was the most abundant species. It was found in dry roadside verges as well as in wet deciduous woods. The red colourform seemed to occur mainly in woodlands and shrubs. Platyarthrus hoffmannseggii is known for its association with ant nests. In the clayey soils however it was frequently found without any ants or nests nearby. It is essentially a specialised soil-dwelling species. Like the small trichoniscids it seemed unable to colonise the heavy clay soil area (fig. 58). The species is probably common and widespread in the Ooijpolder, thus being badly under-recorded. Armadillidium vulgare was recorded from synanthropic habitats like old brickstone factories. In ‘natural’ habitats it is associated with the more or less calcareous microsites. Regularly adults and juveniles were observed living in quite humid conditions such as wet ditches and canals. Porcellio scaber was less abundant than expected. The distribution map shows its preference for the drier soiltypes. In synanthropic habitats the species always dominated, together with O. asellus. In woodlands on the free-draining slopes of the lateral moraine it was found most frequently. As reported by many authors Porcellio spinicornis essentially is synanthropic in the northern regions of Europe. In the monitored area its xerophilic nature is reflected by the distribution map (fig. 66). Although large and conspicuous, it is often difficult to find at daytime. Yet, on the whole P. spinicornis seemed rare due to the scarseness of suitable habitats. It was common at the old brickstone factories. Once it has been found in a greenhouse and in a church. The other records are from gardens where it occurred on walls with mortar. There is no doubt that the urban habitats were under-recorded, since in the city of Nijmegen the species was widespread. Porcellionides pruinosus, like P. spinicornis, is a synanthropic species. However, in contrast to P. spinicornis its presence was primarily related to sheep dung and heaps of mushroom compost. Once it was found in crevices of brickstone churchwalls. The distribution map of Trachelipus rathkii (fig. 72) shows a vicariant pattern with those of P. scaber and O. asellus. In the sequel to this paper will be discussed that this might well be the result of competition. Trachelipus rathkii, like H. riparius, can survive submerged for several weeks, explaining its abundance on the flood plains. In the polder it was recorded mainly in the open fields, avoiding wooded areas. Part 2 of this paper will be published in a next volume of Nederlandse Faunistische Mededelingen. The records will be analysed in detail, e.g. relations with soil types, groundwater level and the amounts of calciumcarbonate in the soil. The first and second order habitats will be discussed, as well as: the association of the mite Bakerdania elliptica with Oniscus asellus, the fenology of woodlice, breeding biology, sex ratio, deseases, parasites and predators.