Folia Historico-Naturalia Musei Matraensis - A Mátra Múzeum Természetrajzi Közleményei 10. (1985)
De Wilde, J. J. — Van Goethem, J. L. — Marquet, R.: The distribution of the species of the genus Deroceras Rafinesque, 1820 in Belgium (Gastropoda, Pulmonata, Agriolimacidae)
MATERIAL AND METHODS In total more than 12,000 specimens representing circa 2,600 lots of Deroceras species were identified and recorded on U.T.M. grid maps with 10 x 10 km squares. This material comprises all Deroceras specimens present in the collections of the Koninklijk Belgisch Instituut voor Natuurwetenschappen /K.B.I.N. /. The total number of 10 x 10 km U.T.M. swuares in Belgium is 375. This number includes the trapezoidal partitions at 6° E, as well as the peripheral squares covering not only Belgian territory but also a part of a neighbouring country. At the end of 1982, 94% of all U.T.M. squares in Belgium were sampled. The percentages in the upper right corner of the distribution maps indicate the number of positive U.T.M. squares for the concerned species compared with the total number of sampled U.T.M. squares for the same year. Legends • : Pre 1950, collected alive or observed alive. • : 1950 onwards, collected alive or observed alive. • • : Records concerning a single particular year as indicated. RESULTS All records of landsnails before 1972 /based on the collections of the K.B.I.N./ are indicated on fig. 1. About 50% of all U T M squares had been sampled at that time, most of the records covering the period from 1935 to 1950. Figs 2-3 present the known distribution patterns before 1972 of D. reticulatum and D. laeve, both species being widespread in Belgium, although D. laeve is less common than D. reticulatum. D. caruanae was recorded for the first time in Belgium only in 1968: Brussels /Woluwe/, in garden, 1 juvenile 6 adult specimens, leg. J. VAN GOETHÉM & A. LIEVROUW /see VAN GOETHÉM, 1974/. In the next three years the species was, as far as we know, not found elsewhere in the country /fig. 4/. For the period 1972-1982, series of maps /figs 5-48/ are given showing year by year all records of landsnails /=sampled U T M -squares/ gathered by the Recent Invertebrates Section /K.B.I.N./ and by the "Laboratorium voor Algemene Dierkunde /R.U.C.A./". Comparison of the corresponding maps compiling all the records for a given species, confirms the widespread occurrence of D. reticulatum and D. laeve in Belgium. The percentages of the positive U T M squares for both species vary slightly. These variations can be explained by the fact that year after year not always the same degree of diversity in biotopes could be taken into account by sampling landsnails. On the other hand, very dry weather conditions during certain years might be responsible for low percentages of D. laeve /e.g. figs 19,23; 1976 = exceptionnally dry summer/. After its first discovery in Belgium, D. caruanae was recorded again in 1972 in 6 different U.T.M. squares /fig. 8/. From then, the number of records increases rapidly each year. With a few exceptions /1978, 1980/ this species was found every year even more frequently than D. laeve. Although in 1978 the survey was almost exclusively concentrated in the four northern provinces, a parcentage of 28% /fig. 32/ seems lower than could be expected. However, the scarcity of D. caruanae at that time in West Flanders and in Limburg could explain this low number. The low percentage for 1980 /20%, fig. 40/ clearly demonstrates its low frequency in the southern part of Belgium. See dispersal of. P. caruanae . The situation at the end of December 1982 is given by figs 49-52. They are directly comparable with figs 1-4. 104
