of a species reflect the soil layer preferred by it, but cer- tain species have a life strategy that does not correspond to their life form (HGVAR, 1983, TAKEDA, 1995). On the other hand, a differentiation in the vertical preferences within a species has been reported (WOLTERS, 1983) indi- cating that smaller individuals tend to inhabit deeper lay- ers. In this study the following questions were addressed : are there vertical preferences for Collembola and under what circumstances does an avoidance behaviour occur? Are there species specific preferences and are they related to life forms? Does an intraspecific differentiation in ver- tical preferences occur that can be related to body size? MATERIAL AND METHODS Site description The investigation site is located in north-eastern Greece, in the western part of the delta of the river Nestos (24¡ 43« N, 40¡ 53« E). The climate in the area is of the subcool-subhumid Mediterranean type, as defined by NAHAL (1981). Four types of deciduous forests can be dis- tinguished in the area : a. Hardwood riparian forest, dom- INTRODUCTION A number of studies state that the majority of Collembola live in the few upper centimeters of the soil habitat, notably in the litter and humus layer (T AKEDA, 1978 ; HGVAR, 1983 ; WOLTERS, 1983 ; BERG et al., 1998). The migration of Collembola to deeper layers is a well known avoidance behaviour, which is attributed to differ- ent factors according to the climatic characteristics of the areas where the different studies were conducted (HOPKIN, 1997). The classification of Collembola to ecomorphological life forms was introduced by GISIN (1943) and has been widely used ever since. Based on morphological charac- teristics, such as presence and number of eyes, pigmenta- tion, length of the legs and the furca, species are characterised as euedaphic (species that are permanent soil dwellers), hemiedaphic (species that that live in the superficial soil layers and leaf litter) and atmobios (species that live in the surface and on vegetation). It was originally assumed that the morphological characteristics Vertical distribution of Collembola in deciduous forests under mediterranean climatic conditions Vassilis Detsis Laboratory of Soils and Agricultural Chemistry, Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece ABSTRACT. This study was conducted in the deciduous forests in the delta of the river Nestos. Soil samples were taken in various seasons and were divided into three layers: O layer, 0-2.5 cm soil depth and 2.5-5 cm soil depth. The Collembola caught were counted and identified and the biomass was determined. Soil moisture and temperature were also recorded. In most seasons the majority of the animals was found in the O and the upper soil layer. Under adverse condi- tions in summer (low soil humidity) a vertical migration to deeper layers was observed. No similar response was observed in winter. These patterns persist at the level of the whole Collembola population and at the species level as well. Only minute differences, if any, were observed in the vertical distribution pattern of the most abundant species, irrespectively of the life form they belong to. Generally a preference of smaller ani- mals for the deeper layers was observed. KEY WORDS : Collembola, vertical distribution, life forms, body size. Corresponding author: V. Detsis, e-mail lsos2kok@auadec.aua.gr Belg. J. Zool., 130 (supplement 1) : 55-59 November 2000