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Pitfall traps provide a convenient method of investigating the ecology of adult Carabidae and have been used in studies on such topics as the seasonal incidence of adults, the spatial pattern of distribution in populations, the relative numbers of a species in different vegetation types, in describing the Carabid fauna of habitats, and in work on daily rhythms of activity (Hikimiuk 1948; van der Drift 1951; Scherney 1955, 1960; Gilbert 1956; 1958; Dawson 1957; Kabacik 1957; Skuhravy 1956; Williams 1959b; Kirchner 1960; Thiele 1960; and others). These traps suffer from the disadvantage that catches depend both on the density of the population being sampled and the activity of individuals in it. Briggs (1961) concluded that in the Carabid Harpalus rufipes (Deg.) the size of the population played a minor part in determining numbers trapped, and Morris (1960) has stressed the desirability of obtaining estimates of absolute population in sampling programmes, that is the number of individuals per unit of habitat. As pitfall catches vary with the amount of locomotor activity shown by Carabidae they are influenced by weather, and Briggs (1961) and Greenslade (1961) found a correlation between catches and temperature. The problem of the relation between catch, population size and activity is not dealt with here, but Taylor's (1963) recent discussion of temperature and insect flight and suction trap catches is relevant. There are, however, other factors which may result in variation in pitfall catches of Carabidae, the two main ones, which have not been considered by some workers, are the effect of ground vegetation in impeding Carabid movement and reducing catches, and the differential susceptibility of species to trapping according to their behaviour. Taking these factors into account the cases in which it is legitimate to draw conclusions from pitfall trapping are discussed. This is necessary for despite their drawbacks pitfall traps are often the only method available in studies on Carabid populations. Although direct quadrat counts are to be preferred they may be precluded by low density or other factors. On the Imperial College Field Station, Silwood Park, Berkshire, where the work described in this paper was carried out, quadrat counts of Carabidae were made in three habitats (Greenslade 1961), and the densities of adults were: beechwood, 1-33 per m2 (6 Carabidae in 4-46 m2 sampled); grass heath, 28 per m2 (52 in 1 86 m2); arable land, 80 per m2 (82 in 1 02 m2). With the sparse woodland population direct counts are not practicable on account of the time and labour they involve, while in other habitats quadrat sampling may result in excessive destruction of the site being studied, especially as Carabid distributions are often highly aggregated and centred on restricted favourable areas (Greenslade 1963a, 1964a, e).