Scale-related use of habitat by the tenebrionid beetle, Physadesmia globosa, was studied beneath tree canopies on the Kuiseb River's floodplain in the Namib Desert. Beetle activity and relative density were estimated using pitfall traps installed in soil-litter subcanopy substrates, which provide the insects with shelter and food. Considerable variability in both large-scale (ca. 0,5 km) use of subcanopy space among different trees, and in intermediate-scale (1-5 m) use of quadrants under the same trees, was recorded between late August and mid-November, suggesting that quality of substrate influences habitat use. We tested beetle responses to substrate quality at two scales. At the intermediate scale we placed different substrates in circles around traps in field arenas and beneath canopies. No preferences were noted for 'good' or 'poor' litter from selected trees, or for Acacia erioloba leaf and flower litter, Salvadora persica leaf litter, or bare soil. However, in a separate experiment, crops of P. globosa confined for 48 h beneath acacias contained significantly more flowers than leaves, despite the obvious abundance of leaves. Therefore, we placed beetles in small laboratory arenas containing leaf litter on which flowers were positioned 0,5, 2,5, and 5,0 cm apart. Flowers were again chosen significantly over leaves. We conclude that nutritional components of the substrate are meaningful to P. globosa only as they are encountered, while non-nutritional compounds determine habitat use at larger scales. Keywords: Namibia, Namib Desert, Gobabeb, Kuiseb River, desert biology, desert ecology, desert fauna, species richness, detritivores, habitat use, foraging behaviour, feeding behaviour, pitfall-trapping. ACCESSION NUMBER: 574.5'688 SEE.