The effect upon the three-dimensional dispersion of experimental populations of Tribolium castaneum (Hbst.) of internal pockets of grain at 18 per cent, moisture content, either mouldy or free from mould, and superficial ones of 25 per cent, moisture content, in a larger bulk of 14 per cent, moisture content, was investigated at 25°C. The behaviour under these circumstances of insects reared and maintained at 70 per cent, relative humidity was compared with those conditioned at 40 per cent. R.H. for 14 days. It was found that insects reared at 70 per cent. E.H. did not accumulate in the damp grain whereas conditioned ones did. The underlying response was shown to be hygrokinetic and the mechanism an orthokinetic one. The rate of movement of unconditioned adults in grain of 18 per cent, moisture content was approximately twice that in grain of 14 per cent, moisture content. Conditioned adults moved very slowly in the damper grain. There were no significant variations in irregularity of pathway (klinokinesis) under any of these circumstances. When grain of 18 per cent, moisture content supporting a mould growth was used, unconditioned adults did, however, accumulate in it due to trophic behaviour.

While there is considerable plasticity of behaviour in this species a synthesis is attempted of the causal behavioural mechanisms underlying the spatial structure of populations in ecological situations involving humidity gradients; the results are also discussed in relation to grain storage conditions, and the detection and control of the species.