Estimations have been made of the weight of material accumulated by adults of Phormia (Protophormia) terraenovae R.-D. from deposits of DDT and other lipophilic particles on different types of substrate on which the flies were exposed.

Uptake of dry DDT particles is shown to be similar to that of dry lipophilic dye particles of similar size so that conclusions on the contamination of insects by dry lipophilic dyestuff may be extended to contamination by DDT dust.

The presence of oil greatly reduces the uptake of DDT particles from a fibrous substrate, the uptake decreasing logarithmically with increase in the amount of oil applied to the substrate. The viscosity of the oil has no more than a very small effect on the uptake of lipophilic particles from oily deposits and the solubility of the particles in the oil has no significant effect.

A reduction in the uptake of dry lipophilic particles from filter paper and from a hard, waxy substrate occurs when a wetting agent solution is applied immediately before the particles. When the particles are applied after the drying of the wetting agent on the substrate, a reduction in uptake also occurs on the non-absorbent, waxy surface, but is not so large as when the particles are applied before drying of the wetting agent. No reduction in uptake occurs on the absorbent substrate under these conditions.

On the basis of the experimental results the variations in the adhesion of lipophilic particles to substrates are discussed.

While differences in the affinity of particles for substrates of different types cause variations in adhesion, the physical structure of the surface may be more important in determining the amount picked up from it. The presence of oil or wetting agent on the surface increases adhesion, probably by increasing both the affinity of the lipophilic particles for the substrate and the area of contact between the particles and the substrate.