1. Experiments are described in which Trichogramma females were provided with Sitotroga eggs arranged at various distances from 0.05 to 0.40 in. The frequency with which the parasite found the neighbouring egg varied inversely with the distance between the eggs. The relation between frequency of contact and distance between the hosts depends chiefly, but not entirely, on chance.
2. The rate of finding the eggs varied inversely with the distance between them. By consideration of the conditions of the experiments, this relation between the rate of finding and the distance between the eggs is seen to indicate that the Trichogramma does not seek at random over the whole area available for movement but restricts its search to the neighbourhood of hosts.
3. From experiments with eggs of four sizes it is shown that frequency of neighbouring contact is correlated positively with the size of the eggs. The influence of size is largely mechanical; but not entirely, for small eggs of Sitotroga are found less efficiently in proportion to their size than are larger eggs.
4. When provided simultaneously with hosts of two different sizes, a Trichogramma finds more of the larger than of the smaller hosts. If the hosts are arranged alternately, the amount of selection that occurs by finding depends entirely upon the relative sizes of the eggs. If they are arranged in separate groups, however, relatively more of the larger eggs are found.