The work described in this paper forms part of an investigation into the conditions which influence the breeding of Pieris brassicae (L.) in captivity. Observations have been made on the behaviour of the females at the time of oviposition, on factors which influence oviposition, on the appearance and weight of the eggs and on their fertility and hatching.

Females which are ready to lay seek out green surfaces and, as they respond eagerly to plants from which they are separated by glass, it seems that plant odour plays little part in the attraction. Once they have alighted, however, the insects drum on the surface with their fore legs to test its suitability. Normally they lay only on plants which contain the mustard-oil glucosides, but they have been observed to oviposit on broad bean (Vicia faba), on which the larvae do not survive. Sinegrin applied to green paper stimulates the female to lay. Provided she is standing on an acceptable surface she will oviposit on any other surface, for example, filter paper or glass. The eggs are normally deposited on the under surface of the leaves. This is largely due to a preference for the physical underside but the insects also seem to prefer the morphological under surface of the leaf to the upper surface. When laying an egg, the female locates one previously laid with the tip of her abdomen and so builds up the regularly arranged batches.

The females lived and oviposited as well in small cages as in large cages. They laid more eggs per day, and more eggs in a batch, at 30°C. than at 20°C. Both numbers increased until the female was about six to seven days old and then declined. Oviposition occurs at low light intensities. Fertilised females laid very many more eggs than virgin females. Oviposition occurred two to three days after copulation, and most females oviposited six or seven times in eight days. The number of eggs laid by starving insects is low: it is higher for insects given water or one per cent, honey solution and very much higher for insects given ten per cent, honey solution. Sucrose solutions are as satisfactory as honey solution.

When first laid, the eggs of P. brassicae are yellow in colour and become more orange as they develop. Some batches of newly laid eggs are of a distinctly darker yellow than others but, as it is believed that the eggs are fertilised only just before oviposition, it seems that this colour difference cannot be due to the eggs being in somewhat different stages of development. The number of ribs on the shells seems to vary in different cultures.

Batches of eggs which are laid within an hour of each other may begin to hatch several hours apart, and the time taken for all the eggs in one batch to hatch was found to range from two hours to about seven. A fertilised female lays scarcely any infertile eggs. The fertility after one mating falls below 100 per cent, after about 14 days, but normally the female mates again before this time. Temperature naturally affects the time taken by the eggs to hatch. The shortest time was about 3¾ days at 28°C.; the longest observed was 17 days at 12·5°C.

The eggs cannot be stored for more than ten days at 3·5°C. and 50 per cent, relative humidity. Eggs will develop and hatch at very low humidities. If the eggs are detached from the surface on which they are laid by the use of acetone their capacity to develop and hatch in air dried over phosphorus pentoxide is substantially reduced.