1. The hatching of N. battus under laboratory conditions was investigated. A high hatch of ripe eggs was obtained on exposure to constant temperatures in the approximate range 8–15° C., and very little hatching took place at temperatures outside this range.

2. These, and other published results can be explained in terms of a hypothesis transferred from the study of the breaking of insect diapause. This hypothesis supposes that two processes underly the completion of hatching—a cold process with low temperature optimum and a warm process with a higher temperature optimum and which can only proceed as the cold process is completed.

3. It is not easy to apply these findings directly to field events, but the attempt to do so leads to the suggestion that the apparent synchrony of hatching and the presence of susceptible lamb hosts in the north-east of the British Isles may be partly fortuitous, and that in slightly different climatic circumstances many eggs might hatch in the autumn or during the winter.

I wish to thank Mr A. V. Cunningham of Threepwood, Galashiels, and Mr J. M. M. Cunningham of the Edinburgh School of Agriculture for their generous help in making sampling of their flocks possible, and Dr J. T. Stamp for advice on the presentation of this paper.