Page:The Zoologist, 4th series, vol 6 (1902).djvu/344

290 of under-cooling and freezing does not cause death on the first occasion, but, if the process be repeated for a second time, the insect dies when the critical point is reached for a second time. This is indicated by the following experiments on Aporia cratœgi, the black-veined white:—

1st lot.—K= –10°, N = –1·2°. When N was reached the animals were removed from the ice-chamber, and lived.

2nd lot.—K= –8°, N = –0·8°. On under-cooling again to –6·5°, and removing the animals, they still lived.

3rd lot.—K= –6·8°, N = –1·1°. On under-cooling again to –10·0, death occurred.

These facts may be graphically represented thus:—

Both the critical point and the normal freezing-point vary not only in different species, but in different individuals of the same species, and at different life-stages of the same individual. Indeed, many factors play a part in determining the nature and relations of these points, such as the rapidity with which the cooling takes place, the sex of the insect, the quantity of food it has eaten, and the amount of time it is kept at any particular temperature.

The number of degrees lying between the critical point and the normal freezing-point is complicatedly dependent on the rapidity of cooling, but the alternatives are so various that it is impossible at present to draw any concise conclusion with regard to them. It is an extremely interesting discovery that males have normally a greater difference between their critical and normal freezing-points than females; but this difference is