290 



THE ZOOLOGIST. 



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 cratcegi, 

 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 : — 



-10 



K 2 = Death 



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 



