70 NINETEENTH REPORT STATE ENTOMOLOGIST OF MINNESOTA—IQ22 
method for measuring temperatures. Using needle-type thermocouples 
with potentiometer that records temperature in millivolts, it was pos- 
sible to follow the internal temperature of individual bugs, as well as 
the temperature of the surrounding air; to note the amount of under- 
cooling which each could withstand, and finally the point at which 
actual freezing took place. For details of this method see White 
(1910), Harvey (1919), and Taylor (1920). 
The bugs used in these experiments were all kept in the same 
hibernating chamber at a temperature averaging very close to 4°C., from 
October 15, 1921, until the hour of the experiment. The first attempt 
to freeze hibernating bugs was made on March 28, 1922, when ether was 
used to lower the temperature of the freezing chamber. This was 
accomplished by drawing air through the ether which surrounds the 
freezing chamber. With this particular apparatus the lowest tem- 
perature which could be obtained was —17°C. Four different bugs 
were reduced to this temperature and kept there for periods of fifteen 
to thirty minutes, but without attaining actual freezing. The freezing 
of undercooled tissue or liquids is always indicated by a rebound in 
temperature to the actual freezing point. One of these bugs was placed 
in the freezing chamber three different times, the periods totaling 45 
minutes, during which the internal temperature was read at points be- 
tween —15 and —17°C. This bug revived when warmed to room 
temperature and was alive and crawling about the next day in spite 
of the fact that it had been pierced with a thermocouple. The thermo- 
couple, which had been reduced to the thickness of an ordinary pin, 
was always inserted through the anus and extended to a point near 
the base of the abdomen. The ether freezing chamber was discarded 
at this point since it was evident that lower temperatures must be 
reached in order to freeze the hibernating insects. 
On the following day, March 29, carbon dioxide gas was used to 
reduce the temperature in the freezing chamber. With this arrange- 
ment one bug was cooled to a point where the internal temperature 
was read at —26°C. but without freezing taking place. Unfortunately, 
at this point the gas supply gave out, so it was not possible on that 
date to find the limit of undercooling which Perillus could withstand. 
It was two weeks before a new tank of carbon dioxide gas could be 
obtained, so the next attempt to freeze the bugs was not made until 
April 15. On this date some more of the hibernating bugs were taken 
from cold storage, where they had remained all winter undisturbed, 
