STUDIES ON PERILLUS BIOCULATUS FAB. 69 
of food all through September. By September 24, only 55 bugs were 
left alive. It was found on September 28 that 11 more were dead. 
Some of this number doubtless were bugs of the first brood which 
had matured in July, while the remaining 44 must have been of the 
second brood and of similar age to the preceding lots. On October 
1 this lot of bugs was placed in cold storage beside Lot A and Lot 207. 
An examination on October 19 showed that only 3 males and 6 females 
were left alive. These figures indicate that 79 per cent of the bugs 
died between September 2 and October 19. On May 19, 1922, only 
one female was found alive. 
On the basis of these observations it would seem that when the 
bugs are kept active thoughout September, and with plenty of food 
available, they are not in fit condition to go immediately into hiberna- 
tion when cold weather overtakes them. The bugs that were kept 
inactive and without food during September, in a temperature of from 
60° to 68°F., were in the best condition by October to pass into the 
hibernating stage. 
Under natural conditions of hibernation the mortality of the 
bugs must be very high during most years. It has been the writer’s 
experience during six or seven years’ observation in potato fields in 
New York and Minnesota, that very few bugs appear in the fields 
the following spring, where the preceding fall they were very abundant. 
Under natural conditions it appears that probably not more than 5 
per cent of the bugs which go into hibernation come out safely in 
the spring. In New York and Minnesota this may well be due to the 
fact that many of the bugs seek hibernation in situations where their 
fatal minimum temperature is reached during the cold winters. In 
the spring of 1921 the largest number of bugs appeared in the field 
that the writer has ever observed. The fall of 1920 was very favor- 
able for the bugs that went into hibernation, and following this the 
winter was unusually mild. The mild winter probably allowed many 
bugs to hibernate safely in situations where during ordinary cold win- 
ters their fatal temperature would have been reached. Such con- 
ditions would seem to account for the greater abundance of the bugs 
in the spring of 1921. 
Determining the Fatal Minimum Temperature by the Thermoelectric 
Method? 
To determine the fatal minimum temperature of a hibernating 
Perillus, experiments were performed utilizing the thermoelectric 
2The writer is indebted to Dr. R. B. Harvey, of the Botany Department, University 
of Minnesota, for the use of certain apparatus as well as for helpful suggestions in the matter 
of taking temperatures by the thermoelectric method. 
