Conventional and modified methods of obtaining supercooling points of immature stages of insects have been utilized in studies of the cold-hardiness of the European pine shoot moth and the European pinesawfly. A method has been developed to permit visual observation of the freezing process of more than one specimen at a time. A freezing chamber consisting of a hole one inch in depth and one-half inch in diameter is located in the upper end of an aluminum rod partially submerged in a dry ice-alcohol mixture. A small filter paper disc, used as the insect platform, rests upon a #40 copper-constantan thermocouple located near the base of the freezing chamber. The thermocouple enters the chamber through a hole in the wall after several circuits around the circumference of the rod to prevent temperature anomalies attributable to thermal conduction within the wire. The thermocouple is connected to a sensitive recording potentiometer. The wall of the freezing chamber is blackened to prevent reflection of light from obscuring the view of the freezing process, through a binocular microscope mounted above the freezing chamber. The moment of freezing is readily recorded on the temperature trace provided by the potentiometer. At a cooling rate of approximately 5°F. per minute, a correction factor of 2.5°F. must be added to the indicated freezing point to obtain the actual temperature at the surface of the platform. When this correction is applied, the results provide data applicable to statistical analysis of freezing point determinations.
A RECORDING POTENTIOMETER FOR USE IN PHYSIOLOGICAL INVESTIGATIONS