1950] Edwards and Nutting — Thermobia and Grylloblatta 41 
tivity was observable. Up to 20° C the heartbeat was strong and 
regular, but after 15 minutes at 22°C diastole appeared incomplete. 
After 15 minutes at 27°C the heart was almost completely relaxed 
but continued to beat with an increasing amount of fibrillation. This 
type of activity was maintained for nearly 20 minutes while the 
temperature was raised to 29.5°C, at which point all activity ceased. 
Thermobia. — The record of heart rate from 11° to 49.5°C is 
presented in Figure 2. The average Qio calculated from 12° to 42° 
is 2.6. A mass Arrhenius plot (Figure 4), over an intermediate 
range of 15° to 45 °C, gives a slope of 2.7 and a temperature 
characteristic of 12,300 calories. 
As the temperature was lowered the heartbeat became more and 
more feeble, although often quite “deliberate”, (i.e. with definite 
pauses between systole and diastole) until, with most specimens, no 
heart movements were visible after 30 to 60 minutes at 11°C. At 
the opposite end of the scale, normal heart activity was observed up 
to about 42 °C, after which the beat became extremely rapid with 
incomplete diastole and intermittent fibrillation. Cessation of ac- 
tivity occurred between 45° and 49.5°C. General movements of the 
limbs and body of both Thermobia and Grylloblatta approximated 
those observed during the respiration experiments. 
Discussion 
The results show essential agreement with observations on the 
activity of Grylloblatta reported by Mills and Pepper (1937). With 
short exposures, they found that cold prostration set in at —6.2°, 
activity was observable from —5.6° to about 20°, and paralysis 
occurred at 24.9°, with 27.8°C being fatal. Also substantiated is 
their statement that Grylloblatta shows no dormancy at low tem- 
peratures but probably tries to avoid unpleasant conditions by sun- 
ning or seeking shelter. 
The results suggest also an answer to the question of what hap- 
pens to the firebrat when the fire goes out, but do not explain why 
the animal has a temperature preferendum so high. Data on the 
firebrat fit the metabolism — temperature curve of other insects for 
which information is available, suggesting that the wide tem- 
perature tolerance of this insect is not due to any special metabolic 
adaptation. 
All insects thus far investigated have the same general metabolism- 
temperature curve, in all seasons and from all climes. The only 
