Temperature: Metabolic As-pects and Perception 



355 



result from desiccation. The degree of temperature which was lethal within 

 one hour was not affected by humidity for a series of small insects. Specimens 

 of Tenehrio larger than 100 mg. survived better in dry air than specimens 

 smaller than 30 mg. (Fig. 88). However, for heat death in 24 hours, tempera- 

 tures effective against those species which lack mechanisms for water retention 

 were much lower in dry air than in moist air (Fig. 89). ^^^ For example, the 

 louse Pediculus survived 24 hours at 38° at 90 per cent relative humidity, but 

 died at 33° at per cent humidity. The results indicate that the primary cause 

 of death was dehydration and not high temperature per se. The cooling effect 

 of evaporation in dry warm air is entirely passive and occurs in dead insects. 

 A balance sheet of heat relations of a live grasshopper^'''* shows that essentially 

 no heat is lost by radiation and conduction at low humidities, and that at 90 

 per cent humidity loss of heat by vaporization still exceeds loss by other 

 channels. The presence of cuticular wax and spiracular closing favors water 

 retention but decreases cooling. 



lO 



30 so 



TIME (minutes) 



Fig. 90. Influence of muscular activity on the body temperature of a female Cecropia 

 moth, 4 days after emergence. In the diagram A to B denotes periods of wing movement. 

 From Oosthuizen.^^'' 



Insect heat of metabolism is considerable, especially in flight muscles. 

 Thermocouple measurements of butterflies (Vanessa) and of lamellicorn 

 beetles and some other insects show temperatures similar to that of the 

 environment in resting insects but rapid increase in temperature prior to 

 flight^^' ^"^' '•'- (Fig. 90). In flight, breathing accelerates, and the temperature 

 rises more in wing muscles than in the rest of the body. Some insects show 

 warming-up movements of the wings, others show muscle action potentials but 

 no visible movement. The duration of warming-up is longer at low than at 

 high air temperature; Vanessa warmed up for over 6 minutes at 11°, IVz 

 minutes at 23°, 18 seconds at 34°, and not at all at 37°. Muscle temperature 

 in this butterfly is about 35° at take-off and 37° during active flight. ^^ The 

 heat produced corresponds to a metabolism of 30 1. 02/kg./hr. before flight.^"** 



Metabolic heat is used in varying amounts in colonial Hymenoptera, and 



