362 



POPULATIONS 



favorably low, the nest temperature was 

 not far from the optimum. 



Michal reports observations made on a 

 population of mealworms over ten days 

 with surrounding air temperatures and nest 

 temperatures recorded three times each 

 day. We have constructed a graph, based 

 on his data, wliich summarizes the findings 

 (Fig. 129). From this graph, Michal's 

 table, and discussion the following points 

 can be made; 



the optimal temperature (33° C.) is ap- 

 proached when external temperatures are 

 low, and but slightly elevated when these 

 temperatures are high. So far as we can 

 discover, Michal does not discuss the 

 source of the extra heat increment pro- 

 duced by the aggregation. Possibly this 

 stems from the increased metabolic activity 

 of the clustered larvae plus heat conserva- 

 tion brought about by the insulating prop- 

 erties of the medium. This hypothesis 



TEMPERATURE 

 OF CULTURE 



/ TEMPERATURE 

 EXTERNAL TO CULTURE 



8 



10 



3 4 5 6 7 



TIME IN DAYS 



Fig. 129. Temperatures taken within an aggregation of meal worms (larvae of Tenebrio 

 molitor) plotted against temperature taken at the same time above the surface of the 

 culture. 



1. The nest temperature is always above 

 the outside air temperature. 



2. The greatest divergence between nest 

 and air temperatures occurs when the lat- 

 ter is low. The temperature within the lar- 

 val aggregation may be as much as 10 de- 

 grees higher. For example, at an air tem- 

 perature of 17° C, the nest temperature 

 was found to be 27° C. 



3. The least divergence between the 

 two temperatures occurs when the air tem- 

 perature is high, although the nest tem- 

 perature is always shghtly higher. For ex- 

 ample, at air temperature readings of 35°, 

 30°, and 30°, respectively, nest readings of 

 36°, 34°, and 33° C. were recorded. 



Our particular interest in Michal's work 

 lies in its demonstration that a nonsocial 

 insect population ameliorates its local ef- 

 fective temperature by a relatively simple 

 coaction, an aggregation probably induced 

 by thigmotaxis. The regulation of nest 

 temperatures assumes a pattern such that 



would not account for the close approxima- 

 tion of nest and air temperatures when the 

 latter are high. 



Several other insect examples of the 

 modification of microclimate by population 

 activities deserve brief mention. Hase 

 (1926) observed that wax-moth caterpil- 

 lars {Galleria melonella) live in dense 

 colonies in honeycombs and that the tem- 

 perature within such colonies may be 17° 

 to 22.7° C. higher than that of the sur- 

 rounding atmosphere. The case is not com- 

 pletely analyzed, but it is suggested that 

 this extra heat is produced partly by fer- 

 mentative processes and partly by the body 

 temperature of the larvae themselves. 



Cases of group thermal control are well 

 known among the social insects. Wellen- 

 stein (1928) and Steiner (1929) worked 

 on nest temperatures of the ant Formica 

 nifa rtifopratensis. They noted a zone 

 within the nest at a depth of 15 to 50 

 centimeters in which the temperature re- 



