356 



Comparative Animal Physiology 



the optimal temperature for brood development is high. The optimum for 

 ants (Formica nifa) covers a range of 23 to 29°; the optimum for Vespa is 30 

 to 32° whereas for honeybees the brood optimum is 34.5-35°, and brood 

 survival is limited to the range of 32 to 36°.-'- '^"^ In the summer when the air 

 temperature rises above the optimum for the brood, the bee workers frequently 

 transport water into the nest, and aid evaporation by fanning. Ants transport 

 brood to cooler chambers in the nest. By these means the brood temperatures 

 are kept lower than the air temperatures. 



In winter most colonial Hymenoptera cluster together, and the temperature 

 of the cluster may be kept well above the air temperature (Fig. 91). If the 

 temperature falls to 8-10° C. bees show great uneasiness, the outer bees being 

 more active than those in the interior of the cluster. Heat is conserved also 

 by the insulating wall of the nest, by closing nest openings (ants), and by 

 other physical means. There is much species variation in the insulating value 

 of the nest covering. Ants also build special hibernating chambers; their body 

 water content is decreased and metabolism reduced in hibernation. Cold rigor 



December 



January 

 1 2 



20 



lO^- 

 O 



22 23 24 25 26 27 26 29 30 31 



20 21 22 23 24 2J 26 



Fig. 9 1 . Temperature of honeybee cluster in winter. From Himmer.*'* 



occurs in bees at about 7°, in wasps and ants at about 0°. Apparently food 

 absorption is reduced critically at these temperatures, since bees die at tempera 

 tures above freezing with their stomachs full of sugar. Hymenoptera combine 

 chemical and physical means in maintaining a relatively constant temperature 

 of the nest. 



In addition to producing metabolic heat, insects can absorb radiant heat 

 from the sun and from a radiating substratum such as a hot rock. Summaries 

 of available data of absorption and reflection are found in the literature;'"''' ^'- 

 the most complete measurements are by Rucker.'*^^ There is little correlation 

 between color as seen in visible light and the transmission by the body wall of 

 infrared (wave length 1-3 /x). The absorption of infrared, hence of radiant 

 heat, ditters less from insect to insect than does absorption of difl:erent visible 

 wave lengths. In general, however, dark insects absorb more infrared than 

 light-colored ones. For example, the dark elytra of the beetle CMrahus reflect 

 less than half of the infrared and reflect least at 2.15 jn, whereas the white 



