INFLUENCE OF HEAT ON ANIMALS. 85 



name of warm-blooded animals, their temperature being kept up to nearly 

 100, when that of the sea is about 80. It is uncertain, however, to 

 what extent it would be depressed, by a lowering of that of the sur- 

 rounding medium. The greatest power of developing heat in cold- 

 blooded animals appears to exist, when their bodies are reduced nearly 

 to the freezing-point ; and when that of the surrounding air or water is 

 much below it. Thus Frogs have been found alive in the midst of ice 

 whose temperature was as low as 9, the heat of their own bodies being 

 33 ; and it has been observed that even Animalcules contained in water 

 that is being frozen, are not at once destroyed, but that each lives for 

 a time in a small uncongealed space, where the fluid seems to be kept 

 from solidifying, by the caloric liberated from the Animalcule. 



123. The peculiar condition of the class of Insects, in regard to its 

 heat-producing power, exhibits in a very striking manner the connexion 

 between an elevated temperature and vital activity. In the Larva 

 state of Insects, the temperature of the animal follows closely that of 

 the surrounding air, as in the cold-blooded classes generally ; but it is 

 usually from \ to 4 above it. In the Pupa condition, which is one of 

 absolute rest in most insects that undergo a complete metamorphosis, 

 the temperature scarcely rises above that of the surrounding medium ; 

 except at nearly the close of the period, when it is about to burst its 

 envelopes and come forth as the perfect Insect. The temperature 

 which different Insects possess in their Imago state, varies in part ac- 

 cording to the species, and in part with the condition of the individual 

 in regard to rest or activity ; but the same principle is evidently ope- 

 rating in both cases, since the variation existing amongst different 

 species, in regard to their heat-producing power, is closely connected 

 with the amount of activity natural to them. The highest amount is 

 to be found in the industrious Hive-Bee and its allies, and in the elegant 

 and sportive Butterflies, which are almost constantly on the wing in 

 search of food; next to these come the Beetles. of active flight ; and 

 lastly those which seldom or never raise themselves upon the wing, 

 but pursue their labours on the ground. The temperature of individual 

 Bees has been found to be about 4 above that of the atmosphere, when 

 they are in a state of repose ; but it rises to 10 or 15, when they are 

 excited to activity. When they are aggregated together in clusters, 

 however, the temperature which they possess is often as milch as 40 

 above that of the atmosphere. When reduced to torpidity by cold, 

 they still generate heat enough to keep them from being frozen, unless 

 the cold be very severe ; and they may be aroused by moderate excitement 

 to a state of activity, in which the temperature rises to a very conside- 

 rable elevation. Now although the increased production of heat is in 

 these cases, as in hybernating Mammals similarly aroused, the conse- 

 quence of the increased activity, there can be no question that it is a 

 condition necessary to the continuance of that activity ; since we find 

 that, if the temperature of the body be again reduced by external cold, 

 the activity cannot be long maintained. 



124. Whilst the foregoing facts exhibit the connexion between an 

 elevated temperature, and the most active condition of the muscular 

 and nervous systems, in cold-blooded animals, there is abundant evi- 



