NATURE AND SOURCES OF ANIMAL FOOD. 143 



by everything which augments the general energy of the 

 system ; whilst, on the contrary, everything which tends to 

 retard the operations of life, such as bodily and mental inac- 

 tivity, sleep, or depression of spirits, tends also to render the 

 demand for food less imperious. Thus, cold-blooded animals, 

 particularly Reptiles, can sustain a very prolonged abstinence, 

 when the general activity of their functions is kept down by 

 a low temperature; and hybernating Mammals, which pass 

 the winter in a state of torpidity, require no food during the 

 continuance of their lethargy. But with this exception, 

 warm-blooded animals require a constant supply of nutriment, 

 not merely for the maintenance of their proper heat, but also 

 for the repair of the waste resulting from that continuous 

 activity which the uniform temperature of their own bodies 

 enables them to keep up. This is the case with Man and 

 the Mammalia generally, and still more with Birds, whose 

 temperature is higher, and whose movements are more active 

 and energetic. It is also more the case with young animals 

 than with adults; since in the former the changes in the 

 tissues, in consequence of the increase they are undergoing, 

 take place with much more rapidity than in the latter, the 

 bulk of whose bodies remains stationary. Hence, if children, 

 young persons, and adults be shut up together, and deprived 

 of food, the younger will usually perish first, and the adults 

 will survive the longest. The Italian poet Dante has given 

 a terrible picture of such an occurrence, in his history of the 

 imprisonment of Count Ugolino and his children. 



141. The difference in the demand for food between the 

 young growing animal and that which has arrived at maturity, 

 is very remarkable in the case of Insects. There are no 

 animals more voracious than the larva or caterpillar; and 

 there are none that can sustain abstinence, with little dimi- 

 nution of their activity, better than the imago or perfect 

 insect. The larvaa of the Flesh-fly, produced from the eggs 

 laid in carrion, are said to increase in weight 200 times in 

 the course of 24 hours ; and their voracity is so great as to 

 have caused Linnaeus to assert, that three individuals and 

 their immediate progeny (each female giving birth to at least 

 20,000 young, and a few days sufficing for the production of 

 a third generation) would devour the carcase of a horse with 

 greater celerity than a Hon. The larva of the Silk-worm 



