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 larve of the Flesh-fly, produced from the eggs 
laid in carrion, are said to increase in weight 200 times in 
course of 24 hours; and their voracity is so great as to 
have caused Linnezus 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 lion. The larva of the Silk-worm 
