144 NATURE AND SOURCES OF ANIMAL FOOD. 
weighs, when hatched, about 1-100th of a grain; previously 
to its first metamorphosis it increases to 95 grains, or 9,500 
times its original weight. The comparative weight of the 
full-grown caterpillar of the Goat-moth to that of the young 
one just crept out of the egg, is said to be as 72,000 to 1. 
Tor this enormous increase a very constant supply of material 
is necessary, and many larvae perish if left unsupplied with 
food for a single day. On the other hand, a black beetle 
(Melasoma) has been known to live seven months, pinned 
down to a board; and another beetle (Scarabaeus) has been 
kept three years without food,— and this without manifesting 
any inconvenience or loss of activity. There are many perfect 
insects which never eat after their last change, but die as soon 
as they have performed their part in the propagation of the race. 
142. The nature of the food of animals is as various as the 
conformation of their different tribes. It always consists, 
however, of substances that have previously undergone organ¬ 
ization. There are some apparent exceptions to this, in the 
case of animals which seem to derive their support, in part at 
least, from mineral matter. Thus, the Spatangus (an animal 
allied to the Echinus, § 119) fills its stomach with sand; but 
it really derives its nourishment from the minute animals 
which this contains. The Earthworm and some kinds of 
Beetles are known to swallow earth; but only to obtain from 
it the remains of vegetable matter that are mixed with it. 
By some races of Man, too, what seems to be mineral matter 
is mixed with other articles of food, and is said to be nutri¬ 
tious ; this may be beneficial, in part, by giving bulk to the 
aliment, and thus exciting the action of the stomach (§ 205); 
but it has been found, in one case at least, that the supposed 
earth consists of the remains of animalcules, and contains no 
inconsiderable portion of organic matter. 
143. There are many instances in which, no obvious sup¬ 
plies of food being afforded, the mode of sustenance is obscure ; 
and it has been frequently supposed that, in such cases, the 
animals are sustained by air and water alone. But it will 
always be found that, where food is taken in no other way, 
a supply of the microscopic forms of animal or vegetable life 
is introduced by ciliary action (§ 45); and it is on these, 
indeed, that a large proportion of the lower forms of aquatic 
animals depend entirely for their support. 
