144 NATURE AND SOURCES OF ANIMAL FOOD. 



weighs, when hatched, about 1-1 00th 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. 

 For 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 (Scarabseus) 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. 



