VALUE OF BIRDS TO MAN. 31 
weighs one-twentieth of a grain; when ten days old, it weighs one-half 
a grain, or ten times the original weight; when twenty days old, it 
weighs three grains, or sixty times the original weight; when thirty days 
old, it weighs thirty-one grains, er six hundred and twenty times the 
original weight; when forty days old, it weighs ninety grains, or eight- 
een hundred times the original weight; and when fifty-six days old, it 
weighs two hundred and seven grains, or forty-one hundred and forty 
times the original weight. 
When a worm is thirty days old, it will have consumed about ninety 
grains of food; but when fifty-six days old it is fully grown, and has 
consumed not less than one hundred and twenty oak leaves, weighing 
three-fourths of a pound; besides this, it has drunk not less than one- 
half an ounce of water. So the food taken by a single silkworm in 
fifty-six days equals in weight eighty-six thousand times the primitive 
weight of the worm. Of this, about one-fourth of a pound becomes 
excrementitious matter, two hundred and seven grains are assimilated, 
and over five ounces have evaporated. What a destruction of leaves 
this single species of insect could make, if only a one-hundredth part 
of the eggs laid came to maturity! A few years would be sufficient for 
the propagation of a number large enough to devour all the leaves of 
our forests.} 
When we consider the dangers arising from the immense 
numbers, fecundity and voracity of insects, the fact that 
insects new to cultivated crops are continually appearing 
becomes a source of grave apprehension. 
THE GREAT LOSS TO AMERICAN AGRICULTURE BY 
INSECT RAVAGES. 
Economic entomologists, who are constantly increasing 
our knowledge regarding insect pests, discover every year 
new species attacking important crops or trees. Dr. Lintner 
made a list of the insects injuring apple trees in the United 
States, which was published in the appendix to his first 
report as entomologist of New York State. It contained 
one hundred and seventy-six species, while large though 
lesser numbers have been found on the plum, pear, peach, 
and cherry. . 
The study of the insect enemies of the forest trees of the 
United States has not yet progressed far enough to deter- 
+ The American Silkworm, by L. Trouvelot. American Naturalist, Vol. I, 
p. 85. 
