VALUE OF BIRDS TO MAN. 31 



weighs one-twentieth of a grain ; wlien ten daj's 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, or six hundred and twenty times the 

 original weight; Avhen foi'ty days old, it weighs ninety grains, or eight- 

 een himdred times the original weight ; and when fift3?-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 ^jart 

 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. 1 



When we consider the dangers arising from tlie immense 

 nmiibers, fecundity and voracity of insects, tlie 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 j^ear 

 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. 



