THE MASTERY OF THE AIR 223 
the air means new opportunities of reaching 
suitable places for laying eggs or bringing up 
the young. The rooks’ nests swaying on the 
tree-tops, what a shrewd idea! 
THE FLIGHT OF INSECTS 
The problem of flight has been solved four 
times by animals, and each time in a different 
way. The first solution was that discovered 
by insects. In insects the two pairs of wings 
arise as hollow, flattened sacs, which grow out 
from the upper part of the sides of the body. 
They arise from the thorax region behind the 
head, a region with three rings or segments, 
each bearing a pair of legs. The wings grow 
out from the second and third rings of the 
thorax, and they have nothing to do with 
limbs. While the wings of a bird are trans- 
formed fore-limbs, the wings of insects are on 
a different line altogether; but we do not 
know to what they can be compared—they 
are just insects’ wings! 
The insect’s body is very lightly built, and 
the secret of the insect’s flight is the extremely 
rapid vibration of the wings, like the pro- 
peller of an airship. A watch ticks sixty 
