upported, like a fan, by ribs of chitin, which may be accompanied by
air-tubes, blood-channels, and nerves. The insect's body is lightly
built and very perfectly aerated, and the principle of the insect's
flight is the extremely rapid striking of the air by means of the
lightly built elastic wings. Many an insect has over two hundred strokes
of its wings in one _second_. Hence, in many cases, the familiar hum,
comparable on a small scale to that produced by the rapidly revolving
blades of an aeroplane's propeller. For a short distance a bee can
outfly a pigeon, but few insects can fly far, and they are easily blown
away or blown back by the wind. Dragon-flies and bees may be cited as
examples of insects that often fly for two or three miles. But this is
exceptional, and the usual shortness of insect flight is an important
fact for man since it limits the range of insects like house-flies and
mosquitoes which are vehicles of typhoid fever and malaria respectively.
The most primitive insects (spring-tails and bristle-tails) show no
trace of wings, while fleas and lice have become secondarily wingless.
It is interesting to notice that some insects only fly once in their
lifetime, namely, in connection with mating. The evolution of the
insect's wing remains quite obscure, but it is probable that insects
could run, leap, and parachute before they could actually fly.

The extinct Flying Dragons or Pterodactyls had their golden age in the
Cretaceous era, after which they disappeared, leaving no descendants. A
fold of skin was spread out from the sides of the body by the enormously
elongated outermost finger (usually regarded as corresponding to our
little finger); it was continued to the hind-legs and thence to the
tail.

It is unlikely that the Pterodactyls could fly far, for they have at
most a weak keel on their breast-bone; on the other hand, some of them
show a marked fusion of dorsal vertebrae, which, as in flying birds, must
have served as a firm fulcrum for the stroke of the wings. The quaint
creatures varied from the size of a sparrow up to a magnificent spread
of 15-20 feet from tip to tip of the wings. They were the largest of all
flying creatures.

The bird's solution of the problem of flight, which will be discussed
separately, is centred in the feather, which forms a coherent vane for
striking the air. In Pterodactyl and bat the wing is a web-wing or
patagium, and a small web is to be seen on the front side of