There are no animals thoroughly aerial, but many insects spend much of
their adult life in the free air, and the swift hardly pauses in its
flight from dawn to dusk of the long summer day, alighting only for
brief moments at the nest to deliver insects to the young. All the
active life of bats certainly deserves to be called aerial.
The air was the last haunt of life to be conquered, and it is
interesting to inquire what the conquest implied. (1) It meant
transcending the radical difficulty of terrestrial life which confines
the creatures of the dry land to moving on one plane, the surface of the
earth. But the power of flight brought its possessors back to the
universal freedom of movement which water animals enjoy. When we watch a
sparrow rise into the air just as the cat has completed her stealthy
stalking, we see that flight implies an enormous increase of safety. (2)
The power of flight also opened up new possibilities of following the
prey, of exploring new territories, of prospecting for water. (3) Of
great importance too was the practicability of placing the eggs and the
young, perhaps in a nest, in some place inaccessible to most enemies.
When one thinks of it, the rooks' nests swaying on the tree-tops express
the climax of a brilliant experiment. (4) The crowning advantage was the
possibility of migrating, of conquering time (by circumventing the arid
summer and the severe winter) and of conquering space (by passing
quickly from one country to another and sometimes almost girdling the
globe). There are not many acquisitions that have meant more to their
possessors than the power of flight. It was a key opening the doors of a
new freedom.
The problem of flight, as has been said in a previous chapter, has been
solved four times, and the solution has been different in each case. The
four solutions are those offered by insects, extinct Pterodactyls,
birds, and bats. Moreover, as has been pointed out, there have been
numerous attempts at flight which remain glorious failures, notably the
flying fishes, which take a great leap and hold their pectoral fins
taut; the Flying Tree-Toad, whose webbed fingers and toes form a
parachute; the Flying Lizard (_Draco volans_), which has its skin pushed
out on five or six greatly elongated mobile ribs; and various "flying"
mammals, e.g. Flying Phalangers and Flying Squirrels, which take great
swooping leaps from tree to tree.
The wings of an insect are hollow flattened sacs which grow out from the
upper parts of the sides of the second and third rings of the region
called the thorax. They are worked by powerful muscles, and are
supported, 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 vertebræ, 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 the bird's
wing. But the bird's patagium is unimportant, and the bird's wing is on
an evolutionary tack of its own--a fore-limb transformed for bearing the
feathers of flight. Feathers are in a general way comparable to the
scales of reptiles, but only in a general way, and no transition stage
is known between the two. Birds evolved from a bipedal Dinosaur stock,
as has been noticed already, and it is highly probable that they began
their ascent by taking running leaps along the ground, flapping their
scaly fore-limbs, and balancing themselves in kangaroo-like fashion with
an extended tail. A second chapter was probably an arboreal
apprenticeship, during which they made a fine art of parachuting--a
persistence of which is to be seen in the pigeon "gliding" from the
dovecot to the ground. It is in birds that the mastery of the air
reaches its climax, and the mysterious "sailing" of the albatross and
the vulture is surely the most remarkable locomotor triumph that has
ever been achieved. Without any apparent stroke of the wings, the bird
sails for half an hour at a time with the wind and against the wind,
around the ship and in majestic spirals in the sky, probably taking
advantage of currents of air of different velocities, and continually
changing energy of position into energy of motion as it sinks, and
energy of motion into energy of position as it rises. It is interesting
to know that some dragon-flies are also able to "sail."
The web-wing of bats involves much more than the fore-arm. The double
fold of skin begins on the side of the neck, passes along the front of
the arm, skips the thumb, and is continued over the elongated palm-bones
and fingers to the sides of the body again, and to the hind-legs, and to
the tail if there is a tail. It is interesting to find that the bones of
the bat's skeleton tend to be lightly built as in birds, that the
breast-bone has likewise a keel for the better insertion of the pectoral
muscles, and that there is a solidifying of the vertebræ of the back,
affording as in birds a firm basis for the wing action. Such similar
adaptations to similar needs, occurring in animals not nearly related to
one another, are called "convergences," and form a very interesting
study. In addition to adaptations which the bat shares with the flying
bird, it has many of its own. There are so many nerve-endings on the
wing, and often also on special skin-leaves about the ears and nose,
that the bat flying in the dusk does not knock against branches or other
obstacles. Some say that it is helped by the echoes of its high-pitched
voice, but there is no doubt as to its exquisite tactility. That it
usually produces only a single young one at a time is a clear adaptation
to flight, and similarly the sharp, mountain-top-like cusps on the back
teeth are adapted in insectivorous bats for crunching insects.
Whether we think of the triumphant flight of birds, reaching a climax in
migration, or of the marvel that a creature of the earth--as a mammal
essentially is--should evolve such a mastery of the air as we see in
bats, or even of the repeated but splendid failures which parachuting
animals illustrate, we gain an impression of the insurgence of living
creatures in their characteristic endeavour after fuller well-being.
We have said enough to show how well adapted many animals are to meet
the particular difficulties of the haunt which they tenant. But
difficulties and limitations are ever arising afresh, and so one fitness
follows on another. It is natural, therefore, to pass to the frequent
occurrence of protective resemblance, camouflage, and mimicry--the
subject of the next article.