Next to water as moulding environment comes to air, for the creature which inhabits either is surrounded on all the sides by a homogenous medium so that it becomes uniformly modified to offer least possible resistance to the attainment of speed.

Aerial creatures must return to the trees or earth or water when they wish to rest and hence their adaptations are always double. Flight may be passive and held up by sustaining organs the creature merely glides from a higher level to a lower level without any locomotive force. True flight implies power and there is a sustained movement through the air. It has evolved three times in the vertebrates-in pterodactyles (Reptilla), birds (Aves) and bats (Mammalia).

The following adaptations are noteworthy:

1. Adaptations for Gliding

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The gliding flights are performed by fishes (Exocoetus) amphibians (Rhacophorus), reptiles (Draco) birds (Ostriches) and mammals (lemurs). The following adaptations are noted:

(i) Development of Patagia. Except in fishes, the patagium is a fold or series of folds of skin supported by ribs. In Draco, there is a pair of large wing-like membranes, which extends outwards from depressed body and each supported by 5 or 6 ribs. In flying lemur (Galeopithecus), the patagium extends from the back of the neck to the tip of the tail even including digits.

(ii)Enlargement of Pectoral Fins. In the flying fish (Exocoetus) large pectoral fins are present, which serves as organ of flight. Pelvics are smaller while the lower lobe of tail is longer. Exocoetus flies up to 200-300 meters. Another example are Pantodon. (African flying fish). Gigantopterus (Extinet flying fish) etc.

(iii) Webbing of Feet. The tree frog, Rhacophorus is the only amphibian in which volant adaptations is found. It has webbed feet by which it has long prolonged leaps. The digits terminate in adhesive pads. Rudiments of patagia infront and behind the arms also present.

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2. Adaptations for True Flight

True flight is seen birds, insects and few mammals like bats and flying squirrels etc. Following modifications are seen here.

(i) Feathers:

Birds develop peculiar structures known as feathers which show an extreme adaptation for flight. Each feather consists of a quill or calamus and the rachis or shaft.

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On the rachis a double series of barbs are developed carrying a double series of barbules which in their turn give rise to barbicels, these terminate distally, into hook lets. These catch in the folded margins of the next proximal row, thus producing a firm surface.

(ii) Wings:

There are three types of wings: (a) bat wings (b) pterodactyle wings and (c) bird wings (a) In bat-wing humerus is well developed, radius is long and ulna is vestigial. The polex is free and clawed. In microchiroptera, second finger is distinct but not free from the third and is attached distally, to support the anterior margin of the wing.

Fourth and fifth digits are also well-developed (b) In Megachiroptera; the second digit is free from third and possesses a claw like the first. In pterodactyl wing, radius and ulna are more or less equal while the former or radius is somewhat smaller. The wing is flexed between the metacarpal and proximal phalynx rather than the wrist as in birds and bats, (c) In bird-wings, digits are

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(iii)Pneumatic Bones:

Hollow air filled bones are found in birds and pterodactylas. In birds, air sacs not make them light but act as accessory respiratory organs.

(iv) Sternum and Pectoral Girdle:

Sternum or breast bone is well developed bearing a keel or carina for the attachment of pectoral muscles which support the wings. Pectoral girdle is rigid and develops clavicles.

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(v) Brain and Sense Organs:

Birds depend for their main contact with the world upon vision rather than small. The optic lobes are large. The cerebellum is much developed and convoluted in nature indicates the delicate sense of equilibrium and the great power of muscular co-ordination belonging to birds. The enormous development of corpora strata in the cerebrum also adds to the extraordinary manoeuvrability to attain stability in flight.