AERIAL ADAPTATIONS IN VERTEBRATES
The aerial adaptations are concerned with the flight. The flight is a form of locomotion in the air under which the body has to be firstly prevented from falling down and secondly moved forwards, the speedier the better. Thus, aerial adaptations must include modifications in the animals body for reducing the weight of the body and also for the formation of organs capable of executing the flight.
Types Of Flight In Vertebrates
1.Passive or gliding type flight
- This type of movement involves no propulsion other than the initial force of jumping. Gliding is characterised by leaping or jumping from a high point and held up by some sustaining organs, then to glide to lower level. Thus, there is no locomotive force other than gravity. Here the ‘wings’ are made of petagia which do not flapped (i.e., do not move up and down) by the muscular action.
- The gliding flights are performed by various lizards (e.g., Draco volans or flying dragon, fishes (e.g., Exocoetus), birds (e.g., ostriches, etc.,) mammals such as flying phalangers (Petaurus sciurens, a marsupial, flying squirrel etc., and amphibians (e.g., Rhacophorus).
All these forms are found to possess the following adaptaptions for the gliding :-
(i) Development of patagia :-
- The sustaining surface (“wing”) for the gliding is a fold or series of folds of skin, called patagium. The patagium lies between forelimbs and hindlimbs and can be folded like a fan against the body when is not in use. In Draco, the patagium is supported by ribs. Ptychozoon is another gliding lizard which is commonly known as “the flying or fringed gecko” and in which lateral expansion of skin (patagium) extends along the side of neck, body, tail and limbs and between toes. Flying snakes (e.g., Chrysopelea) leap by the concave ventral side of body. The extinct reptiles pterodactyls of Mesozoic era were volant creatures akin to birds, containing true flight patagia. Their patagia were extensions between limbs supported by ribs.
- Among mammals, flying squirrel, Sciuropterus has highly developed patagium. Its wide hairy tail is further supplemented by the hairy fringe on the patagium and along the rear of the thighs. In flying lemur (Galeopithecus volans) the patagium extends from side of the neck to the tip of the tail even including digits, which are also webbed. In the bats, the patagium is supported mainly by the elongated forelimbs and the second, third and fifth digits. The first digit remains free.
- Traces of patagia are also found in front and behind the arms (forelimbs) in birds which have adequate supporting function.
- Flying frog too contains rudiments of patagia in front and behind the limbs.
(ii) Enlargement and high insertion of pectoral fins
- In flying fishes (Exocoetus) are trim-built creatures with large parachute-like pectoral fins which are highly inserted on the body. The pelvic fins are much smaller in size. The lower lobe of tail is also invariably longer, helping in leaping. The pectoral fins do vibrate. Exocoetus can fly up to 200–300 metres to escape from the large fishes such as tunny and albicore). Other genera of flying fishes are Dactylopterus, Pantodon and Pegasus. Pegasus is a little marine fish found along the coasts of Japan, China, India and Australia. They skim along the surface of the water for 40 feet or more.
(iii) Webbing of feet.
- In flying frog (Rhacophorus paradailis, the feet are webbed which sustain the prolonged leaps. Flying frog’s digits terminate in adhesive pads which help in adhesion to trees.
2. Active or true flight
- It is the aerial flight caused by the action of wings. True flight is found in insects, pterodactyls, birds and bats. In all of them the nature of development and structure of wings are quite different and their analogy suggest that the flight has evolved independently in different groups. In true flights the power is implied and the movement in air is sustained.
Varieties of Wings
Bat wing :- In a bat wing, humerus bone is well developed, radius is long and curved and ulna is vestigial. The pollex (thumb) is free and clawed for crawling and climbing. In smaller insectivorous bats (Microptera), the second finger is not free from third, but is attached to it distally. These two support the anterior margin of the wing. The fourth and fifth digits are well developed. In frugivorous bats (Megachiroptera, Fig. 18.9), the second digit is clawed and free from the third.
Pterodactyl wing :- In pterodactyl’s wings, the radius and ulna bones are nearly equal. The next segment consists of a heavy fourth metacarpal bearing great wing finger and three small metacarpals supporting the first, second and third clawed digits. There is also present a pteroid bone which is directed towards the shoulder and supposed to support the anterior margin of a prepatagium which lies in front of arm from the wrist to the neck. The single wing finger (i.e., fourth) is huge and forms the entire anterior support of the patagium beyond the wrist.
Bird wing :- In birds forelimbs are modified into flight structures, the wings. The wings of birds are more specialized of all modern wings. In a wing of bird, digits are reduced to three and these are fused together to help in flight. The metacarpals are three which are unequally developed and co-ossified. The digits are represented by one or two phalanges which support the so-called bastard quills (i.e., three, four or five feathers on the first digit (homologous of the thumb) of a bird’s wings.
Aerial Adaptations Of Birds
Bird flight is characterized by the flapping of the wings. In flying, the bird lifts its body and drives forward by beating its wings in a characteristic way. An object moved swiftly in the air is affected by two forces : 1. upward pushing (lifting) and 2. downward pull (drag) due to the action of gravity. Hence, for successful flight, enough force must be applied to neutralize the drag and also to move forward. Birds wings are slightly concave, so are able to produce the air-current for producing the lifting force.
Birds have the following adaptations for true flight :
- Body contour :- The streamlined body is spindle-shaped or boat shaped, encountering least aerial resistance and can easily be passed through the air. Thus, the beak is pointed; head is compact; neck is long and mobile and wings are attached high upon the thorax.
- Development of feathers :- The entire body of birds is invested with a close covering of feathers, constituting the plumage. The feathers form the exoskeleton of birds. These nature’s “master pieces” are light, elastic, waterproof and most important in flight. Bird’s feathers are classified into quill feathers, contour feathers, down feathers and filoplumes. Quills are flight feathers. Flight feathers of wings are called remiges and those of tail are called rectrices. Structurally, flight feather consists of a basal quill or calamus and distal rachis or shaft. The rachis bears a leaf-like vane (or vexillum) consisting of series of lateral barbs. Barbs consist of double row of barbules connected with each other by barbicles or hooklets (through interlocking arrangement). The barbs, barbules and barbicles form a sort of net which help in flight.
- Presence of wings :- In birds the fore-limbs are modified into wings helping in flying. The hindlimbs or legs are large and variously adapted for walking, running, scratching, perching, food-capturing, swimming.
- Pneumatization of bones :- The bones of birds are hollow and air filled. They also contain many air cavities. These add buoyancy during flight.
- Occurrence of flight muscles and keeled sternum :- In birds, specific flight muscles are developed which connect the wings with limb bones. Each wing is depressed or lowered by an enormous muscle called pectoralis major. It is elavated or raised by pectoralis minor (the tendon of which is inserted on the head of humerus). The sternum or breast bone is well developed and bears a median keel or carina for the attachment of pectoralis muscles.
- Development of air sacs :- They act as air reservoir during respiration and serve as balloons to provide lightness and buoyancy to the body. Air sacs also help in internal perspiration, thus, helping in the regulation of body temperature.
- Brain and sense organ’s specificity :- Cerebrum is well developed (for controlling manoeuvrability) and optic lobes become enlarged (for controlling the great development of sight) and olfactory lobes are reduced (i.e., power of smell is reduced). Bird’s eyes are large and bear characteristic sclerotic plates to resist variable air pressure. Eyes also contain pectens) which are comb-like, vascular and pigmented structures) to regulate fluid pressure within the eye (i.e.,accomodation).
- Beak :- The conversion of forelimbs into wings is compensated by the presence of a bill or beak. The beak is horny and lacks teeth.
- Mobile neck :- The neck of birds is very long and flexible.
- Single ovary :- Presence of a single functional ovary of the left side in the female bird also leads to reduction of weight which is very essential for flight.
- Absence of urinary bladder :- Birds do not have a urinary bladder which is present to store the urine temporarily in other animals. Further, birds excrete a semisolid excreta which chiefly contains the insoluble uric acid and urates (urecotelic excretion). These features help in reducing the weight of body.