- Animal inhabiting the aerial environment show many Volant (flying) adaptation for the aerial mode of existence.
- These animals are known as volant forms.
- Fishes, amphibians, reptiles, birds and mammals among vertebrates, and insects among invertebrates exhibit volant adaptations.
- Flight is of two types: i) active or true flight b) passive flight or gliding.
A)Active or true flight
- It involves long and sustained flight through the air.
- It implies power. The true fliers move the wings with varying degrees of rapidity.
- True flight is seen in birds, bats and insects.
- The birds possess boat shaped or spindle shaped (streamlined) body without any protruberances.
- Thus, they offer the least amount of resistance for movement in air.
- The forelimbs are modified into wings in birds.
- In bat the wings are made up of folds of skin, which are known as patagium.
- The upward and downward strokes of the wings help in the elevation of the bird and its subsequent propulsion.
- Changing in direction during flight is caused by tail feathers.
- The different parts of wing work as a single unit.
- A row of quill feathers called remiges is present on the posterior ridge of the wing.
- The body of bird is covered with feathers, which are beneficial to the birds in many ways.
- They act as blanket to insulate the body from external environment. Thus, they maintain high body temperature which is essential for high metabolic rate during flight.
- They form a smooth and closely fitting covering which reduces friction.
- It also makes the body light.
iv)Reduction in body weight
- The bones are hollow and air filled i.e. pneumatic type.
- The large pneumatic bones of the skeleton are responsible for reducing the body weight.
- It also offers large surface of attachment for muscles.
- Birds possess three types of light muscles.
- Pectoralis major, pectoralis minor and coracobrachialis are the types of muscles.
- Pectoralis major and coraco-brachialis by their contraction serve to pull the wings downwards while the pectoralis minor serves to pull the wings upwards.
vi)Presence of neck and beak
- The neck is highly mobile and mouth is produced into beak in birds.
- The highly mobile neck is used in feeding, nesting, offence and defense.
- Beak helps in picking up grains like forceps.
- It also helps in tearing and cutting.
- The hind limbs are adapted for walking.
- They are strong and shifted anteriorly to support the body weight and show bipedal locomotion.
- The hind limbs bear 4- clawed digits three in front and one behind.
viii)Energy for flight
- Blood is oxygenated twice during respiration due to which a high rate of metabolism occurs.
- Thus, birds are provided with larger amount of energy which is essential for long and sustained flight.
- The hind limbs are well suited for perching mechanism in birds.
- When a bird lands on a perch, the bending of legs exerts a pull on the flexor tendons which make the toes flex automatically and to grip the perch.
- The perching is so efficient that the birds can sleep while sitting on trees without any fear of falling.
- The sternum is broad, usually with a longitudinal ventral keel for the attachment of flight muscles.
- Birds bear additional respiratory air sacs which supply more oxygen for rapid oxidation during flight.
- They also provide buoyancy during flight.
- The tail of a bird is short and bears a series of long feathers which can be spread in a fan like manner.
- It serves as a rudder in steering and balancing organ in perching.
- In birds, the brain has enlarged optic lobes, which perform sharp vision.
B)Passive or gliding flight
- Animals cannot fly for long time in this type.
- The animals simply take an initial leap or jump from a high point to lower level and just glide in air with the help of certain organs.
- It may sometimes cover a horizontal distance of many meters.
- There is no involvement of locomotive force other than gravity.
- Here, the wings are made of patagia which cannot move up and down by muscular action.
- This type of flight is performed by a number of arboreal animals like the flying fish (Exocoetus), flying frog (Rhacophorus), flying dragon(Draco volans), birds (Ostrich), mammals such as flying phalangers (Petaurus sciureus), flying squirrel, flying lemur, etc.
- They show following adaptation for the gliding flight.
i)Enlarged pectoral fins
- Flying fishes possess very large and highly developed pectoral fins.
- They are used as parachutes to make along glide in air above the water surface.
- The lower large lobe of caudal fin helps in leaping by accelerating speed.
ii)Development of patagia
- Patagium is fold of skin present between forelimbs and hind-limbs.
- In flying lizards (dragon), the patagium extends on either side of the trunk and is supported by 5 to 7 pairs of long ribs.
- Flying gecko (Ptychozoon) is another flying lizard in which lateral expansion of patagium extends along the side of the neck, body, tail, limbs and between toes.
- Flying snakes leap by the concave ventral side of body.
- In flying frog, the webbed feet helps in long leaps among the trees.
- The webbed feet serve to increase the area of the supporting column of air.
- It also partly checks the speed before landing and further reduces the impact.
- The digits terminate in adhesive pads which help in sticking to smooth surfaces.