Air has so much less buoyancy than water that organisms emerging from water or land are immediately subjected to a stronger pull of gravity which holds them to the earth. They are also exposed to evaporation of water from their bodies, which threaten loss of water from the protoplasm and consequent death due to desiccation. Changes in temperature are much more drastic in air and there is a greater danger of chilling or overheating. Further, periods of light exposure are much longer and much more intense in air than in water or upland. Even more, the problem of mineral supply becomes acute in air. Only a few micro-organisms, plants and animals have invaded the air.
The essential function of dispersal of fruits and seeds of certain flowering plants is performed by the air currents. These fruits and seeds have various kinds of adaptations for such wind dispersal. Various animals of different groups exhibit adaptations for the aerial mode of life. The animals which can fly in air, are said to have flight or Volant adaptations. Volant Adaptations of Animals:Animals which use atmosphere as the medium of living, have twofold adaptations, one kind of adaptations for aerial mode of life (i.
e. , Volant adaptations) and others for trees, earth or aquatic environment, to which, they have to return for rest, feeding, reproduction and other essential activities of life, since, no animal can permanently dwell in air. Flight or Volant adaptations fall into following two types:
(a) Adaptations for gliding type flight:
Gliding is characterized by leaping from a high point and held up by certain sustaining organs, then to glide to lower level. Such gliding flight is performed by following animals: fishes like Exocoetus peeilopterus, Dactylopterus, Gastropelecus and Pegasus volitans, amphibians like Rhacophorus (flying frog) , reptiles like Rhamphorhynchus (extinct flying dinosaur called pterodactyl of mesozoic era), flying lizards like Draco dussumieri, Draco volans and Ptychozoon ; birds like Archaeopteryx (extinct bird), Struthio (ostrich) , etc., and mammals like flying phalangers (Petaurus), rodents (Anomalurus, Pteromys, Sciuropterus, and Eupetaurus), and lemurs.
Morphological adaptations for gliding include (i) Development of a fold of skin called patagium between fore-limbs and hind- limbs. This patagium provides sustaining surface for gliding. Patagia of both sides can be folded like a fan against the sides of the body when not in use.
(ii) In flying fish, pectoral fins become enlarged in the form of parachutes and are highly inserted on the body. (iii) In flying frog (Rhacophorus pardalis) the feet are webbed which sustain it in the prolonged leaps. Its digits terminate in adhesive pads which help in adhesion to the trees.
(b) Adaptations for true flight:
In true aerial flight, the power (i.e., muscular power) is implied and the movement in air is sustained. Adaptations for true flight occur in various insects like; dragon flies, locusts, moths, butterflies, wasps, flies, bees, etc.
, most birds like vultures, kites, parrots, owls, crows, house-sparrows etc., and some mammals like insectivorous bats (Eptesicus, Desmo- dus and Magaderma), frugivorous bats or flying foxes (Pteropus medius, and Cynopterus). Volant adaptations for true flight include following morphological and physiological adaptations: 1. Development of feathers in birds which provide many advantages during flight. 2. In birds, the fore-limbs are modified into flight structures, the wings. In bat wing, humerus bone is well developed, radius bone is long and curved ulna is vestigial.
The thumb or pollex is free and clawed for crawling and climbing. In insects, wings are the lateral membranous folds of exoskeleton. They are attached with the thorax and are made up of thin, light but tough and elastic chitin.
3. The, bones of birds are hollow, spongy and contain many air- cavities. Such pneumatic bones reduce the weight of body and also provide buoyancy during flight.
Birds lack teeth and instead of them have a much lighter beak. 4. Birds also possess air- sacs which have air for respiration and are buoyant.
5. All flying animals have a compact and sleek body build-up and also have well developed flight muscles. 6.
Nervous system and sense organs of flying animals are well developed. 7. For the reduction of body weight, female birds have only one ovary.