Flight
Flying organisms include insects, birds, and bats, all of which evolved the ability to fly (and the wings that flight requires) independently. Flying squir­rels, flying fish, and other animals that only glide are not considered capable of true flight. In general, flight requires an animal to generate enough lift to overcome the force of gravity. Unless it is hovering, the an­imal also needs to generate directional thrust, to move once it is in the air. The different groups of animals manage these tasks in different ways.
Insects
Among the many other titles insects hold (including being the most nu­merous and diverse group of animals) they can claim the title of the first flying organisms, having taken to the air tens of millions of years before the pterosaurs (extinct flying dinosaurs), and hundreds of millions of years be­fore birds and bats. Most insects can fly, or are descended from flying an­cestors, and are grouped in the subclass Pterygota (“having wings”). The more primitive, nonflying insects are grouped in the class Apterygota (“not having wings”). Unlike wings of the other flying animals, insect wings are not modifications of legs but rather separate appendages, outgrowths of the thorax. It is not known how insect wings evolved—the fossil record is not that complete—but there are many hypotheses, including the ideas that wings first evolved for gliding, as solar collectors, or as gills on aquatic ju­venile insects.
Insects manipulate their wings using two kinds of muscles: direct, which are attached to the wing, and indirect, which alter the shape of the thorax. In flight with the indirect muscles, the wing acts as a lever, with a part of the thorax as its fulcrum, and tilts up or down as the thorax changes shape.
Many insects are so small that the relative thickness of the air is too great for them to fly as birds, bats, and airplanes do. Instead, because of the viscosity of the air, they move in a way more akin to swimming than glid­ing or soaring.
Vertebrates
Flight has evolved independently in vertebrates at least three times: in pterosaurs, birds, and bats. Although scientists know that pterosaurs, like bats, flew on wings consisting of skin stretched from the hand to the body, it is not known how they kept such large bodies airborne. Bird wings, on the other hand are made up of flight feathers. Both birds and bats provide most of the thrust for flight with their wing tips, tilting them on both the down stroke and the upstroke so that they cut into the air at an angle and pull the body forward. Most of the lift, however, is provided by the base of the wing. In both birds and bats, as in airplanes, the wing is thicker at the front, convex on the top, and concave or flat on the bottom. As this shape slices through the air, a low-pressure zone is formed by the faster-moving air on top of the wing, and the higher pressure air beneath the wing pushes up on the wing, creating lift. To lighten their bodies and minimize the amount of lift they have to create, both birds and bats are usually relatively small, and birds have hollow bones.
References
Borror, Donald J., Dwight M. DeLong, and Charles A. Triplehorn. An Introduction to the Study of Insects. Philadelphia, PA: W. B. Saunders, Co., 1989.