Wings

Insect wings likely evolved from bilateral, dorsal flaps called gill plates on thoracic and abdominal segments of an aquatic ancestor. These gill plates are seen on some fossilized early insects, like those in the order Palaeodictyoptera. During the evolution of gill plates into winglike appendages, the appendages grew larger and became restricted to the second and third thoracic segments. Some early insects used these winglike appendages to sail on the surface of ponds or glide in gentle updrafts and air currents.

Fly Wings

In the fruit fly Drosophila, each wing develops from a packet of epithelial cells called the imaginal disk. The imaginal disk grows into a wide, baglike structure that flattens out to become a transparent wing only a few cells thick. Hemolymph-filled veins along the anterior wing margin and within the wing support the fragile wing and supply the wing cells with nutrients and water.

Drosophila powers its single pair of wings quite differently than the more primitive dragonflies and grasshoppers with two pairs of wings. Flies have internal dorsoventral (vertical) muscles on each side of the body that pull the top of the thorax down when they contract. This moves each wing up because the wing bases are inserted into the top of the thorax. Contracting longitudinal muscles (running along the length of the thorax) force the top of the thorax back to its original position. This causes each of the wings to move down. Wing movement in most advanced flying insects is associated with the extremely rapid deformation of the thorax. Houseflies can flap their wings up to two hundred times per second, whereas gnats can flap up to one thousand times per second.

The wings of true flies not only flap up and down, but they also alter their angle of attack while moving forward during the down stroke and moving backward during the upstroke. These complex movements provide both lift and thrust and require several highly evolved muscle groups.

Lift

For many insects, simple wing flapping cannot generate sufficient lift for flight or for hovering. This is especially true of tiny insects or insects that have small wing size to body-weight ratios, such as bumblebees. Some flying insects are less than one millimeter long and have extremely small wings that have difficulty moving through air. The viscosity of the air prevents the development of sufficiently small leading-edge vortices. So, how do insects produce sufficient lift for flight and hovering?

Beginning with wings above the body, the downstroke moves the wings down and forward through the air at a high attack angle that produces lift. Leading and lagging edge vortices that lower the air pressure above the wing cause lift. As the wing moves down and forward, it rotates slightly, creating larger vortices above the wing that increase the lift. The leading edge vortex begins near the base of the wing and develops quickly toward the tip. Air entering the vortex moves from the base of the wing to the tip. The upstroke provides insects with additional lift. In the upstroke, the wing rotates so that the leading edge is still ahead of the trailing edge. The wing moves up and backward through the air. During the upstroke, lift is created by air reentering the wake created by the downstroke. This moving air pushes against the wing and provides lift. Lift is the sum of “delayed stall” during the downstroke and “wake capture” on the upstroke. In most insects, wing curvature also plays a small role in the development of lift. Most insect wings do not remain flat or rigid during flight or hovering.

Unlike airplanes, which use wings only for lift and have propellers or jet engines for thrust, insects use their wings to generate both lift and thrust. Although lift is generated during both the downstroke and upstroke, thrust is generated only on the upstroke. In fact, during the downstroke, there is a force on the insect that slows it or causes it to hover.

Pterosaur Wings

The first pterosaurs appeared in the fossil record about 215 million years ago. They were a very successful group of reptiles, surviving until the mass extinction that marked the Cretaceous-Tertiary boundary 66 million years ago. These flying animals evolved from a population of tetrapods, carnivorous reptiles. Their wings consisted of leathery skin supported by forelimbs, body trunks, and hind legs. The leathery skin of the wings was reinforced by stiff fibers that ran from the front of the wing to its trailing margin.

The forelimbs of these flying reptiles consisted of an upper arm bone (humerus), two forearm bones (radius and ulna), wrist bones (carpals), palm bones (metacarpals), bones of three claw-tipped fingers (phalanges), and the bones of a fourth, very long, clawless finger (phalanges). The first, second, and third finger bones are the thumb, the index finger, and the middle finger. The greatly elongated palm and fourth finger bones along the distal front edge of the wing were longer than the upper and lower arm combined and supported about three-quarters of the wing.

The wings of all pterosaurs were long and sickle-shaped, yet they could fold up like a fan along the sides of the body. Pterosaurs and their wings varied in size. The smallest pterosaurs, like Ambopteryx longibrachium and Nemicolopterus crypticus, were about the size of a starling, with a wingspan of about one foot, whereas the largest animal, Quetzalcoatlus northropi, was about twenty-six feet in length from bill to trailing feet, with a wingspan of nearly forty feet. The elongated palm and fourth finger bones were over fifteen feet. Quetzalcoatlus northropi was the largest flying animal ever, weighing 200 to 500 pounds (90 to 225 kilograms). The long, thin, swept-back, sickle-shaped wings suggest that the pterosaurs were rapid flyers and consummate gliders. Hatzegopteryx thambema, only slightly smaller than Quetzalcoatlus northropi, was also a large flying animal with a long bill similar to a stork. The main distinguishing feature between these two large flying animals was their jaw articulation and the Hatzegopteryx thambema's stocky beak and shorter neck.

Bird Wings

Approximately 155 million years ago, birds began to evolve from a population of feathered, bipedal, meat-eating dinosaurs. Their wings consisted of feathers supported by the forelimbs. The forelimbs of these early, birdlike animals consisted of an upper arm bone (humerus), two forearm bones (radius and ulna), wrist bones (carpals), palm bones (metacarpals), and bones of the three claw-tipped fingers (phalanges).

In modern birds, the wing bones of the palm and fingers have changed significantly. The palm and first finger bones have been fused and greatly shortened so that a single, small thumb bone protrudes near the wrist. The feather or feathers originating in the skin at the end of the thumb are known as the alula. The palm bones associated with the second and third fingers have partially fused at their ends to become a bone roughly resembling a fused miniature radius and ulna. The second finger that supports the feathers forming the tip of the wing has become shorter. The third finger has been reduced to a single, tiny bone. The long feathers protruding from the skin covering the palm and second finger are called primaries, whereas the long feathers attached to the forearm are known as secondaries. Feathers attached to the upper arm are called tertiary feathers.

Wing flapping is powered by muscles that stretch from the base of the humerus to the large, keel-like sternum. Muscles attached to the top of the humerus base and to the sternum pull the wings up, whereas muscles attached to the bottom of the humerus base and the sternum pull the wings down. The modern chicken is closely related to the dinosaur, but not a pterosaur as one might expect due to their shared wings. Scientists believe the chicken is most closely related to the Tyrannosaurus rex and shared more similarities in skeletal structure with the giant land dinosaur than any flying prehistoric species.

Wing Shapes for Special Tasks

A bird’s lifestyle and habitat determine the selection of wings, which may be divided into several categories. Birds that live in forested or densely wooded habitats or birds that prey on flying insects and other fast-moving animals have short, broad wings, sometimes described as elliptical wings. Elliptical wings allow birds to carry out rapid, intricate maneuvers. These wings have a high degree of camber and extensive slotting. Highly cambered wings provide greater lift than more flattened wings. The separation of primary feathers at the wing tip provides most of the needed thrust by acting as miniwings. Generally, the primary feathers that constitute the wing tip generate most of the thrust and even some lift.

The first finger (thumb) has one or more feathers parallel to the anterior wing margin, which can be raised above the front of the wing to eliminate air turbulence above the wing. While turbulence significantly decreases lift, the raising of the alula decreases turbulence and increases lift.

Sparrows, finches, cuckoos, barn owls, warblers, grouse, and similar birds have elliptical wings that develop a high degree of camber and slotting. Because of these wing characteristics, birds with elliptical wings are capable of intricate maneuvers. Pigeons have broad wings for maneuverability but also elongated, pointed tips to increase their speed. To muffle the noise generated when barn owls swoop down on prey, their flight feathers have developed fringed edges, and their coverts have become soft and downy.

Large birds of prey, such as vultures, buzzards, eagles, condors, swans, and storks, have high-lift wings. Their wings are wide and highly cambered, with extensive slotting at the tips. The slightest updraft of air provides lift for these heavy birds. The inner flight feathers provide most of the lift within a thermal, whereas the long primary feathers that resemble fingers are used for maneuvering and creating thrust as well as additional lift. The larger the bird, the fewer wing beats per second. Large vultures flap their wings about once each second.

Many marine birds that live along the shore and fly long distances to find food, as well as many migrating birds, have nearly flat, narrow, long, pointed wings. These wings are associated with rapid flying and, in some cases, with hovering. Peregrine falcons, gulls, geese, ducks, swifts, and swallows are examples of fast-flying birds. Long, narrow, swept-back wings are found in some birds that develop high speeds, such as swifts, swallows, and falcons. The common swift flies up to five hundred miles daily, gathering insects for its chicks. The Arctic tern is known for its long migration from the North American Arctic via Europe and Africa to Antarctica, approximately eleven thousand miles. The peregrine falcon is an extremely rapid flyer, sometimes reaching speeds of 175 miles per hour when it dives in pursuit of other birds.

Kingfishers, hummingbirds, and kestrels are both fast and capable of hovering. Hummingbirds beat their wings forty to eighty times each second, depending on their size. In addition, they can hover and fly backward for short distances. To achieve these feats, the wing bones evolved quite differently than in other birds. Hummingbird wing bones are mostly hand bones, as the forearm and upper arm became extremely short. The wrist joint and elbow became rigid so that the wing only rotates at the shoulder. Their wings are thin, flat, and pointed and can become highly slotted. When these birds hover or fly backward, they obtain lift on both the up- and downstrokes of the wing, somewhat like insects. Larger birds usually only generate lift on the downstrokes.

Accomplished gliders, such as the albatrosses and frigate birds, have nearly flat, extremely long, narrow wings. These wings efficiently create lift from updrafts and surface air movements over water and land. These marine birds, which fly long distances when feeding or during migrations, have very long, slender, pointed wings. Each of these birds catches updrafts and can glide long distances without using much energy flapping their wings.

Over sixty million years ago, penguins had longer legs, beaks, and wings, which they used to fly. However, possibly because of a diving adaptation to accommodate a new food source, they stopped flying, and their wings evolved to become more like flippers to help them swim up to 25 miles (40 kilometers) per hour. The wings of many flightless birds, like ostrich, emu, cassowaries, and rheas, evolved to aid in balance, temperature regulation, mate selection, direction guidance, and slowing down when sprinting.

Bat Wings

Bats evolved from tetrapod, insectivorous mammals sometime after the Cretaceous-Tertiary mass extinction 65 million years ago. Their wings consist of elastic, leathery skin supported by forelimbs, body trunk, and hind legs. The upper arm and forearm support the proximal half of the extended wing, while the elongated hand and digit bones support the distal half of the wing like the struts of an umbrella. Greatly elongated palm bones and digits two, three, four, and five provide the outer wing struts. The thumb, midway along the anterior margin of the wing, is a mere stump. Depending upon the bat, sometimes the thumb has a claw the bat uses to hold onto its perch.

Bats are extremely agile fliers because they can alter the camber and shape of their wings. The skin between the body and the fifth finger and the tail membrane generates most of the lift. The skin between the second and fifth fingers produces forward thrust. Highly maneuverable bats have relatively short, broad wings, whereas migrating bats have exceptionally long, narrow, pointed wings. These long wings increase lift and allow for extended gliding, but bats with long wings are not as agile as those with broader wings. Tail membranes are continuations of the wings and are used for sudden turns and changes of direction. The tail membrane is controlled by the back legs. The tail membrane is also used during landings to brake and help flip the bat upside-down so it can attach to its roost. The bat uses both its wings and its tail membrane to stall as it lands.

Some species of bats are strong enough to take off from the ground, but most initiate their flight from an elevated roost. Although bats do not have a keeled sternum like birds, some can take off from the ground, and most are very agile fliers.

Principal Terms

Camber: the degree to which wings are convex on their top and concave on their underside

Lift: the upward force that is developed by moving wings, which opposes the pull of gravity

Slotting: the separation of primary feathers at the tip of the wings

Thrust: the forward force that is developed by engines, rotors, or moving wings, which pushes planes, helicopters, or flying animals and which opposes drag

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