Insect flight

Insects are the only group of invertebrates that have evolved wings and flight. Two insect groups, the dragonflies and the mayflies, have flight muscles attached directly to the wings. Other insects have the flight muscles attached to the thorax, making it oscillate, sometimes at a faster rate than the arrival rate of nerve impulses, and indirectly causing the wings to beat. Clap 1: wings close over backClap 2: leading edges touch, wing rotates around leading edge, vortices formClap 3: trailing edges close, vortices shed, wings close giving thrustFling 1: wings rotate around trailing edge to fling apartFling 2: leading edge moves away, air rushes in, increasing liftFling 3: new vortex forms at leading edge, trailing edge vortices cancel each other, perhaps helping flow to grow faster (Weis-Fogh 1973) Insects are the only group of invertebrates that have evolved wings and flight. Two insect groups, the dragonflies and the mayflies, have flight muscles attached directly to the wings. Other insects have the flight muscles attached to the thorax, making it oscillate, sometimes at a faster rate than the arrival rate of nerve impulses, and indirectly causing the wings to beat. Some very small insects make use not of steady-state aerodynamics but of the Weis-Fogh clap and fling mechanism, generating large lift forces at the expense of wear and tear on the wings. Many insects can hover, maintaining height and controlling their position. Some insects such as moths have the forewings coupled to the hindwings so these can work in unison. Insects first flew in the Carboniferous, some 350 million years ago. Wings may have evolved from appendages on the sides of existing limbs, which already had nerves, joints, and muscles used for other purposes. These may initially have been used for sailing on water, or to slow the rate of descent when gliding. Unlike other insects, the wing muscles of the Ephemeroptera (mayflies) and Odonata (dragonflies and damselflies) insert directly at the wing bases, which are hinged so that a small movement of the wing base downward, lifts the wing itself upward, very much like rowing through the air. Dragonflies and damselflies have fore and hind wings similar in shape and size. Each operates independently, which gives a degree of fine control and mobility in terms of the abruptness with which they can change direction and speed, not seen in other flying insects. This is not surprising, given that odonates are all aerial predators, and they have always hunted other airborne insects. Other than the two orders with direct flight muscles, all other living winged insects fly using a different mechanism, involving indirect flight muscles. This mechanism evolved once and is the defining feature (synapomorphy) for the infraclass Neoptera; it corresponds, probably not coincidentally, with the appearance of a wing-folding mechanism, which allows Neopteran insects to fold the wings back over the abdomen when at rest (though this ability has been lost secondarily in some groups, such as in the butterflies). In the higher groups with two functional pairs of wings, both pairs are linked together mechanically in various ways and function as a single wing, although this is not true in the more primitive groups. There are also exceptions to be found among the more advanced Neoptera; the ghost moth is able to unlock its pair of wings and move them independently, allowing them to hover like dragonflies. What all Neoptera share, however, is the way the muscles in the thorax work: these muscles, rather than attaching to the wings, attach to the thorax and deform it; since the wings are extensions of the thoracic exoskeleton, the deformations of the thorax cause the wings to move as well. A set of dorsal longitudinal muscles compress the thorax from front to back, causing the dorsal surface of the thorax (notum) to bow upward, making the wings flip down. A set of tergosternal muscles pull the notum downward again, causing the wings to flip upward. In a few groups, the downstroke is accomplished solely through the elastic recoil of the thorax when the tergosternal muscles are relaxed. Several small sclerites at the wing base have other, separate, muscles attached and these are used for fine control of the wing base in such a way as to allow various adjustments in the tilt and amplitude of the wing beats. Insects that beat their wings fewer than one hundred times a second use synchronous muscle. A synchronous muscle is a type of muscle that contracts once for every nerve impulse, which is more efficient for fast flight.