Insect wing movement is a complicated process that involves upstrokes, downstrokes, wing rotation, and wing flexing. First, let's examine the two types of muscle arrangements that produce the up and down movements of wings. These muscles are known as direct and indirect flight muscles. A flight is powered by direct flight muscles, both the up and downstrokes are achieved by muscles attached directly to sclerites at the base of each wing. One of these muscles is attached to the wing base proximal to the pivot point. As this muscle contracts, the wing pivots on a point located on the thorax and swings upwards. The second muscle, is attached at the wing base distal to the pivot point, and moves the wing downwards when it contracts. These two muscles work in tandem. As one contracts, the other relaxes moving the wings up and down, generating lift and allowing the insect to fly. Because wing movement is facilitated by muscles attached directly to the wings, some insects with direct flight can move each wing independently allowing them to quickly change direction and speed. Insects that use direct flight include members of the palaeoptera, the mayflies, dragonflies, and damselflies, and some neoptera such as cockroaches. Since dragonflies and damselflies are predatory, this aerial agility is essential for chasing down and capturing insect prey. Most insects power flight using indirect flight muscles. In this case, the wings move because of changes in the shape of the thorax and not because of contraction of muscles attached to sclerites. This is because indirect flight muscles are attached to ridges within the thoracic exoskeleton, rather than directly to the basis of the wings. There are two groups of indirect flight muscles. The dorsoventral muscles, which generate the upstroke, and the dorsal longitudinal muscles, which create the downstroke. The dorsoventral muscles, as the name suggests, are attached to the notum and sternum within the insects thorax. When these muscles contract, they deform the shape of the thorax and pull the notum downwards. This force is the wing joint down, which forces the wingtips upwards. The dorsal longitudinal muscles are attached to ridges internally on the anterior and posterior walls within each thoracic segment. As these longitudinal muscles contract, they cause the notum to go upwards. This forces the wing joint to move upwards against the fulcrum, bringing the wingtips down. As we saw with direct flight muscles, indirect flight muscles also work in tandem. When one set of muscles contract, the other relaxes, moving the wings up and down and lifting the insect into the air. A lot of energy is conserved during insect flight because elastic components of the cuticle store and release energy as the insect flies, resulting in greater efficiency. As the insect flies, energy is used for the muscular contractions that drive each wing stroke. At the end of each stroke, the wing has momentum that must be opposed to move the wing in the opposite direction. The energy of this momentum is stored in the cuticle or in the flight muscles themselves and is released during the opposing stroke to minimize energetic inputs. This pattern continues throughout the insects flight and ensures that little energy is wasted during the wing strokes. The energy from the momentum of the wing is often stored in elastic elements of the cuticle using the protein resilin. All winged insects fly by beating their wings. However, this does not mean that insect flight only involves flapping the wings up and down. Remember that joints at the base of the insect wing contained cuticular sclerites, each of which is associated with individual muscles. Instead of powering flight, these muscles and sclerites work together to twist and rotate insect wings, which fundamentally changes how the wings interact with the air. Each wing beat can be divided into three types of movement. The first component, is the vertical flapping motion to lift and propel the insect. The second type of movement, involves the rotation of the wing around its base, which allows the insect to control air movement around the wings for pitch control. This also allows the wings to be beat in a three-dimensional pattern down and forward, then up and backward in a figure eight, rather than simply vertically up and down. The third and final movement involves the wing flexing for finer flight control. Together, these three movements determine the angle and orientation of wings pushing air down and backwards, thus providing the lift and thrust required for flight. As the wings moved downwards, muscles attached to the sclerites contract simultaneously, tipping the leading edge of the wing down. During the upstroke, contraction of another set of muscles causes the leading edge of the wing to rotate upwards. This allows the wing tips to move in a figure eight pattern during flight, a process that influences the amount of force and power the wings can produce. In addition to allowing fine tuned movements, the muscles attached to wing sclerites, also play an important role in wing folding. Wing beat frequencies vary greatly among insect species. In some cases, insect wings can reach as many as 1,000 beats per second. That's faster than the nervous system can receive and register signals. In contrast, slower flying insects have wing beat frequencies that average only 10 to 50 beats per second. This variation is due to different types of flight muscles called synchronous and asynchronous muscles. These muscles differ in the number of contraction cycles they undergo from a single nerve impulse. In synchronous muscles, one nerve impulse produces one contraction cycle. In asynchronous muscles however, a single nerve impulse can produce multiple muscle contractions cycles and generate a rapid wing beat frequency. In some of the more derived neopteran insects that power their flight using indirect muscles, asynchronous flight muscles will contract automatically when stretched beyond a certain threshold. All that is needed to initiate flight in these insects is a single nerve impulse, a start signal of sorts. The longitudinal or dorsoventral muscles contract in response to being indirectly stretched by the other contracting muscle, so in movement will continue until a stop signal is given. Now that we understand some of the intricacies of insect flight muscles, the next video will focus on what types of techniques researchers have used to study flight in insects.
