Insect flight
The flight of insects is the oldest form of locomotion through the air and includes various flight techniques . There is no fundamental difference to the flapping flight of birds with regard to the forces of lift and propulsion, which are decisive for flying : the upward and downward wings serve as a drive and, in the case of larger wings, as wings , their leading edges are stiffened, while the remaining part remains elastic . Due to the smaller average size of the flying insects , the flapping frequency of their wings is, however, much higher than that of the vertebrates .
The wings of the insects are wings made of chitin . The flying insects are the most species-rich group in the entire animal kingdom. Most species have two pairs of wings.
Importance of viscosity
Depending on its size and flight speed, the viscosity of the air is of different importance for the insect. These three variables can be combined to form the Reynolds number , which indicates the ratio of inertia to viscous forces.
The viscous forces are particularly dominant for small insects, for them the air is as tough as water due to its size and flight speed (see also web links: “When the air becomes sticky” ). You therefore have no aerodynamic wings, but z. T. only brush-like resistance generators such as the bristle or fringed wings .
Larger or faster flying insects usually have transparent skin wings. The rear pair of these can be reduced to so-called swinging bulbs, which only rotate against the wings to balance the masses and are used as a gyroscopic instrument (in dipteras such as mosquitoes and flies).
With flies the wing downstroke takes place from top back to bottom front with horizontal wing position. As a result, the air resistance is maximal in the vertical direction and minimal in the horizontal direction. This creates a high level of lift and a minimal amount of back force. When the wing is upstroke, the wing is moved from the lower front to the upper rear with the wing positioned vertically. This results in a minimum air resistance in the vertical direction and a correspondingly low downforce, as well as a maximum resistance in the horizontal direction and, accordingly, a maximum propulsion. This division of the tasks of the antagonistic muscle groups into lift and propulsion results in an approximate balance of forces, which is of considerable importance for the mutual contraction of the antagonists in the indirect flight muscles through stretching.
A horny cover wing can also be formed from the front pair of wings, which, as a fixed wing, generates lift in flight, while the larger, skin-like rear wing pair is responsible for propulsion (e.g. Coleoptera, beetle ).
But there are also numerous species in which both pairs of wings are fully developed. The dragonfly wings of dragonflies (Anisoptera) is regarded as a masterpiece of nature. On each of the four wings, with the help of the shape of the trachea and membranes in the air flow, a low- resistance laminar profile (i.e. a wing profile with a high thickness reserve) is formed by embedded eddy current rollers, which can be used for gliding, stationary flapping flight and high-speed flight. Trim tanks that are embedded in the wings (pteristigmata) allow additional control. Although the wing makes up only 2 percent of the total weight of the dragonfly, it enables incomparable stability and excellent flight characteristics, while it makes up more than 50 percent of the surface of the animal. This ratio of wing weight and surface area to total weight is unmatched in all of human aviation, but in which other mechanisms are also used for flying.
Smaller insects usually use the technology of the leading edge vortex to gain lift in flapping flight . The up and down of its sharp-edged leading edge of the wing creates a vortex along this edge that can be used directly for lift, as the rear part of this resulting vortex roll sucks the wing up on the downstroke and pushes the wing up on upstroke. This explains the excellent flight characteristics of butterflies (Lepidoptera), some of which can complete several intercontinental flights of up to 4000 km in flapping flight in their lifetime, such as the monarch butterfly .
Flight muscles
The wings are moved either with the indirect or with the direct flight muscles. The direct "drive" is seen as the original.
Values from 0 (dragonfly in gliding flight) to 1046 Hz in small mosquitoes (genus Forcipomyia , Ceratopogonidae ) were measured as wing beat frequencies for various insects . However, since it is impossible for a muscle to be controlled at this frequency by direct nerve impulses, since the latency and absolute refractory period of the action potential of a nerve limits the maximum possible frequency to around 800 to 1000 Hz, the muscles of insects with indirect flight muscles contract independently Overextension. Since very small insects can only generate the air forces necessary for flight with correspondingly high wing flapping frequencies, they always have indirect flight muscles. The airworthy representatives of the more original, direct type all have a certain minimum size (dragonflies).
Indirect flight muscles
The name comes from the fact that the muscles do not attach directly to the wings, but rather the muscle power is transferred indirectly to the wings via the skeletal elements of the thorax . The muscles responsible for the downstroke of the wings are arranged along the thorax (longitudinal muscles), the muscles responsible for the upstroke pull from the "back" of the thorax to its "stomach" (dorsoventral muscles). The deformations of the thoracic skeleton transfer muscle power to the wings.
During the flight, action potentials of motor neurons are observed at fairly regular intervals , which lead to the release of calcium ions from the sarcoplasmic reticulum in the flight muscles . However, the calcium concentration in the cytoplasm of the muscle cells (myoplasm) is not the trigger for the contractions in this specific muscle type. The contraction of a muscle cell is triggered by its expansion, which takes place during flight operations by its antagonist . The presence of a certain myoplasmic calcium concentration is a necessary but not a sufficient condition for the contraction of the indirect flight muscles. In the resting system, however, the first muscle contraction may be triggered by an initial release of calcium, which is caused by an action potential of a motor neuron.
The system is stopped by the lack of action potentials of the motor neurons , since the calcium concentration in the myoplasm then drops so sharply through the absorption of ions that contractions can no longer occur.
The contractions of the muscle cells proceed differently than usual with muscle cells, because the myosin heads bend over due to a change in conformation, but do not detach from the actin in order to start anew at a further actin segment ("biological gear" of the muscles). Therefore the shortening of the flight muscle is extremely small and has to be transferred to the wing by a mechanical translation . This translation is achieved, among other things, by a chain of one-armed levers , in which the force is transferred from the front part of the back of the thorax, the scutum, to the rear part of the back (scutellum), and from there to the front again via the tergal levers attached to the scutellum on both sides to be brought to the wing joints.
Because of this mechanism, in which both wings can only be opened or flapped at the same time, the fly does not have the possibility of generating the propulsive forces, which must be different on both sides when turning, by a different flapping frequency. Instead, it can change the amplitude of the flapping of the wing and the angle of attack of the wing separately on both sides. This is achieved through a highly complicated wing joint construction and a number of thin adjusting muscles that can adjust the joint appropriately.
The wing beat frequency of insects with indirect flight muscles is determined by the air and inertial forces that oppose the muscle contraction. The flight muscles either contract completely or not at all because of the stretch-triggered contraction. However, when the wing flap amplitude or wing position changes, which are used for flight control, the air resistance on the wing that opposes the contraction force changes. This then changes the insect's wing flapping frequency. In the experiment of flies fixed in the thorax, the dependence of the flapping frequency on the mechanical properties of the entire system, which can be understood as a resonance system, can be observed by shortening the wing or by weighing the wing down with wax.
The control of the flight muscles can be extremely primitive and be coupled to the reporting of ground contact of the tarsi of the insect's legs (such as in the fly). To start, the fly simply jumps off the ground, none of the legs report any ground contact and the flight muscle is then started.
When landing, one or more legs report contact with the ground and the wing muscles are stopped. If a fly is attached to its thorax without its legs touching the ground, its wings often flap for a long time.
The indirect flight muscles of flies and other insects are one of the tissues that have the highest energy expenditure. Electron microscope images show an extreme density of mitochondria , which serve as " energy power plants " for all cells . Due to the enormous amount of waste heat produced during flight, an efficient cooling system is required. Bees and probably also flies use the very efficient evaporative cooling here - just like mammals . The insects pump the hemolymph, heated by the flight muscles in the thorax, into the head and choke out a drop of liquid that emerges from the trunk and then evaporates. In flies that are attached to the upper side of the thorax, this drop can be observed very nicely after a while. The tendency of insects to fly is greatly reduced when the air humidity is high.
Direct flight muscles
With more primitive insects, such as dragonflies, one finds a direct drive (in contrast to the other flying insects). The wings are here directly connected to the antagonistic muscles. The control of the movement in the direct approach to the wings is analogous to the control of the skeletal muscles in vertebrates (e.g. flexors / extensors in arm muscles). The contractions are triggered directly by the nervous system via the action potentials of the motor neurons. As with walking movements, certain nerve connections are present in the central nervous system of such insects, which are also called pattern generators and which generate the contraction behavior of the flight muscles as action potential sequences. The drive with direct flight muscles allows the four wings to move independently. This makes dragonflies extremely agile fliers, which enables them to catch prey in the air.
Air combat techniques
Recent studies of dragonfly flight have shown that dragonflies are able to keep their position constant relative to the head and complex eye of the insect they want to attack. As a result, the position of the approaching dragonfly in the eye of its victim remains constant, the attacking dragonfly is not noticed. Furthermore, male dragonflies fight with each other with the reinforced leading edges of their wings in order to have better chances of reproduction by driving away competitors.
literature
- Nachtigall, Werner: Flight of insects . Springer Berlin, 2003, ISBN 354000047X
- Nachtigall, Werner and Rolf Nagel: In the realm of the thousandth of a second, the fascination of insect flight . Gerstenberg, 1988, ISBN 3806720436
- Hagemann & Steininger (ed.): Everything that flies - in nature, technology and art . Small Senckenberg Series No. 23, 1996, ISBN 3-7829-1143-1
Web links
- "When the air gets sticky" (PDF) (134 kB)
- Flutter for science - bionic research on insects (PDF; 305 kB) . By Dörte Ostersehlt In: Biology lessons 332 2008. pp. 22 to 27
- University of Florida Book of Insect Records. Collection of measured flight speeds of various insects.
- "Photos of insects in flight"
Individual evidence
- ↑ Student boys in biology. Bibliographisches Institut & FA Brockhaus AG, Munich 2009. P. 192f.
- ^ CW Scherer: Book of Insect Records, Univ. Florida, Cape. 9, 1995
- ↑ Konrad Dettner, Werner Peters: Textbook of Entomology . 2nd Edition. Elsevier, Munich, ISBN 3-8274-1102-5 , pp. 249 .