The Engine
Just like an aeroplane the engine of an insect must be light but powerful enough to generate lift (Dickinson 1985). The engine in insects is the flight muscles. There are three main types of flight muscles.
1. Direct muscles: are attached to the wings and control wing movement (Champman 1998). Movement is synchronous as the muscles are activated in time with electrical signalling from the brain.
2. Indirect muscles: are attached to the body cavity or thorax of an insect and move insect wings by changing the thorax’s shape (Champman 1998). These muscles are asynchronous.
3. Accessory muscles: are muscles that control the thorax’s shape and wing position (Champman 1998).
While, some insects are capable of using only synchronous muscles for flight they are unable to have wing beat frequencies higher than 100Hz (Dudley & Ellington 1990). A bumblebee exhibits wing beat frequencies greater than 150Hz and therefore requires the energy from indirect muscles to fly (Dudley & Ellington 1990). Indirect muscles are not controlled by electrochemical signalling but stretch activation (Dickinson 1985).
Stretch activation enables more frequent muscle movements as it doesn’t require the slow diffusion of chemicals to form the cross bridging need for muscle contraction (Dickinson 1985). Instead rapid stretch in the muscle activates these cross bridges and brings about contraction. Two complementary indirect muscles are positioned in the thorax, the dorso-ventral muscle and dorsal longitudinal. In bumblebees when the dorso ventral muscle contracts it pulls the tergum and moves the wing up (Champman 1998). The downward stroke is created by the dorsal longitudinal muscle contracting and bowing the tergum allowing the wing to move down (Champman 1998).
1. Direct muscles: are attached to the wings and control wing movement (Champman 1998). Movement is synchronous as the muscles are activated in time with electrical signalling from the brain.
2. Indirect muscles: are attached to the body cavity or thorax of an insect and move insect wings by changing the thorax’s shape (Champman 1998). These muscles are asynchronous.
3. Accessory muscles: are muscles that control the thorax’s shape and wing position (Champman 1998).
While, some insects are capable of using only synchronous muscles for flight they are unable to have wing beat frequencies higher than 100Hz (Dudley & Ellington 1990). A bumblebee exhibits wing beat frequencies greater than 150Hz and therefore requires the energy from indirect muscles to fly (Dudley & Ellington 1990). Indirect muscles are not controlled by electrochemical signalling but stretch activation (Dickinson 1985).
Stretch activation enables more frequent muscle movements as it doesn’t require the slow diffusion of chemicals to form the cross bridging need for muscle contraction (Dickinson 1985). Instead rapid stretch in the muscle activates these cross bridges and brings about contraction. Two complementary indirect muscles are positioned in the thorax, the dorso-ventral muscle and dorsal longitudinal. In bumblebees when the dorso ventral muscle contracts it pulls the tergum and moves the wing up (Champman 1998). The downward stroke is created by the dorsal longitudinal muscle contracting and bowing the tergum allowing the wing to move down (Champman 1998).
The types of muscles used in indrect flight. Image provided by Willmer et al.(2000)
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The action of indirect muscles in flight. Image provided by Siga (2007).
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Evolution of Asychronous Flight Muscles
Insects evolved asynchronous muscles to achieve greater wing beat frequency (Hunter 2007). Iwamoto et al. (2006) studied the myofibril filaments of flight muscles in different insect species and discovered that asynchronous muscles contain a symmetrical crystalline structure (Hunter 2007). These structures provide insects with greater wing control and power. Iwamoto et al. (2006) then found that the hummingbird hawk moth, whose wing beat frequency is at the maximum threshold for synchronous flight, had traces of the crystalline structure (Hunter 2007). It is suggested that as the insects were selected for greater wing control and power the myofibril structures became more dominant eventually forming asynchronous flight muscles (Iwamoto et al. 2006).
A Bumblebee and a Hummingbird Moth which share more than coluration in common but also a crystaline myofibril structure. Image Provided by Flannery (2008).
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Types of myofibrils structures in different insect species. Image B is the bumblebee which like all insects that use indirect flight muscles exhibit a crystalline structure. Image provided by Iwamoto et al (2006).
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