Earlier we compared wingbeat frequencies for birds, bats and insects and saw that
some fast-flying insects such as flies mosquitoes, wasps and bees have wingbeat
frequencies of 100 to more than 1,000 Hertz (wingbeats/sec).
Let us see what physiological modifications insects have developed
to allow such extreme rates of wingbeat.
One adaptation is the way the indirect flight muscles respond to nervous control.
With the exception of the insect indirect flight muscles,
animal muscles are synchronous in that they contract
maximally once for each impulse from a motor neuron.
Insects with slow wingbeats in the 5 to 50 Hertz range have synchronous flight muscles.
Indirect flight muscles of insects with rapid wingbeats are asynchronous and do not
undergo only a single contraction for each motor nerve impulse.
Instead, the motor nerve impulse "activates" indirect flight muscles.
Activation is a state of constant excitation such that if the muscle fiber undergoes a stretch
there is an immediate, rapid, opposing increase in tension,
and a sharp nearly isometric contraction by the activated fiber
followed by an instantaneous relaxation in the tension.
If there are repeated quick, sharp stretches of the activated fiber,
it can contract 15 to 40 times from a single activating motor impulse.
Let us see what this means with a set of the antagonistic indirect flight muscles
using a lateral view from inside the thorax
Here we shall show changes in the
thorax that are closer in magnitude to real movements to demonstrate the
antagonistic responses of the indirect flight muscles.
In the beginning, the changes will be shown slowly.
At the start of flight, nerve impulses activate the indirect flight muscles as described earlier.
One set of muscles is stretched at the start of the cycle, and upon activation, they develop tension.
In our example, the dorso-longitudinal muscles are
stretched and start the contraction.
During the wingbeat cycle, the increasing
tension in the dorso-longitudinal muscles shortens the notum causing it to dome upward,
At the equilibrium point, the notum snaps upward flipping the wing into the downstroke.
The release of load on the dorso-longitudinal muscle instantly relaxes it's tension,
but the simultaneous tug on the activated
dorso-ventral muscles stimulates an antagonistic increase in tension.
The sudden increase in the activated dorso-ventral muscles snaps the rigid
notum back down - flips the wing up and counter stretches
the activated dorso-longitudinal muscles to restart notal shortening.
Previously the, movements have been slowed and exaggerated to demonstrate their complexity,
and how the click mechanism occurs.
Now we shall show a more realistic series of wingbeats
from the perspective of time and the magnitude of movement.
Asynchronous muscles shorten by only 3 percent during the wingbeat
cycle, so their contraction is nearly isometric, and the stretch-tension-release
cycle of the two antagonistic muscles comprises a rapidly oscillating
resonance system that is self-perpetuating in response to motor neuron activation.
Depending on the species, asynchronous muscles can
contract between 15 and 40 times in response to activation by a single motor neuron impulse.
Activation may result from a persistently high level of
released calcium since indirect flight muscles
have a reduced sarcoplasmic reticulum for calcium storage.
The extended presence of calcium permits
constant recycling of the actin-myosin crosslinks
to produce repeated stretch-tension-release cycles
without recurrent nervous stimulation.
Finally let us describe how an insect starts
this antagonistic resonating muscle system to initiate flight