Unlike birds and bats, the flight muscles of nearly all insects do not attach
directly to the wings, but produce wingbeats by distorting the thoracic cuticle.
Using muscles to create wingbeats indirectly is unique to insects,
and allows some insect species to develop remarkable rates of wingbeats
that are 10 to 100 times faster than those of birds and bats.
In this scene we will explain how this is possible.
Let us begin by examining the organization of the muscles in the thorax of insects.
Only in the dragonflies, damselflies and mayflies -- the most primitive flying insects -- do any
of the muscles involved in the wingbeat actually attach directly to the wing.
Here is a cross-section for a thorax from a primitive insect such as a
dragonfly compared with that of a typical thorax from the more advanced flying insects.
In both the primitive and advanced insects, indirect dorso-ventral muscles
produce the upstroke, but in the primitive orders,
muscles attach directly to the base of the wing to produce the downstroke.
However, in the advanced flying insects,
both the upstroke and downstroke are produced by indirect muscles
through distortion of the thoracic cuticle.
Let us examine the structures and relationships of the insect flight muscles
and thoracic cuticle to see how indirect flight muscles produce wingbeats.
As discussed earlier, the wings are extensions of the cuticle of the meso- and meta-thoracic segments.
The following is a composite generalized description of a typical
wing-bearing thoracic segment and its functions.
Specific adaptations of these structures and functions can be found among the
various insect orders and species.
The armored body characteristic of most
species of adult insects is comprised of rigid cuticular plates called sclerites.
Thoracic segments consist of four major sclerites:
a dorsal notum,
two lateral pleurons
and a ventral sternum.
The thoracic sclerites are subdivided by sulci or furrows that form internal
strengthening ridges for muscle attachment.
The notum is divided into a scutum
and a scutellum.
The wings attach to the thorax in the membranous area between the notum and the pleuron.
The wing dorsal and ventral cuticles are
membranous and flexible at the points where they join the notum.
Anterior and posterior notal wing processes act as articulation points for the wing.
Movements of the thorax for flight are transferred to the wing by three sclerites
in the membranous area which form a hinge between the notal wing processes and the wing base.
A pleural wing process also acts as a fulcrum point during wing movement.
There are two pairs of indirect muscles in each wing-bearing thoracic segment.
Dorso-longitudinal indirect flight muscles attach to large, flattened
plates called phragma that are present internally at the front and back of each notal sclerite
in wing-bearing segments.
A second pair of dorso-ventral indirect muscles attach to the notal and sternal sclerites.
Thoracic cuticle is rigid and acts like an elastic box.
Movements of the thorax and wings will be exaggerated to illustrate the actions that occur during a wingbeat.
When the dorsal longitudinal muscles contract,
they shorten the thoracic segment causing the notum to bulge upward.
Shortening of the notum draws this scutellum forward and pushes up on the
hinge sclerites and the scutum at the base of the wing, which lifts the
wing base over the pleural wing process and flips the wing into the downstroke.
Subsequently, the dorsal ventral muscles contract and pull the notum back down
causing the thorax to expand lengthwise.
This pulls the scutellum back and
the scutum down so that the wing base is drawn back over the pleural wing process
and the wing flips into the upstroke.
flipping of the wing up or down
over the pleural wing process is called the "click mechanism" for insect flight.
Once the wing surpasses its equilibrium point on the pleural wing process, it
snaps into its final, extended position without expending further muscular energy.
Birds and bats require muscular energy expenditure throughout their
entire wingbeat, but insects drive the click mechanism by recovering elastic
energy stored from deformation of the cuticle during the preceding wingbeat.
If the insect thorax were flexible laterally, the thorax would be similar to
a soft rubber ball, and bow in and out as the wings move smoothly through their
wingbeat cycle without a click mechanism.
This would mean that, like birds and bats,
insect wing movements would require energy expenditure throughout the entire
cycle,
Instead, the insect thoracic cuticle has lateral stiffness that holds the pleurons
rigid so that the wings pivot over the plural wing processes and click into
either a stable up, or down, position.
To explain lateral stiffness of the pleurons,
let us view a cross section of the thorax at the level of the pleural sulci.
Cuticular ridges called apodemes are formed on each side of the thorax by inward
folds along the pleural and sternal sulci, and the two ridges are connected by a set
of sterno-plural muscles.
The sterno-pleural muscles control the flexibility of the
pleural sclerites and produce lateral stiffness to make the thoracic sides rigid.
With lateral stiffness, once the flight muscles overcome the inertia of the wing
and move the wing base past its equilibrium point on the pleural wing process,
elastic properties of the thoracic cuticle "click" the wing into its
new stable position with no further exertion by the muscles.
The wing has two stable positions:
fully extended up,
or down.
Here is a magnification of the wing
hinge to demonstrate the details of the movements of the wing in cuticle during the wingbeat.
Here we see the main sclerites involved with winged movements
the notum,
the scuteller lever,
the notal hinge,
the pleural wing process
and the first and second axillary sclerites.
At the start of the downstroke, the longitudinal muscles cause the notum to shorten and move upward
which pulls the scuteller lever forward and upward to lift the notal hinge.
The notal hinge pushes outward on the first axillary sclerite,
pivots the wing over the pleural process on the second axillary sclerite,
and flips the wing into the downstroke.
The contracting longitudinal muscles
release their tension as the wing clicks into its downstroke.
Deformation of the cuticle during the downstroke stretches the antagonistic
dorso-ventral muscles to develop an immediate tension that starts the upstroke.
Contraction of the dorsal ventral muscles pulls down and lengthens
the notum which pulls back and down on this scuteller lever drawing the notal hinge back and down
over the pleural process to click the wing back into the up position.
The advantage of this is that once the wing has moved past its
equilibrium point on the pleural wing process, the flight muscle no longer has
to exert energy and tension for the wing to attain its full stroke.
The wing clicks into its new, full position based on energy that was stored in elastic elements
of the cuticle and antagonistic muscle by their distortion during the previous stroke.
80% of that stored energy is recovered during the counter-stroke.
The energy is stored in a unique, rubber-like protein called resilin.
Resilin stores and releases energy with 97% efficiency and is found in the
membranous regions of the wings and legs where it is stretched during movement.
Next, we shall demonstrate how the indirect flight muscles can attain such rapid rates of wingbeat.