Flight I: Structure & Function of Wings

• Wings in living insects serve a number of functions,

including active flying, gliding, parachuting, altitude
stability while jumping, thermoregulation, and sound
production. Today’s lecture covers the structure and
function of wings in modern insects.

• Understanding the evolution of wings requires an

understanding of the adaptive value of the intermediate or
transitional stages in their development. The next lecture
covers the evolution of wings and problems associated
with its study.
 Structure of wings
•  Cross section through the wing.
Membrane is two layers of
integument. Veins including nerve,
blood space and tracheae. Wings
do not contain muscle.

•  Venation. Irregular network of

veins found in primitive insects.
Longitudinal veins with limited
cross-veins common in many
pterygote groups. Extreme
reduction of all veins common in
small insects. Longitudinal veins
concentrated and thickened toward
the anterior margin of the wing.
This gives increased efficiency and
support.
 Structure of wings
•  Pterostigma. Darkened area on
forewing in Hymenoptera,
Pscoptera, Megaloptera and
Mecoptera and on both wings in
Odonata. Functions as inertial
mass in flight. Reduces wing flutter
during gliding in odonates, thereby
increasing flight efficiency.
Provides passive control of angle of
attack in small insects, which
enhances efficiency during flapping
flight.

•  Wing folding. Flexion lines reduce

passive deformation and enhances
wing as an aerofoil. Fold lines used
in folding of wings over back.
 Mechanisms of wing movement
•  Wing coupling. Orthoptera and
Odonata wings are not anatomically
coupled. Coordination of forewings and
hindwings in flight is accomplished by
pattern-generator neurons in the central
nervous system. Details of anatomical
wing-coupling varies among taxonomic
groups, suggesting that it evolved
independently several times.

•  Halteres in Diptera. Derived from the

hindwings. Functions to maintain stability
in flight.
 Two general mechanisms of wing
movement
•  Direct mechanism. Downward
movement of the wing is the result of the
contraction of muscles attached directly
to the wing. This flight mechanism is
under control of synchronous flight
muscle. Because each wingbeat is
control by a nervous impulse, the direct
mechanism of insect flight is said to be
neurogenic in origin.

•  Indirect mechanism. Downward

movement of the wing is the indirect
result of the contraction of muscles
attached to the thorax. This flight
mechanism is under the control of
asynchronous flight muscle. Because
several to many wingbeats occur for
every nervous impulse, the indirect
mechanism of insect flight is said to be
myogenic in origin.
 Two general mechanisms of wing
movement
•  Taxonomic distribution of direct and
indirect flight mechanism. Direct mechanism
of wing movement is found in the Palaeoptera
and the Blatteria. Indirect mechanism of wing
movement is found in the Hymenoptera (bees),
Diptera, some Coleoptera and some
Hemiptera. Other groups (some Coleoptera
and Orthoptera) use a combination of direct
and indirect mechanisms to move wings.

•  Efficiency of flight production. Muscles used

in flight arise in the coxa in many insects and
also function in leg movement during terrestrial
locomotion. Elastic properties of wing hinges,
wing muscles and thorax greatly enhance flight
efficiency. Elasticity of these structures is due
to the presence of the protein resilin. In locust,
86% of the energy used in the upstroke can by
recovered during the downstroke. Elasticity of
the thorax means that wings are in stable
position only at the top of the upstroke or at the
bottom of the downstroke.
 Movement of the wings
•  Stoke plane is the plane in which
wings move relative to the long axis of
the body. Stroke plane determines the
rate of forward movement during flight.
Insects control turning movements by
changing the stroke plane on one side
of the body relative to that on the other
side of the body. Stroke plane in locust
averages about 30o. Hovering requires
an average stroke plane of 0o.

•  Amplitude of wingbeat is the distance

in degrees travelled by the wing tip
from the top of the upstroke to the
bottom of the downstroke. Greater
amplitude produces greater power
output. Insect control turning
movements by varying the amplitude of
the wingbeat on both sides of the body.
 Movement of the wings
•  Wingbeat frequency is the number of
wingbeats per second (Hz). Insects with
synchronous flight muscles have low
wingbeat frequencies (≤ 50 Hz) relative to
insects with asynchronous flight muscle
(100-1000 Hz). Wingbeat frequency is
also negatively correlated with body size.
The greater the wingbeat frequency the
great the power output and the greater the
lift production.

•  Wing twisting occurs when the relative

position of the leading and tailing edges of
the wing changes during the wingbeat.
Both passive and active (=muscular)
forces are responsible for changing wing
twisting. Wing twisting controls the angle
of attack which control lift and forward
movement of the insect in space.
 Aerodynamics
•  Relative wind is the movement of air
relative to the wing. Its two components
are due to 1) the airspeed of the insect
and 2) the velocity of the wing in the stroke
plane.

•  Angle of attack is the angle at which the

relative wind strikes the chord of the wing.
Insects control the angle of attack by
active and passive twisting of the wing.
Changes in the angle of attack are used to
control the force of relative wind.

•  Force of relative wind has two

components. Lift is the vertical force
produced by relative wind. This force is
what gets insects into the air and keeps
them there. Thrust is the horizontal force
produced by relative wind. This force
moves insects forward through the air.
Forward thrust is resisted by profile drag
(the cross-sectional area the insect
presents to the air) and mostly by induced
drag (development of vortices at the wing
tips.)
 Variations in forward flight

•  Hovering in flight is
accomplished by changing
the stroke plane to nearly
horizontal and maintaining a
positive angle of attack
throughout the wingbeat.

•  Gliding requires a high lift-to-

drag ratio. This is
accomplished mostly by
changing the angle of attack
to maximize thrust and
minimize drag and negative
lift.
Control of flight