The document discusses muscle force, tension, work, energy and power in insects. It defines these terms and provides examples of force measurements in insect mandibular and extensor muscles comparable to human values. Flight muscles store elastic energy due to their stiffness. The force exerted depends on muscle cross-sectional area, with the locust extensor tibiae muscle reaching 15N due to fiber oblique insertion. Twitch duration varies by temperature, fiber type and between antagonistic muscles during flight. Direct power measurements of insect flight muscles show maximum values of 130W/kg and 110W/kg. Oxygen is supplied to flight muscles through a specialized tracheal system with fine tracheoles near mitochondria to support their high metabolic demands.

2. Definitions
 FORCE: Force is an influence causing a mass to
change its state of motion, so that it accelerates or
decelerates.
 A muscle generates a force as it contracts because of
the resistance of the object being moved
 The unit of force is a Newton, N= kgms-2.
 TENSION: Tension is produced by opposing forces
pulling on an object. Tension in a muscle normally rises
to a peak as the muscle begins to shorten and then falls
again as shortening continues.

3.  WORK: Work is the application of a force over a
distance, or a measure of the energy transferred
by a force.
 ENERGY: Energy is the ability to do work. The
unit of energy is the same as the unit of work, the
joule (J). J= Kgm2s-2
 POWER: Power is the rate of doing work, or the
rate at which energy is supplied. The unit of
power is the watt (W). W = kg m2s-2

4. Tension And Force during
Contraction
 The tension exerted by insect muscles is not
exceptional.
 The mandibular muscles of various insects exert
tensions of 3.6–6.9 g cm2, and the extensor tibiae
muscle of Decticus (Orthoptera) is 5–9 g cm2 compared
with the values of 6–10 g cm2 in humans.
 Energy stored in the elastic system, but most of the
energy stored depends on the stiffness of the muscle.
 Flight muscles, and especially fibrillar muscles, as
studied in Lethocerus, Drosophila and other insects,
have a much higher elastic storage because they are
stiffer than other muscles.

5. Fig 1: Mandible of
Orthoptera
tension exerts 3.6–6.9 g
cm2
Fig 2: Extensor muscle
of
locust and other
grasshopper

6.  The force exerted by a muscle is proportional to its
cross-sectional area and, in general, this is not very
great in insects.
 In some muscles, however, such as the extensor
tibiae of a locust, a considerable cross-sectional
area is achieved by an oblique insertion of the
muscle fibers into a large apodeme.
 As a result, this muscle can exert a force of up to 15
N.

7. Twitch duration of the muscles
 The duration of each muscle twitch, the time for it to
shorten and relax, is temperature dependent.
 Efficient flight by Schistocerca requires a wing beat
frequency of about 20 Hz, with a period of about 50
ms, only above 30°C, twitch duration short enough to
avoid significant overlap of antagonistic muscle
twitches.
 Some large insects, such as Lethocerus, go through
extensive “futile” flight muscle contractions to raise
their body temperature prior to flight.
 Twitch duration also varies in different fiber types,
shorter the muscle faster the twitch duration and
viceversa.

8. Power output
 A muscle’s mechanical efficiency is defined as the
ratio of mechanical power output to metabolic energy
consumption.
 Direct measurements of the mechanical power
output of flight muscles of Manduca and Bombus
give maximum values of 130 W kg-1 and 110 W kg-1 ,
respectively.
 In a flying insect, energy is required not only to move
the wing to produce aerodynamic force, but also to
start it moving from an extreme up or down position,
and for braking at the end of a half stroke.
 Actomyosin crossbridges may remain attached
throughout the cycle of elongation and contraction
and function as part of the elastic element energy
storage.

9. Actomyosin crossbridge subunit

10.  Maximum efficiency is achieved only under certain
conditions viz., for the flight muscle of Manduca at
35°C, maximum efficiency occurs at a cycle
frequency of about 30 Hz.
 The power output of a muscle may be increased by
multiple stimulation via the motor axon ie., double
firing.
 For instance, double firing of the axon to the second
basalar muscle of Schistocerca, results in the
doubling the amount of work it does.
 In basalar muscle at 40°C, the force is maximum
when second stimulus is at 8ms followed by first.
 In bush cricket, power output from a tergocoxal
muscle is achieved by 3 firings at 4ms interval.

11. Oxygen supply
 Muscular contraction requires metabolic energy
(ATP), and the muscles have a good tracheal supply.
 It is true of the flight muscles, in which tracheal
system is specialized in case flying insect.
 In most muscles the tracheoles are in close contact
with the outside of the muscle fiber, except in
Odonata and Blattodea.
Respiration at cellular level; which includes muscle cells

12. Oxygen supply to flight muscles
 The tracheoles follow the invaginations of the T-
system and so penetrate to the center of the fibers.
 Fine tracheoles, less that 200 nm in diameter, comes
very close to mitochondria in the wing.
 Probably every mitochondrion in the flight muscles is
supplied by one or two tracheoles of this tertiary
system so that the distance oxygen has to pass
through the tissue.
 But in Odonata and Blattodea, the tracheoles are 10
microns, it will not reach up to the mitochondrion.

13. Oxygen supply to flight muscles in Drosophila sp. through tracheoles.

14.  Locust flight muscles use some 80 liters oxygen kg-1
h-1 during flight and consumption by the flight
muscles of some other insects may exceed 400 liters
O2 kg-1 h-1
 In locusts, ventilation of the pterothorax produces a
supply of oxygen well in excess of needs, and the
specialized system of tracheae and tracheoles in the
muscles ensures that oxygen reaches the site of
consumption.
 Pneumatization mechanism in the tracheoles near
the mitochondria and tracheal intima is permeable.
EK:The metabolic rates of flying insects are commonly
100 times higher than those of resting insects.