Locomotion in insects – Support and Locomotion (Senior 4 Biology) | Ganzaa

Support and Locomotion

Lessons

Senior 4 · Biology

Support and Locomotion

5 lessons

Lesson 5 of 5

Locomotion in insects

25 min

Walk, jump and fly

Try to catch a grasshopper in tall grass. It walks steadily up a stem, then, as your hand comes near, it is suddenly a metre away, and a moment later it may open its wings and fly off. One small animal, with no bones at all, walks, jumps and flies. A large locust can jump about a metre, some 20 times its own length, without using its wings. Its secrets are a hard, jointed exoskeleton that works as both armour and levers, muscles hidden inside it, and some of the fastest-working muscles in the animal kingdom.

Definition

Exoskeleton: a hard outer skeleton, the cuticle, made mainly of chitin and protein, secreted by the epidermis.

Flexor and extensor: an antagonistic pair of muscles that bend and straighten a joint.

Tripod gait: a way of walking in which three legs (the front and back legs on one side and the middle leg on the other) are on the ground while the other three swing forward.

Direct flight muscles: muscles attached to the base of the wings. Indirect flight muscles: muscles attached to the walls of the thorax, which move the wings by changing its shape.

Asynchronous muscle: flight muscle that contracts many times for each nerve impulse it receives.

Resilin: a rubber-like, very elastic protein in insect cuticle that stores and returns energy.

Support: a skeleton on the outside

Each part of an insect's body and legs is a hollow tube of cuticle. A hollow tube is strong and stiff for its mass, and it also protects the body and reduces water loss. At each joint the cuticle is thin and flexible, so the segments can move. Muscles are attached to the inside of the tubes, often to inward projections of the cuticle, and work in antagonistic pairs: inside the femur, a flexor bends the knee and an extensor straightens it. The limits of an exoskeleton, that it cannot grow (so the insect must moult) and that it would become too heavy for a large animal, help explain why insects are small.

Walking

Insects walk with a tripod gait (Figure 1). At every moment three feet on the ground form a triangle around the centre of gravity, so the insect is always stable, even when it stops suddenly. Claws and sticky pads on the feet give grip, which is how flies walk on walls and ceilings.

Figure 1. The tripod gait (seen from above). Blue: legs on the ground; orange: legs lifted.

Jumping: a built-in catapult

A muscle cannot contract both very forcefully and very fast at the same time. Grasshoppers and locusts get round this with a catapult. The huge extensor muscle in the femur of each hind leg contracts slowly while a flexor holds the knee locked; the energy is stored by bending stiff, springy parts of the cuticle. When the lock is released, the stored energy is let go in a fraction of a second, kicking the long hind legs straight against the ground. The take-off acceleration reaches about 20 times the acceleration of gravity.

Worked example 1: the energy of a jump

A locust of mass 2.0 g jumps a horizontal distance of 1.0 m. Taking off at $45^\circ$ and ignoring air resistance, the range $R = \frac{v^2}{g}$ , where $v$ is the take-off speed and $g = 9.81\,\mathrm{m\,s^{-2}}$ . (a) Calculate the take-off speed. (b) Calculate the kinetic energy at take-off.

(a) $v = \sqrt{Rg} = \sqrt{1.0 \times 9.81} = 3.1\,\mathrm{m\,s^{-1}}$ .
(b) $E_k = \frac{1}{2}mv^2 = \frac{1}{2} \times 0.0020 \times 9.81 = 0.0098\,\mathrm{J} \approx 10\,\mathrm{mJ}$ (using $v^{2} = 9.81$ ).
This energy must be released in a few hundredths of a second, which is why it is stored in advance in springy cuticle rather than produced directly by the muscle during the kick.

Flight

Insect wings are not modified legs: they are thin, flat outgrowths of the cuticle of the thorax, stiffened by veins. Two systems of flight muscle work them.

Direct flight muscles (dragonflies): muscles attached to the base of each wing pull it up or down. Each wing can be controlled separately, which makes dragonflies superb at turning and catching prey in the air.
Indirect flight muscles (all other winged insects, including flies, bees, beetles, butterflies and locusts): the wings are hinged between the roof of the thorax (tergum) and pivots on its side walls. Vertical dorsoventral muscles pull the roof down, which flips the wings up; longitudinal muscles then shorten the thorax, the roof bows up, and the wings flip down (Figure 2). The thorax and elastic pads of resilin in the wing hinges spring back like a bow, storing energy on one stroke and returning it on the next.

Figure 2. Indirect flight muscles (thorax cross-section, simplified).

In insects whose flight muscles are synchronous (one contraction per nerve impulse), wingbeat frequencies are typically 5 to 200 per second. In flies, bees and some beetles, the muscles are asynchronous: once switched on, each muscle is set contracting by being stretched as its partner contracts, so the thorax oscillates many times per nerve impulse, and wingbeat frequencies can exceed 1000 per second.

Worked example 2: how many contractions per impulse?

Illustrative values. A fly's wings beat at 200 per second while nerve impulses reach its flight muscles 10 times per second. A bee's wings beat at 230 per second. (a) How long does one wingbeat of the fly take? (b) How many contractions does each fly muscle make per nerve impulse? (c) What does this show?

(a) Period $= \frac{1}{200}\,\mathrm{s} = 0.005\,\mathrm{s} = 5\,\mathrm{ms}$ .
(b) $\frac{200}{10} = 20$ contractions per impulse.
(c) The muscle is asynchronous: a single nerve impulse cannot be produced and recovered from in 5 ms, so after being switched on, the muscles keep each other contracting by stretching each other, using the springy thorax.

Larry Keeley: "Animal Flight: Wing Structures and Wingbeat Mechanisms". Watch for how direct and indirect flight muscles differ, and how the thorax and asynchronous muscles produce very fast wingbeats.

Warning

Common mistakes

Saying insect muscles are attached to the outside of the skeleton. They are inside the cuticle tubes.
Calling insect wings modified legs. All six legs are still there; the wings are outgrowths of the thorax wall.
Saying that in indirect flight the muscles pull the wings. They change the shape of the thorax, and the wings move as levers about the pivots.
Thinking the jump comes straight from a fast muscle contraction. The energy is stored first and released by a catapult.

Tip

Memory hook. In indirect flight, "Down-Vertical muscles push wings up; Longitudinal muscles Lower them": dorsoventral contraction pulls the roof down and the wings up; longitudinal contraction arches the roof and the wings go down.

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Practice: insects

Explain two ways in which an exoskeleton supports an insect, and one disadvantage of it.
Describe the tripod gait and explain why it keeps an insect stable.
Explain how a grasshopper can jump much further than its muscles could make it jump directly.
A grasshopper of mass 0.50 g takes off at $2.0\,\mathrm{m\,s^{-1}}$ . Calculate its kinetic energy at take-off.
Compare direct and indirect flight muscles, giving an example of an insect with each.
A mosquito's wings beat at 500 per second. Calculate the time for one wingbeat in milliseconds.
Explain why asynchronous flight muscles allow higher wingbeat frequencies than synchronous ones.

Show answersHide answers

Any two: the hollow cuticle tubes are strong and stiff for their mass and hold the body in shape; they give internal surfaces for muscle attachment and act as levers at the joints; they protect the body and reduce water loss. Disadvantage: the insect must moult to grow and is vulnerable while the new cuticle hardens; or, an exoskeleton becomes too heavy for a large animal.
Three legs (front and back on one side, middle on the other) are on the ground while the other three swing forward, then they swap. The three feet on the ground always form a triangle around the centre of gravity, so the insect never tips over, even when it stops.
The large extensor muscle contracts slowly while the knee is held locked, storing energy in springy cuticle; when released, the stored energy straightens the long hind legs very fast. This catapult releases energy far faster than the muscle could contract.
$E_k = \frac{1}{2} \times 0.00050 \times 2.0^2 = 0.0010\,\mathrm{J} = 1.0\,\mathrm{mJ}$ .
Direct flight muscles are attached to the wing bases and move each wing directly (dragonfly). Indirect flight muscles are attached to the thorax walls and move the wings by changing the shape of the thorax (fly, bee).
$\frac{1}{500}\,\mathrm{s} = 0.002\,\mathrm{s} = 2\,\mathrm{ms}$ .
Synchronous muscle needs a nerve impulse for every contraction, and nerves and muscles cannot fire and recover fast enough for very high frequencies. Asynchronous muscle, once switched on, contracts each time it is stretched by its antagonist, so it can oscillate many times per impulse with the springy thorax.

Quick check

Put the steps of a grasshopper's jump in order.

Tap the cards in the right order.

Key points

The chitin exoskeleton supports, protects and acts as levers; muscles attach inside in antagonistic pairs; it limits size and requires moulting.
Insects walk with a tripod gait, always balanced on three legs; grasshoppers jump with a catapult that stores energy in springy cuticle.
Dragonflies use direct flight muscles; most insects use indirect muscles that change the shape of the thorax. Asynchronous muscles give wingbeats of hundreds or even over 1000 per second.

1Walk, jump and fly

2Support: a skeleton on the outside

3Walking

4Jumping: a built-in catapult

5Flight

Walk, jump and fly

Support: a skeleton on the outside

Walking

Jumping: a built-in catapult

Flight