BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL SKELETOMUSCULAR SYSTEM

18.5. Locomotion in Selected Invertebrates

18.5.2. Locomotion in Insects

Insects possess an exoskeleton, The Relationship of which to Muscles and locomotion was discussed in Section 18.1.3.

Walking

This mode of locomotion is achieved through the coordinated action of three pairs of limbs (walking legs) — one pair on each of the three thoracic segments. Each leg is a system of hollow cylinders with walls made of rigid Chitin (Fig. 18.1). These cylinders are joined together by flexible membranes. The Articulation of the coxa (the proximal segment of an insect leg) with the body forms a type of ball-and-socket joint; all other leg joints are hinge joints. Flexion and extension of the legs are brought about by antagonistic muscles — flexors and extensors — attached to the inner surface of the exoskeleton on either side of the joint (Fig. 18.1).

When an insect moves, three of its legs (the first and third on one side, and the second on the other) rest on the ground and support the body, while the other three step forward. In this process, the first leg on one side pulls the body forward, the third leg on the same side pushes it, and the second leg on the opposite side acts merely as a prop. The whole sequence is then repeated with the roles of the two sets of limbs reversed.

Many insects have pairs of claws and adhesive pads at the tips of their legs. These pads consist of microscopic hollow tubules that secrete an adhesive fluid, enabling the animals to cling to smooth surfaces. This is why such insects are able to walk up vertical surfaces and even upside down.

Flight

Insect wings are flat outgrowths of the exoskeleton supported by a complex network of Veins. Their movements are controlled by two principal Muscle groups: direct and indirect flight muscles. In insects with large wings (such as butterflies, locusts, and dragonflies), the muscles attach directly to the Base of the wing (Fig. 18.27). These are the direct muscles. They raise and lower the wings, as well as adjust the angle of the wing beat during flight. By positioning the wings at specific angles relative to one another, the insect performs aerial turns. These muscles are also involved in folding the wings after flight has ceased.

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Fig. 18.27. Action of the "direct" flight muscles in large-winged insects such as butterflies and dragonflies.

In large winged insects, such as butterflies or locusts, the wings beat between 5 and 50 times per second (Table 18.4). In these insects, individual muscle contractions are triggered by single nerve impulses, with impulses generated at a frequency equal to the wing beat frequency. Flight muscles operating in this manner are termed synchronous. In the housefly, which has a wing beat frequency of 120–200 per second, the flight muscles must operate far too rapidly for each contraction to be a response to a separate Nerve Impulse. Such muscles are called asynchronous, and they receive only about one nerve impulse for every 40 wing beats; these impulses serve to maintain the muscle in an active state during flight. Such a muscle can sustain contraction longer and generate greater power than a synchronous muscle. An asynchronous muscle is capable of contracting automatically in response to stretch (the so-called stretch reflex) without waiting for the next nerve impulse.

Table 18.4. Wing beat frequency in various insects¹

18.3. The sarcoplasmic reticulum of insect flight muscles exhibits an increased degree of branching, which significantly enlarges its surface area. What is the likely adaptive significance of this feature?

18.4. In which type of muscles would you expect the sarcoplasmic reticulum to be more developed — synchronous or asynchronous? Justify your answer.