Movement in Insects

How muscles move exoskeleton in insects

In insects, muscles attached to the inner surfaces of the exoskeleton bring about movement of the legs, wings and other appendages.

Moving the legs: The exoskeleton in each leg consists of several tube like segments joined end to end. The adjacent leg segments are connected by flexible chitin membranes which can fold and unfold. Pairs of antagonistic muscles connecting adjacent leg segments bring about movements by their contractions.

Insect flight : Flying insects  may have one or two pairs of wings attached to their thoracic segments. They fly by moving their wings up and down independently to beat against the air. During the downstroke, the wings are pulled downwards and forwards; during the upstroke, they are pulled upwards and backwards. These strokes provide both forward thrust and uplifting power so that  the insect is propelled forwards in air.

    In most large insects, two sets of flight muscles are not attached directly to the wing bases. Instead, a vertical pair of muscles connects the roof of the thorax to the floor, and a longitudinal pair connects the front and back of the thorax. When the longitudinal muscles contract, the thoracic segment is shortened from front to back; at the same time, the vertical muscles relax.

This increases the height of the segment, causing the wings to move down. To raise the wings, the height of the thorax is decreased by contracting the vertical muscles and relaxing the longitudinal ones. These two pairs of muscles are known as indirect flight muscles.

Note : The thoracic segment is made of several separate hard plates of exoskeleton which form the roof, sides and floor, joined by strips of soft, flexible membrane.

   Each wing base is linked to an articulating surface which slides over a pivot. the articulating surface is part of the roof of the thorax, while the pivot is on the side of the thorax. This results in a very efficient lever system so that a small change in the height of the thorax brings about a large wing movement.

   A set of direct muscles attached to the floor and sides of the thorax twist and flex the wings during their flapping movement, and thus increase the uplift and the forward thrust.

When an insect soars upwards, the wings beat at a a faster rate. However, as it swoops downwards to rest, the wings beat at a slower rate. The wings of insects beat at a rate of 5 beats per second in certain large butterflies to about 400 to 500 beats per second on mosquitoes. In most grasshoppers, the wings beat at a rate of 15 to 50 beats per second.