4

Flight Morphology and Flight Muscles

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the wings are usually folded on the back and the body rests on three pairs of jointed

legs on the substratum.

The main flight muscles in the thorax can be classified as direct and indirect

flight muscles. Direct flight muscles are present in primitive insects and are attached

to the wing base directly. Hence, they can move their wings by contraction either

downward or upward. However, in insects with indirect flight muscles, the wings are

attached to the thorax and by their contraction, they deform the thorax. As the wings

are the continuation of thoracic chitin, the contraction of the flight muscle causes

the deformation of the thorax which in turn leads to down and up movements of the

wings which usually trace a figure of eight.

Based on the number of contractions made by the thoracic muscles, we have low-

frequency and high-frequency fliers. Low-frequency fliers are known as neurogenic

(synchronous) and high-frequency fliers are known as myogenic (asynchronous).

Flight motor construction in insects has evolved independently. Wing venation also

might have evolved from a single ancestor. The study of flight morphology helps

in understanding the body and wing geometry, scaling laws, mimicking models,

wingbeat frequency and flight at low Reynolds number at a moderate velocity.

The Vein

Each of the wings consists of a thin membrane supported by many longitudinal veins.

The membrane is formed by two layers of integument closely fused, while veins are

formed where two layers remain separate and the lower cuticle is thicker. Within

each of the major veins, there is a nerve and trachea. Since the cavities of veins are

connected with the haemocoel, haemolymph (the colourless blood) can flow into the

wings from the base of the wing to the tip of the wing (Fig. 4.2).

The Transverse Section (TS) of the longitudinal vein of the wing has a covering

of wing membrane running along the epidermis. The study of Fig. 4.2 shows the cut

Fig. 4.2 Transverse section of a longitudinal vein (illustrative)