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Usually in their inactive or passive state the wings are held off horizontally from the body during flight, and are laid upon the back again when the insect alights; but an exception occurs in most butterflies and Neuroptera, among which the wing-joint allows only one movement round the oblique and long axis of the wings. From this cause, too, the insects just mentioned can unfold their wings suddenly.

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The transition of the wings from the active to the resting condition seems to be by way of a purely passive process, which, therefore, usually gives no trouble to the insect. The wing being extended by the tractive power of the muscles, flies back, when this ceases, to its former or resting posture by means of its natural elasticity, like a spiral spring disturbed from its balance. The structure of this spring joint is very different, however.

It usually consists (Fig. 169) of two parts. The wing can move itself up and down in a vertical plane by means of the forward joint, and at the same time can rotate somewhat round its long axis, because the chitinous part mentioned above is ground off after the fashion of a mandrel.