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Agrioninæ, according to Kolbe, can fly with the fore pair of wings or with the hind pair almost as well as with both pairs together. Also the wings of these insects can be cut off before the middle of their length without injuring their power of flight. Butterflies, Catocalæ, and Bombycidæ fly after the removal of the hind wings. Also the balancers of the Diptera must be useful in flying, since their removal lessens the power of flight.

Chabrier regarded the under sides of the shell-like extended wing-covers of the beetles as wind-catchers, which, seized by wind currents, carry the insect through the air. We may also consider the wing-covers as regulators of the centre of gravity of flight.

The observations of insects made by Poujade (Ann. Soc. Ent., France, 1887, p. 197) during flight teaches us, says Kolbe, that in respect to the movement during flight of both pairs of wings, they may be divided into two categories:—

1. Into those where both pairs of wings (together), either united, and also when separated from each other, perform flight. Such are the Libellulidæ, Perlidæ, Sialidæ, Hemerobiidæ, Mymeleonidæ, Acridiidæ, Locustidæ, Blattidæ, Termitidæ, etc.