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2. Into those whose fore and hind wings act together like one wing, since they are connected by hooks (hamuli), as in certain Hymenoptera, or are attached in other ways. Here belong Hymenoptera, Lepidoptera, Trichoptera, Cicadidæ, Psocidæ, etc.

The musculature of the mesothorax and metathorax is similar in those insects both of whose pairs of wings are like each other, and act independently during flight, viz. in the Libellulidæ. On the other hand, in the second category, where the fore and hind wings act as a single pair and the fore wings are mostly larger than the hinder (except in most of the Trichoptera), the musculature of the mesothorax is more developed than that of the metathorax.

To neither category belong the beetles, whose wing-covers are peculiar organs of flight, and not for direct use, and the Diptera, which possess but a single pair of wings. In the beetles the hind wings, in the Diptera the fore wings, serve especially as organs of flight. It may be observed that the Diptera are the best fliers, and that those insects which use both pairs of wings as a single pair fly better than those insects whose two pairs of wings work independently of each other. An exception are the swift-flying Libellulidæ, whose specially formed muscles of flight explain their unusual capabilities for flying (Kolbe).