ROACHES AND OTHER ANCIENT INSECTS
character, though not a distinctive one, since modern dragonflies (Fig. 58) and mayflies (Fig. 60) likewise keep the wings extended when at rest.
The question of how insects acquired wings is always one of special interest, since, while we know perfectly well that the wing of a bird or of a bat is merely a modified fore limb, the nature of the primitive organ from which the insect wing has been evolved is still a mystery. The Paleodictyoptera, however, may throw light upon the subject, for some of them had small flat lobes on the lateral edges of the back plate of the prothorax, which in fossil specimens look like undeveloped wings (Fig. 56). The presence of these prothoracic lobes, occurring as they do in some of the oldest known insects, has suggested the
Fig. 56. Examples of the earliest known fossil insects, called the Paleodictyoptera, having small lobes (a) projecting like wings from the prothorax
A, Stenodictya lobata (from Brongniart). B, Eubleptus danielsi (drawn from specimen in U. S. Nat. Mus.): T1, T2, T3, back plates of three thoracic segments
idea that the true wings were evolved from similar flaps of the mesothorax and metathorax. If so, we must picture the immediate ancestors of the winged insects as creatures provided with a row of three flaps on each side of the body projecting stiffly outward from the edges of the thoracic segments. Of course, the creatures could not actually fly with wings of this sort, but probably
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