[Illustration: FIG. 246.—Micropterous form of _Mesopsocus unipunctatus_. _a_, _a_, Wings. (After Bertkau.)] There are several peculiarities connected with the wings. Frequently they exist, though of no use for flight; some Psocidae that have perfectly-formed wings are so reluctant to use them that, M‘Lachlan says, they will allow themselves to be crushed without seeking to escape by flight. At certain periods, however, some Psocidae float on the wing in considerable numbers, especially in a moist still atmosphere, and then drift about like the winged Aphididae, which are frequently found with them. There is evidence that individuals, or generations, of some of the winged species occur with only rudimentary wings; although this has been denied by Kolbe, there can be no doubt about it. The form figured above (Fig. 246) was described by Bertkau[312] as a distinct genus, but was afterwards recognised by him[313] to be a short-winged form of _Mesopsocus unipunctatus_. It is probable that the adult female of this species has the wings always micropterous, while the male has these organs of the full size. In other species the condition of the rudimentary wings seems to be quite constant. The facts concerning the wings of Psocidae are so peculiar that Kolbe came to the conclusion that the organs exist not because they are of use for flight, so much as because it is the nature of an Insect to develop wings.[314] Some of the species of Psocidae have never any trace of wings. These apterous forms are mostly included in the division Atropinae, and are usually very minute; it has been again and again erroneously stated that they are the young state of winged forms. Hagen kept a large colony of _Atropos divinatoria_ for some years in confinement, so that he saw numerous generations as well as many specimens. He found the apterous condition quite constant. {395}The association of ocelli with wings is nearly constant in Psocidae. The genus _Clothilla_—allied to _Atropos_—possesses very rudimentary wings but no ocelli. Hagen, however, found[315] that in a certain locality no less than 12 per cent of the individuals of this species were provided with ocelli,—a most extraordinary variation. In some of these apterous forms there is found on each side of the prothorax a tubercular prominence which, according to Hagen, can be considered only as the rudiment of a wing that never develops. Though no existing Insect is known to possess rudimentary wings on the prothorax, we have previously mentioned (p. 344) that in the Carboniferous epoch appendages of the nature alluded to were not very rare. A genus of living forms—_Hyperetes_—in which the three thoracic segments are well developed, but in which there are no alar appendages or rudiments, is considered by Hagen to be more primitive than the Psocidae found in amber to which we shall subsequently allude.