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.
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.
The number of described species of Psocidae does not reach two hundred; we have, however, thirty species or more in Britain.[[316]] Nietner observed about the same number in the immediate vicinity of his house in Ceylon. The isolated and remote Hawaiian group of islands is remarkably rich in Psocidae. Two thousand is a moderate estimate of the number of existing species. The largest forms yet discovered belong to the Brazilian genus Thyrsophorus; they attain, however, a breadth of only about one inch with the wings fully expanded. The Cuban genus Embidopsocus is said to be of great interest from its approximation to Embiidae. It is at present very inadequately known.
One (or more) very minute Insects of this family—Clothilla pulsatoria according to Hagen, Atropos[[317]] divinatoria according to some other authors—is widely known under the name of the death-watch, owing to its being believed to make a peculiar ticking noise, supposed to be prophetic of the decease of some individual—a human being we fancy, not a death-watch. It is difficult to believe that so minute and soft an Insect can produce a sound audible to human ears, and many entomologists are of opinion that the sound in question is really produced by a beetle—of the genus Anobium—which lives in wood, and that as the beetle may be concealed in a hole, while the Clothilla is seen running about, the sound is naturally, though erroneously, attributed to the latter. But the rapping of the Anobium is well known, is produced while the Insect is at large, and is said to be a different noise from that of the Psocid; evidence too has been given as to the production of the sound in a workbox when the Psocid was certainly present, and the most careful search failed to reveal any beetle.
Fig. 247.—A, Atropos divinatoria; B, Clothilla pulsatoria. (After M‘Lachlan.)
The Rev. W. Derham, who two hundred years ago was Rector of Upminster, in Essex, and was well known as a distinguished writer and philosopher, gave an account of the ticking of death-watches to the Royal Society.[[318]] This gentleman was a most accurate and minute observer; he was well acquainted with the ticking of the greater death-watch—Anobium—which he describes very accurately, as well as the acts accompanying it, the details he mentions being exactly such as occur at the present time. He not only heard the ticking of the Psocid or lesser death-watch, but repeatedly witnessed it. He says: "I am now so used to, and skilful in the matter as to be able to see, and show them, beating almost when I please, by having a paper with some of them in it conveniently placed and imitating their pulsation, which they will readily answer." He also states that he could only hear them beating when it was done on paper, and that this death-watch will tick for some hours together without intermission, with intervals between each beat, so that it much resembles the ticking of a watch. The act of ticking was accompanied by rapping the front of the head on the paper, but Mr. Derham could not be sure that the sound was produced in that manner, because each stroke was also accompanied by a peculiar shudder, or recoil. After a prolonged ticking he observed that another individual of the other sex made its appearance. The species figured by Mr. Derham more resembles a Hyperetes than it does either of our two known book-lice, Atropos and Clothilla.