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 {396}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. [Illustration: FIG. 247.—A, _Atropos divinatoria_; B, _Clothilla pulsatoria_. (After M‘Lachlan.)]