mite, but the remedy which seems the most efficacious is Balsam of Peru. [Illustration: Fig. 21.--Sarcoptes scabiei, or male acarus of the itch; the lower surface.] [Illustration: Fig. 22.--Sarcoptes scabiei, female; the upper surface.] [Illustration: Fig. 23.--Sarcoptes scabiei, male; the dorsal surface.] Most mammals have their peculiar species of acari, and the horse has 133 two which give rise to different skin affections. Since the presence of these animals constitutes the disorder, it may be easily caught; man may communicate it to domestic animals, and they may give it to him. The itch-insect of man bears the name of _Sarcoptes scabiei_, and no other species than those of Sarcoptes can be transferred from animals to man. These animalcules have at different times been diligently studied by many naturalists, and Dr. Füestenberg has lately published a folio volume, under the title of "Die Krätzmilben der Menschen und Thiere," with large lithographic plates, and illustrations in the text. It is possible that the pustular disease which prevails at Sierra Leone is originated by some peculiar acarus. Another acarus parasitical on man, the _Persian Argas_, is fortunately unknown in Europe. It is said to be common at Miona, and prefers to attack strangers. Its stings produce acute pain, and travellers assure us that they may be the cause of death. This acarus remains but a short time on the person, and generally makes its appearance during the night. It is called also the Miona bug. Fischer of Waldheim has published a very interesting memoir on this parasite. Justin Goudot has also observed another Argas (_A. Chinche_) which torments man in the temperate regions of Columbia. These Arachnida, for they are articulata with four pairs of legs, often make their appearance where we should not expect to find a living organism, and naturalists, under these circumstances, have, with the best faith possible, supposed that they had seen these mites produced spontaneously without parents. We have seen a remarkable 134 instance of this in the _Acarus marginatus_ of Hermann. On the 18th Thermidor, an 2, they were making a _post mortem_ examination at Strasburg of a man who had died of fracture of the skull, and when opening the dura mater, they saw on the corpus callosum, a mite running about which became the type of the species. The appearance of this acarus under such conditions made, as may be supposed, much noise at the time, but we should not be surprised if it had been introduced during the operation by a fly seeking to lay its eggs. In this group is found another interesting acarus, which is developed in man in the sebaceous crypts of the nostrils. The name of Simonea has been given to it, from Dr. Simon of Berlin, who made it his especial study. This genus leads us by its form to the _Linguatulæ_, the structure of which has been so long doubtful. The _Simonea folliculorum_ belongs to the family of the _Demodicidæ_.