Special mention must be made of the _Snow-Flea_. Do not think we are referring to an insect of the same species as our common fleas: the snow-flea approximates much more closely to the lice family than to the fleas, though it hops like the latter. The history of its discovery dates back as far as 1839. At this epoch, M. Desor, a learned Swiss naturalist, had undertaken some researches upon the glaciers. Accompanied by some friends, he set out from the hospice of the Grimsel, and arrived in the vicinity of the glacier of the Lower Aar. He had commenced his observations, when suddenly he heard Agassiz calling him, and shouting, "Come, come, make haste; here are your Mont Rosa fleas." Desor ran to the spot, and saw under a stone the little creatures whom Agassiz persisted in taking to be veritable lice, pretending they had been accidentally brought to these heights. "I recognised with extreme joy," says M. Desor, "the little creatures whose loss I had regretted a year before. They are not pretty, but, on the contrary, very ugly. However, they showed, in opposition to the opinion of Agassiz, that they really inhabited the glacier, and were not merely chance visitors. We found them by thousands under other stones. ... Our guide, with whom the glaciers were old acquaintances, had never seen them before, and the tiny creatures excited his astonishment. What surprised us most was the rapidity with which they penetrated into even the most compact ice, till they resembled blood-corpuscles circulating in their vessels. This fact shows that there exist, in the hardest and most transparent ice, certain capillary fissures which escape an unskilled eye: it also proves that the glaciers, on their surface, and down to a certain depth, are by no means incompatible with the development of organised beings."[23] The tiny insect in question was at first baptized by the name of _Desoria saltans_ (order of the _Thysanouræ_ of Latreille), but has since received definitively the name of _Desoria glacialis_. It belongs to the family of the _Poduræ_, singular creatures which, by virtue of their form, are a link between the earwigs and the spiders. These are its generic characters:-- [Illustration: FIG. 15.--_Desoria glacialis_; _a_, natural size; _b_, enlarged.] The body elongated, cylindrical, garnished with long setiform hairs, and composed of eight segments, six of which are perfectly distinct, and two (the two latter) very short, and scarcely perceptible; four-jointed antennæ, longer than the head; long, slender, cylindrical feet; forked tail, silky, and transversely wrinkled; seven eyes, laterally grouped at the base of each antenna; body without scales. [Illustration: FIG. 16.--_Podura plumbea_; _a_, natural size; _b_, enlarged.] The _Desoria glacialis_, a species at present unique, is of a velvety black, and about one-sixth of an inch in length.