For my part, I must acknowledge that, whether the Ocypete rubra is or is not the transitory state of a more perfect animal, I do not know. But I am sure that the Leptus autumnalis lives and dies on the skin where it has selected its dwelling-place,—a living dwelling-place.
It is impossible to be too circumspect in the determination of certain genera and species, whose different phases of existence are little known, and which seem, so far as their characteristics are concerned, to participate of several orders or classes of articulated animals. The errors which have been committed in this respect ought at least to teach us caution.
Thus, the red, oval, six-legged animalcules, whose mobile heads are furnished with a proboscis shaped like an angular beak, and whose two palpi are large and semi-transparent,—the singular animalcules which, in June or July, are hatched in the spongy stems of certain aquatic vegetables,—notably the Potamogeton natans,—have been described[83] as forming a peculiar genus of Arachnidæ, the genus Achlysia; and this genus, created by Audouin, was ranked along with the Leptus and Ocypete in the family of Microphthiræ. Yet nothing is less exact. These Achlysia are simply the larvæ of a kind of Hydrachna or water-acarus. To be convinced of this, you have but to watch their development. At first very small and pear-shaped, these larvæ, deprived, like all larvæ, of the reproductive organs, rapidly increase in size. At the end of a few weeks you will see them adhering to a leaf of potamogeton; they thrust their proboscis into the stem, and cling to it with their palpi. Little by little, the legs, the proboscis, and the palpi, are drawn back towards the body, abandoning the skin which has hitherto formed for each of these organs a kind of horny sheath. From the larva state, the animal passes into that of the nymph. But this nymph continues to feed and enlarge; proboscis, legs, and palpi grow thinner and harder; claws, ciliæ, and hairs are developed; and, finally, through a fissure in the skin emerges the perfect animal, red as wine, with eight feet, and about two millimetres in length. This animal, placed in the family of the Hydrachnellæ, has been described by De Geer under the name of Acarus aquaticus globosus, and by Dugès under that of Hydrachna globosa, on account of its globular form.
The Cheese Mite.
From the crust of a dry old cheese,—such a kind of cheese as a bon-vivant likes with a glass of "good old ale,"—a very fine powder often crumbles off, like the dust made by wood-eating worms.
Examine this powder with your lens, or if you have good eyes, you may make use of them. You will quickly detect something moving in it, and by degrees you will see that this movement pervades the whole mass; that there is a general stir and commotion in all directions.
But you find it impossible to distinguish clearly the form of the animals which are thus agitated. You are certain, however, that they are not maggots, for they affect moist cheeses; besides, they are visible enough to everybody, and at need can make themselves felt upon your hands, and even upon your face, for they have a faculty of launching themselves to a distance by a little manœuvre familiar enough to serpents: bringing the head round towards the tail, they curve themselves like the spring of a watch, then abruptly uncoiling themselves with the help of some solid appui, they fling forth into the air, and are thus launched to very considerable distances. It is a curious species of locomotion, not unworthy the attention of the mechanician.
To clear up the mystery of a movement whose cause is not apparent at the first glance, let us sprinkle with this impalpable débris,—with this kind of sawdust, or cheese-dust,—a little strip of glass, and place it beneath the focus of a microscope.
