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Our Leptus at first undoubtedly breathed through the skin, as do most of the Poduras, since we have been unable to find tracheæ in them, nor even in the prolarva of a genus of minute ichneumon egg parasites, nor in the Linguatulæ and Tardigrades, and some mites, such as the Itch insect and the Demodex, and other Acari. In the Myriopod, Pauropus, Lubbock was unable to find any traces of tracheæ.
After attaining this form, its small abdomen begins to enlarge until it assumes a globular form, and the mass of mites look like little beads. Mr. Newport was unable to discover the male, and thought that this mite was parthenogenous. It will be seen that the adult Demodex retains the elongated, worm-like appearance of the larva of the higher mites, such as Typhlodromus.
This is a minute little creature, scientifically called Demodex folliculorum, hardly visible to the naked eye, with comparatively large fore body, a more slender hind body and eight little stumpy processes that do duty as legs. No specialized head is visible, although of course there is a mouth orifice.
We have the faintest possible indication in the structure of some mites, and of the Tardigrades and Pentastoma, where there is a striking recurrence, as we may term it, to a worm-like form, readily noticed by every observer, whatever his opinion may be on the developmental theory. In the Demodex we see a tendency of the mite to assume under peculiar circumstances an elongated, worm-like form.
While the Demodex is probably the humblest in its organization of any of the insects we have treated of, there is still another mite, which, some eminent naturalists continue to regard as a worm, which is yet lower in the scale. At first, however, it is found in the liver or lungs of various animals, sometimes in man.
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