The dog harbours a demodex (_D. Caninus_) which causes it to lose its hair. Some years ago, the sheep in Belgium were attacked by one of the acaridæ, the _Ixodes reduvius_, which had been introduced from a neighbouring country, and had multiplied with frightful rapidity. Packard has given an account of an _Ixodes bovis_ on the _Erethizon epixanthus_, and on the _Lepus Bairdii_, and an _Argas Americana_ on cattle coming from Texas; this was published in the sixth report of the United States' Geological survey (1873). According to the observations of Mons. Megnin, the _Tyroglyphi_, the _Hypopi_, the _Homopi_, and the _Trichodactyli_, are transitory forms which ought not to be preserved as generic divisions among the acaridæ. We have found on the small bat (_Pipistrella_) an acaride (_Caris 135 elliptica_) and a new _Ixodes_ which we have described in a special memoir on the parasites of the _Cheiroptera_. Mr. Lucas caught an ixodes on a dog, and kept it alive long enough distinctly to see it lay eggs which proceeded from an oviduct. These eggs formed masses attached to the abdomen of the mother. An acarus (_Dermanyssus avium_) is found on birds, and multiplies with such rapidity that it completely exhausts those on which it has established itself. It has been seen accidentally on man. An instance is recorded of a woman who could not get rid of these parasites, because she passed every day through her henhouse in order to get to her cellar, and the frightened fowls threw down upon her a perfect shower of acaridæ. Not long ago mention was made at the Academy of Medicine at Paris, of a sarcoptes (_S. mutans_), which produces a disease among fowls, especially on the cock and hen, and which passes from these to the horse and other domestic animals. This sarcoptes prefers to live under the epidermis of the feet. Reptiles are not free from its attacks, for it is often seen on lizards and serpents. We have found a very curious one on the skin of a gecko from the south of France. Many insects are always covered with certain species of acaridæ. Every entomologist knows that the body of the "watchman" beetle always has some of these, like little living pearls, which wander especially on the under side of the abdomen. It is the same with a small coleopterous insect that is found abundantly wherever there is any decomposing matter. Léon Dufour gave himself up to the study of some of the parasites of insects, and mentions, among others, a species 136 belonging to the muscidæ, the _Limosina lugubris_, which does not measure a line in length, and which harbours as many as fifteen pteropti under its abdomen. [Illustration: Fig. 24.--Hydrachna geographica.]