A Monograph of the Slug Mite—Riccardoella limacum (Schrank.).

Summary. The history of the classification of the mite is set forth and the chief authorities with those of their works which deal with this species are given. The synonymy of Riccardoella limacum (Schrank), R. oudemansi Sig Thor, and R. jenynsi Sig Thor, the first name having priority, is proved. The last name is shown to refer, most probably, to the male, whilst the original description under the first name was given to the appearance of the animal prior to ecdysis or in certain conditions of the environment. The various stages in the life‐history are described, some for the first time, and the deutonymph is shown to be the nympha generans stage whilst the fully adult form is in process of disappearing from the life cycle. The processes of the whole life‐history are given, as far as is at present known, and the female is shown to be oviparous and viviparous whilst some overwintering larvae are shown to remain in the dry skin of the female. The internal anatomy, as far as it has been possible to make it out, is described and several new variations for acari of this group are recorded, the most important of which is the possibly primitive nature of the respiratory system in the larva which, it is suggested, is possibly representative of the primitive acarine type. The chromosome number is given and compared with what is at present known of this in other species of acari. The general biology of the mite together with a list of its known hosts, geographical distribution, habits, reaction to light, heat and humidity are all described and discussed. The food of the mite about which, hitherto, there has been much vagueness and disagreement, is shown to be the mucus of the host. In this connection the rôle of intracellular symbiotic Haplosporidia in supplying vitamins of the B group is suggested and the presence of these in the egg is noted. The case recorded by Oldham (1931) of some effects of a mite on Arianta arbustorum is discussed and criticised and it is shown to be highly improbable that the mite was R. limacum. The phylogeny of the Ereynetidae is dealt with briefly and the relationship of the genus Riccardoella to others in this family set forth in a tentative scheme which takes account, too, of the singular likenesses between R. crassipes and the larval form of R. limacum. Some of the methods used in these investigations and observations, both successful and unsuccessful, are described.