L evolution des acariens, et les modeles de l evolution des arachnides. - In the present paper a comparative study is made of the evolution of the life cycle in Araclmidea, and of the evolution of Arachnidean chaetotaxy. The terminology vvith reference to the various fonns of moulting animals is completed by the introduction of two new terms (isophenon and stasoid). Four different evolutionary phenomena are distinguished (protelattosis, neoteny, metelattosis and plethomorphosis; with the exception of neoteny these tenns are all new). A model for the evolution of the Araclmidean life-cycle has been developed, in which two periods are distinguished: stasoidy and orthostasy; the last-mentioned period is subdivided into protostasy and alassostasy. The terms prelarva, larva and nymph are redefined. In a comparative study of chaetotaxy, a model has been developed for the numerical evolution of setae. In this model evolution starts from primordiotrichy, a hypothetic ancestral chaetotaxy which is not idionymic (the term adelonymic is introduced here as an antonym of idionymic). Beside primordiotrichy, prototrichy and neotrichy, the term meiotrichy (indicating deficient idionymic chaetotaxies} is introduced here. Three evolutionary phenomena are distinguished: trichodelosis, trichomeïosis and neotrichosis. The various chaetotaxies are classified in two groups: idiotrichy (idionymic) and atactotrichy (adelonymic).