Studies on the Thyasinae of North America (Acarina: Hydryphantidae)

Abstract

The present work deals with the 17 species belonging to the subfamily Thyasinae known to occur in North America. Since there are still large areas of North America which have been poorly collected for water mites, this list is without doubt incomplete. There are several thyasin genera, with a widespread palearctic distribution, which are as yet unreported from this continent. Because so many water mite genera, formerly known only from the Old World, have been collected in North America during the last few years, it seems probable that members of some or all of these European thyasin genera may be taken in future collections. Compared to many other groups of Hydracarina, the Thyasinae have received little attention in North America. The first records from this continent were three species described under the generic name Thyas by Koenike (1895). Two of these species were later placed in other genera. Mitchell (1953) in his treatment of the family Hydryphantidae gave a detailed account of the early work on the subfamily Thyasinae. Cook (1953, 1955) described two new thyasin genera, one of which is now considered by the author to be only a subgenus. Habeeb (1954) erected a new genus and five new species based on specimens from eastern United States and Canada. The new genus and three of the new species are considered synonyms by the present author. Habeeb (1958) also described a new species of Thyasides from New Brunswick. The presence or absence of pigment in the median eye has been considered to be of strong phylogenetic significance and in some classifications the forms with pigmented median eyes have been placed in a separate subfamily Euthyasinae. For reasons which will be given during the discussion of the Panisopsis-complex, the author feels that the median eye character alone is of little taxonomic value, and at best should be used to separate subgenera. Members of the subfamily Thyasinae often exhibit much intraspecific variation, even within a single population. As would be expected, populations of the same species from widely scattered localities show certain differences. However, in view of the extremes of variation often found in members of a species within a given area, it is felt that little would be gained by giving these populations a trinomial name. Likewise, when it can be shown that specimens of our North