The generic characters of the Tetranychidae lie in the tarsal appendages, particularly the claw. The palpi and shape of the male sexual organ are used mainly for the determination of species. These specific characters are depicted in profile; thus, in order to observe them the mite must be mounted and flattened dorso-ventrally. This entails a considerable amount of manipulation, and a high magnification is necessary in order to obtain definition of the minute details.

As material increases and taxonomic information is refined in any group of organisms, it is a natural corollary that the systematic arrangement by its students should be in the direction of progressive limitation, restriction, or subdivision to facilitate either or both recognition and knowledge of natural relationships.

Most of the species described in this paper have been sent to me for identification by various students of limnology to whom I wish to express my appreciation for their cooperation. Types are in the collection of the Illinois State Natural History Survey, unless otherwise stated. I am indebted to Dr. C. O. Mohr of the Survey for considerable assistance with the drawings.