Three new genera and eighteen new species of miniature polydesmid millipedes from the northwestern United States (Diplopoda, Polydesmida, Polydesmidae) Author Shear, William A. Professor Emeritus, Department of Biology, Hampden-Sydney College, Hampden-Sydney VA 23943; present address 1950 Price Drive, Farmville VA 23901. Author Marek, Paul E. 0000-0002-7048-2514 Associate Professor, Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. pmarek @ vt. edu; https: // orcid. org / 0000 - 0002 - 7048 - 2514 pmarek@vt.edu text Zootaxa 2021 2021-05-24 4975 1 81 126 journal article 6194 10.11646/zootaxa.4975.1.3 529d48db-4299-4e2d-9aaf-6c4178c08b18 1175-5326 4804909 DB7C9028-3EDF-454F-88D0-336624AD1DC4 Family Polydesmidae Leach, 1815 Diagnosing the family Polydesmidae in a way that distinguishes its members from other, closely related and similar families is not easy. We contend that the superfamily Trichopolydesmoidea is superfluous and synonymous with Polydesmoidea , bringing Trichopolydesmidae , Macrosternodesmidae , Mastigonodesmidae , Opisotretidae and Fuhrmannodesmidae into consideration in delineating the family. The most recent attempt to diagnosis Polydesmidae is that of Enghoff et al . (2015) . Their account includes qualifying words “usually”, “occasionally” and “often”, and this is necessary because the family presently includes some discordant elements and as has been often stated, the taxa give the characters and not vice versa. Nevertheless, it is possible to list a few characters for Polydesmidae that may provide guidance: 1) presence on the gonopod of a fimbriate pad or projection (pulvillus) at the point where the seminal pore opens, 2) presence of a vesicle in the distal part of the seminal canal, 3) the seminal canal with a distinct arch or loop proximal to the vesicle. One or more of these characters may be absent in species presently considered polydesmids, including a few described here. Nonsexual characters that are helpful include a microspiculate limbus and the presence of sphaerotrichomes on at least some of the anterior legs of males. Species of Macrosternodesmidae , in our view the most closely related family and the only other polydesmoid family in North America, lack these gonopod characters, but many do have sphaerotrichomes; in addition, macrosternodesmids have the gonopod prefemorite transverse with respect to the gonocoxae such that the acropodite makes a right angle with it. In Polydesmidae , the acropodite continues the line of the prefemorite. Characteristically, the acropodites of macrosternodesmids are more complex, with at least three major branches, including an exomere. In the genera and families described herein, even the characters just listed do not always occur together, possibly because of phenotypic simplification associated with a great reduction in size. However, the pulvillus is always present in the species discussed here, the vesicle in only a few, and the loop in the seminal canal in none of the species. Bidentogon species are the exception, retained for now in Polydesmidae , but lacking all three of these important characters, unless our hypothesis about the origin of the solenomere in Bidentogon species is supported.