Over 70% of the slide-mounted aquatic oligochaetes have been identified. Many additional collections of aquatic oligochaetes, presently held in alcohol jars and vials, await final processing, mounting on glass microslides, and identification. A computer database for the collection was begun in 1991. An important acquisition for the INHS Annelida Collection in 1986 was the Walter J. Harman collection of terrestrial oligochaetes. The material in this collection, comprised of 1,600 earthworm specimens representing 22 species, 10 genera, and 3 families, was collected by Dr. Harman from 230 sites in central Illinois during his 1956-1958 survey for his doctoral work at the University of Illinois. Specific surveys for aquatic annelids as well as general surveys for all aquatic macroinvertebrates conducted by M. J. Wetzel and other INHS biologists since 1973 have contributed the vast majority of the identified and unidentified specimens to the INHS Annelida Collection.

Illinois species now deposited in the INHS Annelida Collection include 3 aphanoneurans (1 family, 1 genus), 9 branchiobdellidans (3 families, 6 genera), 86 aquatic or semiaquatic oligochaetes (7 families, 45 genera), 32 leeches (4 families, 19 genera), and at least 22 terrestrial oligochaetes (3 families, 10 genera). One aquatic species, Eclipidrilus asymmetricus (Smith, 1896) (Oligochaeta: Lumbriculidae), is endemic to Illinois; several other species, including Limnodrilus tortilipenis Wetzel, 1987, Rhyacodrilus falciformis Bretscher, 1901, and Rhyacodrilus subterraneus Hrabe, 1963 (Oligochaeta: Tubificidae), and Allonais inaequalis (Stephenson, 1911) (Oligochaeta: Naididae) are species considered rare in Illinois, and known from only a few localities in North America. Another species, Varichaetadrilus angustipenis (Brinkhurst & Cook, 1966) - sporadic in its North American distribution - has been collected from several springs and caves in Illinois, but rarely elsewhere in the state.