For many years the Drunella lata complex of eastern North America consisted of 4 recognized species: 3 described from the Northeast, Drunella lata (Morgan), Drunella cornuta (Morgan), and Drunella cornutella (McDunnough), and 1, Drunella longicornis (Traver), from the southern Appalachian Mountains region. Recently, all 4 were synonymized to a single species (D. lata [Morgan], by priority) because morphological variation was perceived to be continuous and historical characterizations of species were viewed as arbitrary. We used genetic, morphometric, and life-history data to reevaluate D. lata, D. cornuta, and D. cornutella in northeastern North America, where all 3 morphotypes coexist in certain catchments and often in the same reach of stream. Pairwise genetic comparisons of coexisting morphotypes revealed fixed allelic differences at 6 to 10 of 19 allozyme loci, showing reproductive isolation among D. cornuta, D. cornutella, and D. lata even within the same reach of stream. Morphometric studies confirmed their distinction by uncovering significant differences in a suite of characters, some with nonoverlapping ranges (i.e., diagnostic). The 3 taxa also were shown to be segregated ecologically by consistent differences in seasonality, thermal requirements, and spatial distribution. We conclude that D. cornuta, D. cornutella, and D. lata are all worthy of full species status, and so reinstate D. cornuta and D. cornutella. Diagnostic descriptions are provided and existing keys emended.
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