Variations in the Winter Stonefly Allocapnia granulata as Indicators of Pleistocene Faunal Movements1

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Abstract
The winter stonefly, Allocapnia granulata (Claassen), has the greatest east-west range of any species in the genus, occurring from the New England States to the Wichita Mountains in central Oklahoma. Variations discovered in the genitalic structures and wings of the males were plotted against several transects taken across the geographic range of the species. These indicate that the species contains an eastern population from the Atlantic Coast to central Illinois, and a western population occurring from central Missouri to central Oklahoma. The eastern population contains 2, the western 3 subpopulations. All are definable only on a modal basis. A phylogenetic analysis of the variants and their present geographic distribution indicates the following sequence of events. The progenitor of A. granulata probably originated in the Cumberland plateau-Appalachian region. The species spread northward, then divided into northern and southern populations, the northern one evolving shorter wings. Probably at the onset of the last glacial epoch, the more northern population moved south and westward as far as Oklahoma. Soon after, the main eastern and western populations became separated at about the mid-Illinois line. Subsequently the western population broke up into the subpopulations, but moved northward only slightly. As the glaciers dissipated, the eastern population moved northward, colonizing the deglaciated country from Minnesota to the East Coast.