Efficacy of morphological characters for distinguishing nymphs of Epitheca cynosura and Epitheca spinigera (Odonata: Corduliidae) in Wisconsin

Abstract

Attempts to distinguish exuviae and last-instar nymphs of Epitheca cynosura (Say) and Epitheca spinigera (Selys) (Odonata: Corduliidae) using lateral spine characters have proven to be unreliable, and recent use of setae counts on only one side of the prementum or one labial palp have led to confusion because these structures often hold unequal numbers of setae on the two sides of the same specimen. Based on exuviae of 67 reared E. cynosura and 55 reared E. spinigera from lakes throughout Wisconsin, we tested the efficacy of previously used character states for distinguishing these species and searched for new characters to improve the reliability of regional keys. The most reliable diagnostic character was the combined number of setae on both sides of the prementum and on both labial palps (≤ 35 - E. cynosura; ≥ 36 - E. spinigera), which correctly determined 96% of our specimens. For the small percentage of specimens that lie in the region of overlap in total setae number, we found that total exuviae length, cerci ÷ epiproct ratios of females, tubercle distance ÷ epiproct ratios of males, and the shape of the dorsal hook on segment 8 could be used to strengthen determinations.