A method for recognizing the final stadium and the four preceding stadia of dragonfly nymphs (Odonata: Anisoptera) was derived by dividing hind wing sheath length (WSL) by maximum head width (HW). Based on measurements for 15 species representing all seven North American families, five stadia can be delineated, counting backwards from the final (F-0) to the four preceding stadia (F-1, F-2, F-3, and F-4). The ratio WSL/HW over all species ranged as follows: F-0, 0.89–1.39 (mean 1.16); F-1, 0.57–0.88 (mean 0.70); F-2, 0.36–0.61 (mean 0.46); F-3, 0.24–0.44 (mean 0.32); and F-4, 0.15–0.32 (mean 0.23). As a crude guide, a ratio near 1 or greater indicates F-0, about two-thirds indicates F-1, about one-half indicates F-2, about one-third indicates F-3, and about one-fourth indicates F-4. Plathemis lydia (Libellulidae) had the highest WSL/HW ratio in F-0 (1.32–1.39).
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Vol. 126 • No. 4