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Dorsum black, body long and broad, 11th (most lateral) row of punctures (stria) on hardened fore-wings (elytra) close to the margin in the apical third (posterior), uniform net-like pattern (microreticulations) and sparse, uniformly spaced circular pits (micropuncturation) on elytra (visible at 50X magnification) (Oygur and Wolfe 1991). Elytra very shinny, sides of elytra bronzed, rows of punctures (different from micropunctures visible under low power called elytral stria) more deeply impressed laterally than medially (Fall 1922). Dorsum of abdominal segments 4-7 uniformly black, legs uniformly red (Oygur and Wolfe 1991). Most closely resembles Gyrinus picipes and G. dubius. Examination of male genitailia (aedeagus) may be necessary for a definitive identification. Male genitalia (aedeagus) medially ridged, tapering moderately towards apex, margin moderately sinuate. Gyrinus sayi can be separated from G. picipes by the presence of uniform microreticulation which is absent or non-uniform in G. picipes (Oygur and Wolfe 1991). Females of G. sayi have more distinct microreticulations than males, whereas females of G. picipes have the same microreticulations as males. Male length 5.5-6.4 mm and width 2.8-3.2 mm; female length 5.8-6.6 mm and width 3.2-3.6 mm.