Comparative studies encompassing a wide range of aquatic animals have shown that rowing is exclusively used at low Reynolds numbers (Re <1) while flapping is predominantly used at Re > 100, but few studies have been undertaken to document the transition in individual species that traverse the intermediate Re regime using a single set of appendages. Thus, it is not generally known whether a gradual increase in Re within a system results in a gradual or sudden shift between rowing and flapping. Here I document both the kinematics of the appendages and the surrounding fluid of a nymphal mayfly Centroptilum triangulifer that uses a serial array of seven pairs of abdominal gill plates and operates at Reynolds numbers ranging from 2-22 during ontogeny. I found that some kinematic variables (stroke frequency and metachronal phase lag) did not change during ontogeny but that others changed substantially. Specifically, gills in small instars used strokes with large pitch and stroke-plane deviations, while larger instars used strokes with minimal pitch and minimal stroke-plane deviation. Gills in larger instars also acquired an intrinsic hinge that allowed passive asymmetric movement between half strokes. Net flow in small animals was directed ventrally and essentially parallel to the stroke plane (i.e. rowing), but net flow in large animals was directed dorsally and essentially transverse to the stroke plane (i.e. flapping). The metachronal phasing of the gills produced a time-dependent array of vortices associated with a net ventilatory current, a fluid kinematic pattern here termed a "phased vortex pump". Significantly, absolute vortex size did not change with increasing animal size or Re, indicating that the vortex diameter (Lv) decreased relative to intergill spacing (Lis) during mayfly growth. Given that effective flapping requires organized flow between adjacent appendages, I proposed the hypothesis that rowing should be favored when Lis / Lv < 1 and flapping should be favored when Lis / Lv > 1. Significantly, the rowing-to-flapping transition in Centroptilum triangulifer occurs at Re ~5, when maximum dynamic intergill distance equals vortex diameter. This result suggests that the Re-based rowing-flapping demarcation observed in a wide array of aquatic organisms is determined by the relative size of the propulsive mechanism and its self-generated vortices.