As the Reynolds number of a system decreases, traditional pumping techniques become less effective. In nature, oscillating appendage systems exhibit distinct patterns of movement based on their Reynolds number. Studies of pumping by mayfly nymph gill arrays have shown different kinematics over Reynolds numbers from 2 to 22. To understand why and how this pumping mechanism might be optimized, a robotic oscillating plate array was constructed allowing stroke and pitch variation as well as phase lag variation between adjacent gills. Stereoscopic PIV was used to obtain three dimensional velocity data, allowing computation of the net pumping rate and flow induced dissipation for five cases, focusing on the role of the gill plate interactions and their dependence on the phase lag. The results indicate that mayfly gills most likely use a phase lag of 90° because it produces the highest net mass flow rate and has the highest specific flux efficiency.