Viscosity determines the resistance of hemolymph flow through vessels. For flying insects, viscosity is a major physiological parameter limiting flight performance by controlling the flow rate of fuel to the flight muscles, circulating nutrients, and rapidly removing metabolic waste products. The more viscous the hemolymph, the greater the metabolic energy needed to pump it through body cavities and hemolymph vessels. By employing Magnetic Rotational Spectroscopy with nickel nanorods, we showed that viscosity of hemolymph in resting hawkmoths (Sphingidae) depends on wing size non-monotonically. Viscosity increases for small hawkmoths with high wingbeat frequencies, reaches a maximum for middle-sized hawkmoths with moderate wingbeat frequencies, and decreases in large hawkmoths with slower wingbeat frequencies but greater lift. Accordingly, hawkmoths with small and large wings have viscosities approaching that of water, whereas hawkmoths with mid-sized wings have more than twofold greater viscosity. The metabolic demands of flight correlate with significant changes in circulatory strategies via modulation of hemolymph viscosity. Thus, the evolution of hovering flight would require fine-tuned viscosity adjustments to balance the need for the hemolymph to carry more fuel to the flight muscles while decreasing the viscous dissipation associated with its circulation.