A Possible Role of the Boundary Layer in Insect Flight

Abstract

IT has been recently proposed by Greenewalt¹ that the observed wing-beat frequency of insects and birds is equivalent to the characteristic frequency of the flight system as a mechanical oscillator. Evidence for this has come primarily from work on those higher orders of insects in which direct neural control of frequency is absent (‘asynchronous’ type of Roeder²). From this point of view the inverse variation of wing-beat frequency with air density reported in some species by Chadwick and Williams³ and Sotavalta⁴ appears anomalous. Evidence is given here which shows that such an anomaly may be due to a factor hitherto not considered in investigations of the flight of small insects: a variation of effective wing inertia with air density due to a functionally significant additional mass corresponding to a volume of air ‘attached’ to the wings.