Abstract
Hovering insects are divided into two categories: 'normal' hoverers that move the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both downstroke and upstroke, shedding vortex rings each half-stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using computational fluid dynamics, and so it remains to be determined whether insect wakes can be represented by any of the suggested models. Here, we used tomographic particle image velocimetry, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths is best described as a series of bilateral, stacked vortex 'rings'. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared with the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power (Pind) from downwash velocities in the wake. Standard models predicted a Pind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.
Highlights
Hovering is a flight mode utilized by all major extant flying taxa, and most commonly found in nectarfeeding species
These measurements were used as input into aerodynamic power models, as well as to investigate the potential link between kinematics and vortex wake topology
An inclined stroke plane angle has previously been associated with a wake consisting of only downstroke vortex rings (Ellington, 1984b), and the stroke plane angle of M. stellatarum lies well above the cut-off angle (~33° > 20°) for when an inactive upstroke is considered likely (Fig. 1D)
Summary
Hovering is a flight mode utilized by all major extant flying taxa, and most commonly found in nectarfeeding species. In insects hovering is a ubiquitously used flight mode. Pennycuick (1968) that the relationship between power required to fly and flight speed follows a U-shaped function, so that very slow (e.g. hovering) and very fast flight requires more power than flight at intermediate speeds. Empirical data have supported this relationship for vertebrates It has historically been surmised that insects – operating in a different flow regime from birds and bats – are exempt from the problem of costly hovering, and several empirical studies have suggested that hovering requires a similar amount of power as forward flight Studies using quantitative flow measurements to determine the cost of hovering in insects are lacking
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