Interspecific variation in bristle number on forewings of tiny insects does not influence clap-and-fling aerodynamics

Abstract

Miniature insects must overcome significant viscous resistance in order to fly. They typically possess wings with long bristles on the fringes and use clap-and-fling mechanism to augment lift. These unique solutions to the extreme conditions of flight at tiny sizes (< 2 mm body length) suggest that natural selection has optimized wing design for better aerodynamic performance. However, species vary in wingspan, number of bristles (n), and bristle gap (G) to diameter (D) ratio (G/D). How this variation relates to body length (BL) and its effects on aerodynamics remain unknown. We measured forewing images of 38 species of thrips and 21 species of fairyflies. Our phylogenetic comparative analyses showed that n and wingspan scaled positively and similarly with body length across both groups, whereas G/D decreased with BL, with a sharper decline in thrips. We next measured aerodynamic forces and visualized flow on physical models of bristled wings performing clap-and-fling kinematics at chord-based Reynolds number of 10 using a dynamically scaled robotic platform. We examined the effects of dimensional (G, D, wingspan) and non-dimensional (n, G/D) geometric variables on dimensionless lift and drag. We found that: (a) increasing G reduced drag more than decreasing D; (b) changing n had minimal impact on lift generation; and (c) varying G/D minimally affected aerodynamic forces. These aerodynamic results suggest little pressure to functionally optimize n and G/D. Combined with the scaling relationships between wing variables and BL, much wing variation in tiny flying insects might be best explained by underlying shared growth factors.

Fluid–structure interactions of bristled wings: the trade-off between weight and drag

“…After validating the simulations with experimental data, we leveraged the numerics to systematically explore the parameter space, including bristle length, diameter and number. Our main results, presented in figures 5 and 7, offer several insights that supplement prior research on bristled wing morphologies [35,44,47]. First, avoiding reconfiguration of the bristles during flight appears to be a general feature in the biological wings we examined (figure 5).…”

“…Drag and lift force production by bristled wings in linear and rotational translations and clap-and-fling movements have also been characterized [32,40–44]. These studies confirmed that, at low Re , bristled wings enjoy aerodynamic advantages compared to their membraned counterparts because they experience only slightly decreased lift force while greatly reducing wing weight.…”

“…The inter-bristle gap size is an important parameter, determining the type of flow between the bristles and the leakiness of the bristled wing [17,42,44–47]. We define the dimensionless gap size Gfalse^=G/D, where G is the distance from the edge of one bristle to that of its neighbour.…”

“…In particular, the study by Kasoju et al [42] on comb-like wing models with clap-and-fling actuation ( Re c = 5–15) confirmed previous numerical studies [28,38] and further asserted that, compared to membraned wings, bristled ones had enhanced drag-to-lift ratio. Using a similar experimental set-up, Ford et al [43] and Kasoju et al [44] showed that aerodynamic forces are only weak functions of the geometric parameters of the wing, such as the number of bristles and inter-bristle gap. The authors concluded that optimizing aerodynamic force generation during clap-and-fling might not drive the interspecific variations in bristled wing morphology.…”

Aerodynamics of two parallel bristled wings in low Reynolds number flow

“…In recent years, an increasing number of researchers have focused their attention on the smallest insects, with a special focus on their flying principles and aerodynamic mechanisms. According to the existing studies on insect morphology, unlike the larger insects with membranous wings, the smallest insects have distinctive bristled wings, e.g., thrips 4 , or partially bristled wings, e.g., the tiny wasp Encarsia Formosa 5,6 ; the specific morphology of the bristled wings can be found in the works of Huber and Noye 5 , Kolomenskiy et al 6 , Kasoju et al 7 and Zhao et al 8 .…”