Aerial locomotion in flies and robots: kinematic control and aerodynamics of oscillating wings

Dynamic experimental rigs for investigation of insect wing aerodynamics

This paper provides a systematic and critical review of dynamic experimental rigs used for insect wing aerodynamics research. The goal is to facilitate meaningful comparison of data from existing rigs and provide insights for designers of new rigs. The scope extends from simple one degree of freedom rotary rigs to multi degrees of freedom rigs allowing various rotation and translation motions. Experimental methods are characterized using a consistent set of parameters that allows objective comparison of different approaches. A comprehensive catalogue is presented for the tested flow conditions (assessed through Reynolds number, Rossby number and advance ratio), wing morphologies (assessed through aspect ratio, planform shape and thickness to mean chord ratio) and kinematics (assessed through motion degrees of freedom). Links are made between the type of aerodynamic characteristics being studied and the type of experimental set-up used. Rig mechanical design considerations are assessed, and the aerodynamic measurements obtained from these rigs are discussed.

Study of Mosquito Aerodynamics for Imitation as a Small Robot and Flight in a Low-Density Environment

In terms of their flight and unusual aerodynamic characteristics, mosquitoes have become a new insect of interest. Despite transmitting the most significant infectious diseases globally, mosquitoes are still among the great flyers. Depending on their size, they typically beat at a high flapping frequency in the range of 600 to 800 Hz. Flapping also lets them conceal their presence, flirt, and help them remain aloft. Their long, slender wings navigate between the most anterior and posterior wing positions through a stroke amplitude about 40 to 45°, way different from their natural counterparts (>120°). Most insects use leading-edge vortex for lift, but mosquitoes have additional aerodynamic characteristics: rotational drag, wake capture reinforcement of the trailing-edge vortex, and added mass effect. A comprehensive look at the use of these three mechanisms needs to be undertaken-the pros and cons of high-frequency, low-stroke angles, operating far beyond the normal kinematic boundary compared to other insects, and the impact on the design improvements of miniature drones and for flight in low-density atmospheres such as Mars. This paper systematically reviews these unique unsteady aerodynamic characteristics of mosquito flight, responding to the potential questions from some of these discoveries as per the existing literature. This paper also reviews state-of-the-art insect-inspired robots that are close in design to mosquitoes. The findings suggest that mosquito-based small robots can be an excellent choice for flight in a low-density environment such as Mars.

Morphology of hindwing veins in the shield bug Graphosoma italicum (Heteroptera: Pentatomidae)

Light, fluorescence, and electron microscopy were applied to cross sections and -breakage and whole-mount preparations of the anterior hindwing vein of the shield bug Graphosoma italicum. These analyses were complemented by investigations of the basal part of the forewing Corium and Clavus. The integration of structural, histological, and fluorescence data revealed a complex arrangement of both rigid and elastic structures in the wall of wing veins and provided insights into the constitution of transition zones between rigid and elastic regions. Beneath the exocuticular layers, which are continuous with the dorsal and ventral cuticle of the wing membrane, the lumen of the veins is encompassed by a mesocuticular layer, an internal circular exocuticular layer, and an internal longitudinal endocuticular layer. Separate parallel lumina within the anterior longitudinal vein of the hindwing, arranged side-by-side rostro-caudally, suggest that several veins have fused in the phylogenetic context of vein reduction in the pentatomid hindwing. Gradual structural transition zones and resilin enrichment between sclerotized layers of the vein wall and along the edges of the claval furrow are interpreted as mechanical adaptations to enhance the reliability and durability of the mechanically stressed wing veins.

Body side-specific control of motor activity during turning in a walking animal

Animals and humans need to move deftly and flexibly to adapt to environmental demands. Despite a large body of work on the neural control of walking in invertebrates and vertebrates alike, the mechanisms underlying the motor flexibility that is needed to adjust the motor behavior remain largely unknown. Here, we investigated optomotor-induced turning and the neuronal mechanisms underlying the differences between the leg movements of the two body sides in the stick insect Carausius morosus. We present data to show that the generation of turning kinematics in an insect are the combined result of descending unilateral commands that change the leg motor output via task-specific modifications in the processing of local sensory feedback as well as modification of the activity of local central pattern generating networks in a body-side-specific way. To our knowledge, this is the first study to demonstrate the specificity of such modifications in a defined motor task.

DOI:http://dx.doi.org/10.7554/eLife.13799.001

Walking along a curve or turning is a complex manoeuvre for the nervous system, as it must coordinate different leg movements on each side of the body. Rhythmic processes such as walking are controlled by networks of neurons called central pattern generators. The resulting movements can be adjusted by feedback from sense organs in response to environmental conditions. For example, sensory feedback that provides information about the load placed on each leg, allows the animal to control the duration of a stance. How the nerve cells, or neurons, involved in these processes work together to produce complex, flexible movements such as turning is largely unknown.

Previous work on how the brain negotiates turning movements has been carried out mostly in animals that swim or fly. To understand what happens during walking, Gruhn et al. monitored stick insects that walked in a curve on a slippery surface, and recorded the electrical activity within the animals' nervous system as they turned.

By comparing the activity of the nervous system on each side of the body while the insects walked a curve, Gruhn et al. found that the nervous system uses at least three different mechanisms to produce the different movements on the inside and outside. Firstly, the sensory feedback signals that communicate the load on the leg are processed in the legs on the outside of the curve to support forward steps, while they are processed on the inside legs to support forward, sideward, and backward steps. Secondly, the motor activity produced by the central pattern generator is modulated to be stronger for the muscle that moves the leg backward on the outside of the curve. At the same time, this activity is stronger for the muscle that moves the leg forward on the inside of the curve. Thirdly, signals from a front leg influence the movement of the other legs on the same side of the body. This influence is strong on the inside and weak on the outside of the curve.

Together or separately, these three mechanisms could provide the animal with the means to perform turns in all their different curvatures. Future work will need to work out exactly which local neurons process the signals sent from the brain to control movement.

DOI:http://dx.doi.org/10.7554/eLife.13799.002

Mechanical models of sandfish locomotion reveal principles of high performance subsurface sand-swimming

We integrate biological experiment, empirical theory, numerical simulation and a physical model to reveal principles of undulatory locomotion in granular media. High-speed X-ray imaging of the sandfish lizard, Scincus scincus, in 3 mm glass particles shows that it swims within the medium without using its limbs by propagating a single-period travelling sinusoidal wave down its body, resulting in a wave efficiency, η, the ratio of its average forward speed to the wave speed, of approximately 0.5. A resistive force theory (RFT) that balances granular thrust and drag forces along the body predicts η close to the observed value. We test this prediction against two other more detailed modelling approaches: a numerical model of the sandfish coupled to a discrete particle simulation of the granular medium, and an undulatory robot that swims within granular media. Using these models and analytical solutions of the RFT, we vary the ratio of undulation amplitude to wavelength (A/λ) and demonstrate an optimal condition for sand-swimming, which for a given A results from the competition between η and λ. The RFT, in agreement with the simulated and physical models, predicts that for a single-period sinusoidal wave, maximal speed occurs for A/λ ≈ 0.2, the same kinematics used by the sandfish.

Recent developments in the remote radio control of insect flight

The continuing miniaturization of digital circuits and the development of low power radio systems coupled with continuing studies into the neurophysiology and dynamics of insect flight are enabling a new class of implantable interfaces capable of controlling insects in free flight for extended periods. We provide context for these developments, review the state-of-the-art and discuss future directions in this field.

The free-flight response of Drosophila to motion of the visual environment

In the present study we investigated the behavioural strategies with which freely flying fruit flies (Drosophila) control their flight trajectories during active optomotor stimulation in a free-flight arena. We measured forward, turning and climbing velocities of single flies using high-speed video analysis and estimated the output of a 'Hassenstein-Reichardt' elementary motion detector (EMD) array and the fly's gaze to evaluate flight behaviour in response to a rotating visual panorama. In a stationary visual environment, flight is characterized by flight saccades during which the animals turn on average 120 degrees within 130 ms. In a rotating environment, the fly's behaviour typically changes towards distinct, concentric circular flight paths where the radius of the paths increases with increasing arena velocity. The EMD simulation suggests that this behaviour is driven by a rotation-sensitive EMD detector system that minimizes retinal slip on each compound eye, whereas an expansion-sensitive EMD system with a laterally centred visual focus potentially helps to achieve centring response on the circular flight path. We developed a numerical model based on force balance between horizontal, vertical and lateral forces that allows predictions of flight path curvature at a given locomotor capacity of the fly. The model suggests that turning flight in Drosophila is constrained by the production of centripetal forces needed to avoid side-slip movements. At maximum horizontal velocity this force may account for up to 70% of the fly's body weight during yaw turning. Altogether, our analyses are widely consistent with previous studies on Drosophila free flight and those on the optomotor response under tethered flight conditions.

Near- and far-field aerodynamics in insect hovering flight: an integrated computational study

We present the first integrative computational fluid dynamics (CFD) study of near- and far-field aerodynamics in insect hovering flight using a biology-inspired, dynamic flight simulator. This simulator, which has been built to encompass multiple mechanisms and principles related to insect flight, is capable of 'flying' an insect on the basis of realistic wing-body morphologies and kinematics. Our CFD study integrates near- and far-field wake dynamics and shows the detailed three-dimensional (3D) near- and far-field vortex flows: a horseshoe-shaped vortex is generated and wraps around the wing in the early down- and upstroke; subsequently, the horseshoe-shaped vortex grows into a doughnut-shaped vortex ring, with an intense jet-stream present in its core, forming the downwash; and eventually, the doughnut-shaped vortex rings of the wing pair break up into two circular vortex rings in the wake. The computed aerodynamic forces show reasonable agreement with experimental results in terms of both the mean force (vertical, horizontal and sideslip forces) and the time course over one stroke cycle (lift and drag forces). A large amount of lift force (approximately 62% of total lift force generated over a full wingbeat cycle) is generated during the upstroke, most likely due to the presence of intensive and stable, leading-edge vortices (LEVs) and wing tip vortices (TVs); and correspondingly, a much stronger downwash is observed compared to the downstroke. We also estimated hovering energetics based on the computed aerodynamic and inertial torques, and powers.

Assessing sensory function in locomotor systems using neuro-mechanical simulations

Computer simulations are being used increasingly to gain an understanding of the complex interactions between the neuronal, sensory, muscular and mechanical components of locomotor systems. Recent neuro-mechanical simulations of walking in humans, cats and insects, and of swimming in lampreys, have provided new information on the functional role of specific groups of sensory receptors in regulating locomotion. As we discuss in this review, these studies also make it clear that a full understanding of the neural and mechanical mechanisms that underlie locomotion can be achieved only by using simulations in parallel with physiological investigations. The widespread implementation of this approach would be enhanced by the development of freely available and easy-to-use software tools.

Dynamics of in vivo power output and efficiency of Nasonia asynchronous flight muscle

By simultaneously measuring aerodynamic performance, wing kinematics, and metabolic activity, we have estimated the in vivo limits of mechanical power production and efficiency of the asynchronous flight muscle (IFM) in three species of ectoparasitoid wasps genus Nasonia (N. giraulti, N. longicornis, and N. vitripennis). The 0.6 mg animals were flown under tethered flight conditions in a flight simulator that allowed modulation of power production by employing an open-loop visual stimulation technique. At maximum locomotor capacity, flight muscles of Nasonia are capable to sustain 72.2 +/- 18.3 W kg(-1) muscle mechanical power at a chemo-mechanical conversion efficiency of approximately 9.8 +/- 0.9%. Within the working range of the locomotor system, profile power requirement for flight dominates induced power requirement suggesting that the cost to overcome wing drag places the primary limit on overall flight performance. Since inertial power is only approximately 25% of the sum of induced and profile power requirements, Nasonia spp. may not benefit from elastic energy storage during wing deceleration phases. A comparison between wing size-polymorphic males revealed that wing size reduction is accompanied by a decrease in total flight muscle volume, muscle mass-specific mechanical power production, and total flight efficiency. In animals with small wings maximum total flight efficiency is below 0.5%. The aerodynamic and power estimates reported here for Nasonia are comparable to values reported previously for the fruit fly Drosophila flying under similar experimental conditions, while muscle efficiency of the tiny wasp is more at the lower end of values published for various other insects.

The significance of spiracle conductance and spatial arrangement for flight muscle function and aerodynamic performance in flyingDrosophila

During elevated locomotor activity such as flight, Drosophilasatisfies its increased respiratory demands by increasing the total spiracle opening area of the tracheal gas exchange system. It has been assumed that in a diffusion-based system, each spiracle contributes to oxygen flux into and carbon dioxide flux out of the tracheal system according to the size of its opening. We evaluated this hypothesis by determining how a reduction in size and interference with the spatial distribution of gas exchange areas impair flight muscle function and aerodynamic force production in the small fruit fly Drosophila melanogaster. This was done by selectively blocking thoracic spiracles of tethered flies flying inside a flight simulator. Flow-through respirometry and simultaneous measurements of flight force production and wing kinematics revealed a negligible functional safety margin for respiration. Maximum locomotor performance was only achieved by unmanipulated flies, supporting the general assumption that at the animal's maximum locomotor capacity, maximum spiracle opening area matches respiratory need. The maximum total buffer capacity for carbon dioxide in Drosophila amounts to approximately 33.5 μl g–1body mass, estimated from the temporal integral of carbon dioxide release rate during the resting period after flight. By comparing flight variables in unmanipulated and `spiracle-blocked' flies at comparable flight forces, we found that (i) stroke amplitude, stroke frequency and the chemo-mechanical conversion efficiency of the indirect flight musculature were broadly independent of the arrangement of spiracle conductance, while (ii) muscle mechanical power significantly increased, and (iii) mean lift coefficient and aerodynamic efficiency significantly decreased up to approximately 50% with an increasing number of blocked spiracles. The data suggest that Drosophila apparently maximizes the total efficiency of its locomotor system for flight by allowing oxygen delivery to the flight musculature through multiple spiracles of the thorax.

Insect-like flapping wing mechanism based on a double spherical Scotch yoke

We describe the rationale, concept, design and implementation of a fixed-motion (non-adjustable) mechanism for insect-like flapping wing micro air vehicles in hover, inspired by two-winged flies (Diptera). This spatial (as opposed to planar) mechanism is based on the novel idea of a double spherical Scotch yoke. The mechanism was constructed for two main purposes: (i) as a test bed for aeromechanical research on hover in flapping flight, and (ii) as a precursor design for a future flapping wing micro air vehicle. Insects fly by oscillating (plunging) and rotating (pitching) their wings through large angles, while sweeping them forwards and backwards. During this motion the wing tip approximately traces a "figure-of-eight" or a "banana" and the wing changes the angle of attack (pitching) significantly. The kinematic and aerodynamic data from free-flying insects are sparse and uncertain, and it is not clear what aerodynamic consequences different wing motions have. Since acquiring the necessary kinematic and dynamic data from biological experiments remains a challenge, a synthetic, controlled study of insect-like flapping is not only of engineering value, but also of biological relevance. Micro air vehicles are defined as flying vehicles approximately 150 mm in size (hand-held), weighing 50-100g, and are developed to reconnoitre in confined spaces (inside buildings, tunnels, etc.). For this application, insect-like flapping wings are an attractive solution and hence the need to realize the functionality of insect flight by engineering means. Since the semi-span of the insect wing is constant, the kinematics are spatial; in fact, an approximate figure-of-eight/banana is traced on a sphere. Hence a natural mechanism implementing such kinematics should be (i) spherical and (ii) generate mathematically convenient curves expressing the figure-of-eight/banana shape. The double spherical Scotch yoke design has property (i) by definition and achieves (ii) by tracing spherical Lissajous curves.