Wing Morphology and Powered Flight of Insects

Seiichi Sudo a,* Takumi Moriya a, Kyohei Hoshika a, Tetsuya Yano a, and Yoshiaki Shimazaki a

a Faculty of Systems Science and Technology, Akita Prefectural University, Japan

Abstract—This paper describes flapping behavior in powered flight of some selected insects. Free flight by a pair of wings for a dragonfly, free flights of other insects, and tethered flight of a fly are studied by the high-speed video camera system. The morphology of the wings of these insects is also studied. The relationships between wing morphology and flight mechanisms are considered experimentally.

Index Terms— Insect Flight, Flapping Flight, Flight

Analysis, Wing Morphology Fig. 1 Block diagram of experimental apparatus.

studied by the high-speed video camera system. The I. INTRODUCTION relationships between wing morphology and flight An insect‘s wings have two functions. They provide mechanisms are revealed experimentally. force called lift which overcomes the force of gravity, and they flap round and round like propellers driving the insect forward through the air. Dragonflies can II. EXPERIMENTAL METHOD hover, accelerate in almost any direction, and Experiments for the free flight analysis of some manoeuvre precisely at high speed to prey on other selected insects are conducted with the high-speed insects [1]. video camera system. A schematic diagram of the Extensive investigations on dragonfly flight have experimental apparatus is shown in Fig. 1. The been conducted [2]. Azuma et al. [3,4] observed the experimental system is composed of a high-speed video free flight of dragonflies with a high speed 16mm movie camera, a control unit, a video cassette recorder, a video camera, and theoretically analyzed the flight monitor, and a personal computer. In the experiment, a performance at various speeds. Okamoto et al. [5] used pair of dragonfly wings is cut off with scissors. Then the a variety of techniques, such as force transducer dragonfly with one pair of forewings or hindwings is measurements and gliding models with upside-down released in the air. The free flight images of the wings, to create lift-to-drag polar curves for real dragonfly are captured using the high-speed video dragonfly wings. Wakeling and Ellington studied the camera, and series of captured images are analyzed by aerodynamic force in the gliding flight of the dragonfly the personal computer. In this paper, the tethered flights [6], the velocities, accelerations and kinematics of of other insects are also examined for comparison. Test flapping flight [7], and the aerodynamic power required insects for the flight analysis are dragonfly(Sympetrum for dragonfly flight [8]. Sudo et al. [9] examined the infuscatum), flies (Lispe orientalis and Drosophila wing structure and the aerodynamic characteristics of an hydei), and butterfly (Colias erate). in-flight dragonfly, using a scanning electron microscope and a small low-turbulence wind tunnel. III. EXPERIMENTAL RESULTS AND They further measured the curved surface of the DISCUSSION dragonfly wing with a three-dimensional, curved shape measuring system. In spite of many investigations, there A. Free Flight and Tethered Flapping of Small still remains a wide unexplored domain on the free Flies flight by a pair of dragonfly wings. The details of Flies are insects of the Order Diptera. Flies are relation between wing morphology and flights of a among the most maneuverable of all flying animals and variety of insects are also unknown. generate elaborate flight behaviors under visual control In the present paper, free flight by a pair of wings for [10]. They have just one pair of wings; the hindwings a dragonfly and free flights of some selected insects are

* Corresponding author: Ebinokuchi 84-4, Tuchiya, Yurihonjo, Akita 015-0055, Japan, E-mail:[email protected] – 24 – JOURNAL OF AERO AQUA BIO-MECHANISMS, VOL.1, NO.1

t = 0.00 ms t = 0.22 ms t = 0.44 ms

t = 0.67 ms t = 0.89 ms t = 1.11 ms

t = 1.33 ms t = 1.56 ms t = 1.78 ms

Fig. 3 Wingtip trajectory during fly free flight.

t = 2.00 ms t = 2.22 ms t = 2.44 ms

t = 0.00 ms t = 2.96 ms t = 2.67 ms t = 2.89 ms t = 3.11 ms

Fig.2 Free flight behavior of the fly in frontal view. are reduced to small, clubshaped balancing organs called halteres. Flies perform an extraordinary array of t = 0.74 ms t = 3.70 ms complex aerial maneuvers. Tu and Dickinson used a combination of high speed video and electrophysiological recordings to investigate the relationship between wing kinematics and the firing patterns of the first and second basalar muscles of tethered flying blowflies [11]. In this section, free flight t = 1.48 ms t = 4.44 ms behavior of a small fly and flapping behavior of a tethered larger fly are examined with the high-speed video camera system. Figure 2 shows the high speed photographs of the free flight of the fly, Drasophila hydei. The body length of the fly is L=2.96mm, the wing length is lw=2.74mm t = 2.22 ms t = 5.19 ms 2 and the wing area is Sw=1.93mm . In Fig. 2, the fly is carrying out the hovering flight mostly. In this Fig. 4 Tethered flight behavior of the fly in frontal view. experiment, the fly was released in the air by opening the lid of a small bottle. The fly avoided some obstacles of the wing motion during free flight of the fly. The and flew out of the bottle. As shown in Fig. 2, Dipterous plots in Fig. 3 reveal the wingtip orbits of free flight. In insects can defy the force of gravity and take to the air Fig. 3, the fly shows the slower upward flight. In this by several unique specialization that enable them to case, the flapping frequency of the fly is fi=282.7Hz. detect and respond to moving targets with such rapidity. However, the details of the wing motion during flapping Figure 3 shows an example of the measurement results of the fly in Fig. 3 are not clear. In order to examine the

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Fig. 5 Wingtip trajectory of tethered fly flapping in frontal view.

details of the wing motion, the tethered flight of the fly, Lispe orientalis was studied. Figure 4 shows a sequence Fig. 6 Free flight behavior of the butterfly in frontal view. of photographs showing nearly a complete cycle of the the butterfly is going up by h=46mm (vertical height fly‘s wing flapping. The body length of the test fly is from former position) due to the downstroke wing L=7.05mm, and the wing length is l =5.35mm. The live w motion (t=28.9 – 63.6ms). The elevation is greater than fly was tethered with a fishing line bonded to the the body length, but less than the forewing length abdomen tip. It can be seen that fly wings are deformed (l =51.9mm) of the butterfly. During the upstroke of greatly in the final stage of the power stroke (t=2-3ms in w wing motion, the body pitches nose up and wings Fig. 4). This deformation is restored to normal strongly supinate (t=0 – 28.9ms). It can be seen from condition as a spring in the next stage of flapping. Fig. 6 that the butterfly is going up even in the upstroke Figure 5 shows the wingtip orbit during beating of the wing motion. This fact shows the presence of upward fly. The Cartesian coordinate system was chosen, with inertia. Furthermore, the butterfly moves forward during the origin at the fly head, of the sky (upper) direction as the upstroke of wing motion. Figure 7 shows a sequence the z coordinate axis, and the left wing direction as x of photographs of free flight behavior of the butterfly in coordinate. In Fig. 5, the time interval of data plot is set lateral view. It can be seen from Fig. 7 that the butterfly constant. It can be seen that the orbits of a power stroke is performing the advance flight and the rising flight and a recovery stroke during beating are different. simultaneously by the wing flapping motion. Weis-Fogh Especially, the wing speed described the fling as a rigid rotation of the wing dx2  dz2 surfaces about their trailing edges [14]. The fling found v  (1) in the Lepidoptera and Drosophila is called the peel: the dt wings curve along the chords and the separation point is larger in the recovery stroke. moves smoothly from leading to trailing edges, rather like pulling two pieces of paper by their leading edges B. Flight of Butterfly [12,14]. Figure 7 shows that the ―peel‖ phase [12,14] Butterflies and moths make up the second-largest generates thrust. As the flapping down proceeds, only order of insects, the Lepidoptera. Adult Lepidoptera the veins at the leading edges of the forewings and a feed principally on nectar and other liquid food, and part of the membranes near the veins separate from each many are commonly seen on flowers. Their flight is other, and the rest of the wings maintaining contact due usually rather erratic but fairly fast. to their flexibility. Such wavy motion on the wings may Figure 6 shows a sequence of photographs of free generate the velocity component of flow for behind, i.e., flight behavior of the butterfly, Calias erate Esper. The a thrust is generated. As the wings are clapped together body length of the test butterfly is L=32.8mm. In this dorsally, the wings are closed from the leading edges of experiment, the flapping frequency of the butterfly is forewings to the trailing edges of hind wings. Such fi=15.8Hz. It can be seen from Fig. 6 that the stroke wavy motion on the wings may generate the backward plane is vertical and the wing motion is perpendicular to squeeze flow. This phase of wing flapping also the chord. The downstroke of wing motion of the generates thrust. Two pairs of wings of the butterfly are butterfly generates the vertical force [12,13]. In Fig. 6 flapped like a pair of wings as the combined wings. A

10mm 10mm 10mm t=0ms t=2ms t=4ms t=6ms t =t=0ms 0 ms t =t =2ms2 ms t =t =4ms4 ms t =t =6ms6 ms

t=8ms t=10ms t=12ms t=14ms t =t =8ms8 ms t =t=10ms 10 ms t =t=12ms 12 ms t = t=14ms14 ms

t=16ms t=18ms t=20ms t=22ms t =t=16ms 16 ms t = t=18ms18 ms t =t =20ms20 ms t =t =22ms22 ms

Fig. 8 Free flight of a dragonfly with a pair of hindwings.

Fig. 7 Free flight behavior of the butterfly in side view. kinematic variation of the fling found in the Lepidoptera, called the peel, was described in detail by Fig. 9 Trajectories of head and right forewing of dragonfly during free flight with a pair of wings. Ellington [15]. The circulation  around the exposed chord is described as follow;   u l f   (2) L=mgcos (3) c c D=mgsin  where u is the chord‘s unzip velocity, l is the exposed c c where m is the mass of dragonfly and g is the chord length, f() is given by Ellington [15] gravitational acceleration. And the glide angle is 1  f    2 , and  is the half-angle between the determined as follows; wings. D   tan -1 (5) C. Dragonfly Flight with a Pair of Wings L Dragonflies are extremely versatile fliers. Their performance includes actions like high-speed forward, In the simplest situation of dragonfly flight, to fly hovering, and backward flight. Aerodynamic studies of horizontally at a steady speed, the thrust force T must be dragonfly flight have been conducted. In spite of many provided, equal and opposite to the drag D. To maintain investigations [16], there are no published studies on this level flight, the lift force L equals the weight W, L = free flight by a pair of wings of dragonflies. This study W. The power to generate the lift and thrust is supplied investigates the dragonfly‘s free flight by a pair of by the flight muscles. As the acting point of the above- wings. Fig. 8 shows a sequence of photographs showing mentioned four forces, one point in the dragonfly body the free flight behavior of the dragonfly with a pair of is rational for the stable flight. In general, the dynamic hindwings. A pair of forewings was cut off from a properties of insect body shapes in the earth‘s gravity dragonfly for this experiment. Despite of the loss of field are determined by the relative spatial positions of wings, the dragonfly maintains the balance by raising the geometrical center of the shape and the center of the abdomen. The abdomen of dragonfly plays an gravity of the body [17]. In this case for the dragonfly important role in flight balance. In this case, the with only a pair of hindwings, the center of action of lift moment acts on the dragonfly. This moment induces generated by hindwing flapping shifts backward from head lowering pitching on the dragonfly body. In the original point. Therefore, the dragonfly adjusts the general, during steady gliding of the dragonfly, the lift moment by its abdomen. Of course, the dragonfly (L) and drag (D) forces are maintains the balance of the moment by using aerodynamic drag acting to the abdomen.

Likewise, the free flight of the dragonfly with a pair haltere efferent control pathway is based on a neural of forewings was examined. Figure 9 shows the right architecture that may be common among insects and wingtip and head orbits during free flight of the provides a parsimonious explanation for the evolution dragonfly at the two-dimensional coordinate system x-z. of halters from aerodynamically functional hindwings. Orbits in Fig. 9 show slow flight of the dragonfly. The Probably, wing morphology of the fly described by dragonfly is doing its beat to balance the body in the air. Eq.(7) is needed for the aerodynamic function and the The wingtip trajectory is characterized in the beginning evolution of the haltere efferent control system based on of each power stroke. The negative lift is not generated, a neural architecture. The aspect ratio of the wings of because the dragonfly does not descent. In this the butterfly, Colias erate, is described as experiment, it is confirmed that dragonfly can fly with l 2   w 1.51 (8) only a pair of wings. butterfly S D. Aspect Ratio of Insect Wings where S is the total area of right and left butterfly wings. Butterflies are known as the order Lepidoptera In general, powered flight of insects is performed by (meaning scaly wings). Their wings are covered with using extended wings. The character of insect flight thousands of tiny, overlapping scales. As adult depends upon the structure of its wing apparatus. The butterflies are only able to feed on fluids, flower nectar insect‘s particular flight regime is a function of the is the major source of sustenance for most species. selective value of the flight [2]. This paper examined Plants benefit from these associations, because, while free flight of the dragonfly with only a pair of wings and feeding, butterflies transfer pollen from one plant to free flight of the fly and the butterfly. The wing area of another. Whites and brushfooted butterflies fly slowly these insects was also measured. The aspect ratio of among flowers searching for nectar [2]. Brodsky these insects‘ wings suggests the difference of the flight reported that the butterfly‘s flapping flight can be mechanisms. Denoting the wing area S, the aspect ratio divided into three successive stages: during the of the hindwing of the dragonfly, Sympetrum downstroke, force generation can be explained by infuscatum, is described as 2 quasi-steady aerofoil action; during the upstroke and lw (6) supination, by unsteady aerofoil action; and during dragonfly   3.73 S pronation, by a jet mechanism [20]. Srygley and Dragonflies are powerful fliers, capable of adjusting Thomas showed that free-flying butterflies use a variety their wing beat pattern to suit widely different of unconventional aerodynamic mechanisms to generate maneuvers. Dragonflies hunt for their prey by laying in force: wake capture, two different types of leading-edge wait on a branch or plant sticking out of the water [2]. vortex, active and inactive upstrokes, in addition to the Feeding flights proceed differently in plant-feeders and use of rotational mechanisms and the Weis-Fogh ‗clap- predators. Thomas et al. [18] showed that dragon flies and-fling‘ mechanism [21]. fly by using unsteady aerodynamic mechanisms to In this paper, high-speed images show that wavy generate high-lift, leading-edge vortices. That is, in motion of butterfly‘s wings plays an important role in normal flight, dragonflies use counterstroking lift and thrust generation. Probably, wing morphology kinematics, with a leading-edge vortex on the forewing of the butterfly described by Eq.(8) is needed for the downstroke, attached flow on the forewing upstroke, butterfly flight using wavy motion of wings. and attached flow on the hindwing throughout. The flight mechanism of the dragonfly may be Accelerating dragonflies switch to in-phase wing-beats explained based on the blade-element theory of with highly separated downstroke flows, with a single propellers. The sustaining vertical force in the butterfly leading-edge vortex attached across both the fore-and flight obviously results from the pressure drag on the hindwings. Probably, wing morphology described by wings during the downstroke. The flight mechanism of Eq.(6) is needed for the such powerful flight of the fly is a mixture of both of two above-mentioned dragonflies. mechanisms. The aspect ratio of the wing of the fly, Drosophila hydei, is described as References 2 [1] Rüppell, G. and Hilfert, D., The Flight of the Relict lw (7) fly   2.43 Dragonfly Epiophlebia superstes (Selys) in Comparison S with that of the Modern Odonata (Anisozygoptera: Flies are the most widespread of all insects. 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