© 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb205385. doi:10.1242/jeb.205385

RESEARCH ARTICLE Jumping and take-off in a winged fly (, communis) M. Burrows*

ABSTRACT grasshoppers (Orthoptera, Acrididae) (Bennet-Clark, 1975), flea High-speed videos were used to analyse whether and how adults of a beetles (Coleoptera, Chrysomelidae) (Brackenbury and Wang, 1995; winged species of scorpion fly (Mecoptera, ) jump Nadein and Betz, 2016) and many other hemipteran plant-sucking and determine whether they use the same mechanism as that of the bugs such as froghoppers (Hemiptera, Cercopidae) (Burrows, 2006). only other mecopteran known to jump, the wingless snow flea, Such extreme movements require specialisations of the body. For hyemalis. Adult females are longer and heavier than males and have example, mechanical, interacting gears may synchronise movements longer legs, but of the same relative proportions. The middle legs are of the propulsive legs to within 30 µs of each other so that thrust can 20% longer and the hind legs 60% longer than the front legs. A jump be generated in a particular direction and energy is not lost to rotation starts with the middle and hind legs in variable positions, but together, of the body (Burrows and Sutton, 2013). by depressing their coxo-trochanteral and extending their femoro-tibial The second method uses direct contractions of muscles to power joints, they accelerate the body in 16–19 ms to mean take-off velocities the propulsive leg movements. The acceleration times of these of 0.7–0.8 m s−1; performances in males and females were not jumps are usually longer and the final take-off velocities are significantly different. Depression of the wings accompanies these leg generally lower. Many using this mechanism propel movements, but clipping them does not affect jump performance. themselves with elongated hind legs, as exemplified by bush Smooth transition to flapping flight occurs once airborne with little loss of crickets (Orthoptera, Tettigoniidae), which can have propulsive energy to body rotation. Ninety percent of the jumps analysed occurred hind legs that are four times longer than the front legs (Burrows and without an observable stimulus; the remaining 10% were in response Morris, 2003). The leverage provided by these long legs is offset by to a mechanical touch. The performance of these jumps was not the longer time (30 ms) it takes to extend them fully. Many true flies significantly different. In its fastest jumps, a scorpion fly experiences an (Diptera), however, use their middle legs (Card, 2012), which are no acceleration of 10 g, expends 23 µJ of energy and requires a power longer than the other two pairs of legs. For some insects, a further output less than 250 W kg−1 of muscle that can be met by direct muscle variation to this mechanism is to use the middle and hind pairs of contractions without invoking an indirect power amplification legs together; examples are lacewings (Neuroptera, Chrysopidae) mechanism. The jumping mechanism is like that of snow fleas. (Burrows and Dorosenko, 2014), caddis flies (Trichoptera) (Burrows and Dorosenko, 2015b), moths (Lepidoptera) (Burrows KEY WORDS: Locomotion, Flying, High-speed imaging, and Dorosenko, 2015a), praying mantises (Mantodea, Mantidae) Escape movements (Sutton et al., 2016) and ants (Hymenoptera) (Baroni Urbani et al., 1994; Tautz et al., 1994). Effects of this are to distribute the forces INTRODUCTION applied to the substrate over a larger area and to increase muscle To jump from the ground or a plant, many insects, with a few notable mass to reduce the power requirements per unit of muscle. The exceptions, rely on propulsion by the legs, sometimes with the synchronisation between the leg movements does not have to be assistance of wing movements. Two basic strategies provide leg closely controlled, so that in different jumps one pair of legs may propulsion. The first is a catapult method that generates some of the move first and one pair may leave the ground before the other. In fastest leg movements in any and some of the fastest take-off some winged insects, propulsive leg movements can also be velocities. This requires mechanical specialisations to enable the accompanied by the start of flapping movements of the wings, for energy that is built up by slow muscle contractions to be first stored example, in moths (Burrows and Dorosenko, 2015a), and even if without moving the legs, usually in distortions of the skeleton, and the wings are not moved, their shape can influence stability of the then released suddenly to power the rapid movements of the legs. The trajectory in whiteflies (Ribak et al., 2016). power (energy/time) of the muscles is thereby amplified greatly. In Flightless scorpion snow fleas (order Mecoptera, family Boreidae) planthoppers (Hemiptera, Fulgoridae), some of the best exponents of use strategies that incorporate elements of both catapult and direct this strategy, the slow preparatory phase of a jump, is followed by a contraction mechanisms to produce a jumping performance that short acceleration time, often less than 1 ms, in which rapid lies at the boundary between the two; acceleration times are movements of the hind legs generate propulsion. The accelerations approximately 7 ms and take-off velocities are 0.7 to 0.8 m s−1. can exceed 500 g and the final take-off velocity can reach 5 m s−1 Jumps are propelled by the combined movements of the middle and (Burrows, 2009). Other insects that use this mechanism are hind pairs of legs with three factors indicating that there must also be some power amplification by storage of muscle energy (Burrows, Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, England. 2011). First, the power required for the fastest jumps reaches and may exceed the maximal contractile limits of muscle (Askew and Marsh, *Author for correspondence ([email protected]) 2002; Ellington, 1985; Josephson, 1993; Weis-Fogh and Alexander, M.B., 0000-0003-1386-5065 1977). Second, performance is not affected by temperatures of −3°C to +3°C in their natural habitat when jumping on snow, as would be

Received 15 April 2019; Accepted 14 August 2019 expected were direct contractions of the muscles alone involved Journal of Experimental Biology

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(Burrows, 2011). Third, a thoracic region containing the elastic protein resilin and where energy could be stored is associated with A Panorpa communis, male each of the four propulsive legs (Burrows, 2011). This small family (Boreidae) contains only about 30 of the 600 total species in the order Mecoptera (Whiting, 2002), and Whiting indicates that ‘boreids are unique among Mecoptera in their ability to jump up to 30 cm’. To test the hypothesis that jumping may be present more widely in this order of insects and that the same mechanism is used, analysis was undertaken of a larger, winged species, the scorpion fly Panorpa communis, from a genus that itself contains almost half of mecopteran species. Hasken (1939), in his detailed anatomical studies of the skeleton and musculature of this scorpion fly, gained the impression that it could move from the ground in what he described as a clumsy flutter involving the participation of the hind and middle legs (Hasken, 1939). When he removed the wings, a 2 mm clearer propulsive contribution from the legs became apparent. Can B Panorpa communis, female these scorpion flies jump, and if so, are there any common features with scorpion snow fleas in the mechanism that they use and the performance that they generate?

MATERIALS AND METHODS Adult, winged scorpion flies Panorpis communis Linnaeus 1758 were collected during the summers of 2013–2019 from hedgerows and from patches of stinging nettles () or bramble (Rubus fruticosus) in the countryside around Cambridge, UK. They are omnivorous, feeding on pollen and fruit, and are scavengers of 2 mm dead insects and rotting fruit. They belong to the order Mecoptera Dorsal C Panorpa communis, Posterior Anterior and the family . Fossil representatives of these groups male date to the Permian, 250 million years ago. Ventral Photographs of live P. communis (Fig. 1A,B) were taken with a Nikon D7200 camera fitted with a 105 mm Nikon macro lens. The lengths of the legs and of the body were examined in live insects, Coxa and in those fixed in 70% alcohol. Images were captured with a Trochanter GXCAM-5C digital camera (GT Vision Ltd, Stansfield, Suffolk, Coxa Trochanter UK) attached to a Leica MZ16 microscope (Wetzlar, Germany) and Coxa Femur Femur projected onto a large monitor. Leg and body lengths were then Trochanter measured from these images to an accuracy of 0.1 mm with a ruler (Table 1). Body masses were determined to within 0.1 mg with a Femur Mettler Toledo AB104 balance (Beaumont Leys, Leicester, UK). P. communis The body shape of male and female is characterised by Tibia Tibia a head with a large ventrally pointing beak-like rostrum with biting mouthparts at the distal end and two pairs of similarly-sized wings Tibia that project beyond the tip of the abdomen (Fig. 1A,B). Males also Tarsus have an upwardly curved posterior region of the abdomen that ends in a prominently enlarged distal part that is red in colour (Fig. 1A) Tarsus and contains a curved, terminal pair of claspers pointing anteriorly Hind leg Middle leg Front leg that are used to hold females during copulation, which can be prolonged. This structure resembles in appearance, but not in action, Tarsus the stinging apparatus of a scorpion, hence the common name given to members of the family Panorpidae. Sequential images of jumps were captured at a rate of 1000 s−1 and 1 mm an exposure time of 0.2 ms, with a Photron Fastcam SA3 high-speed camera [Photron (Europe) Ltd, West Wycombe, Buckinghamshire, Fig. 1. Form of the body and legs of Panorpa communis. UK] fitted with a 100 mm macro f/2.8 Tokina lens. Images were fed (A,B) Photographs of an adult male (A) and an adult female (B) in side view. directly to a computer. The insects were free to jump in a chamber (C) Drawing of the head, thorax and the right legs of an adult male viewed from the side. with an internal width of 80 mm, height of 80 mm and depth of 25 mm. The front wall was made of optical quality glass, the floor, side walls and ceiling of 12 mm thick, closed-cell foam (Plastazote, then it fell outside the period of analysis and was thus not part of Watkins and Doncaster, Leominster, UK). If an bumped into measured performance. Jumps could occur from any of these the side of the chamber during take-off, then that jump was excluded surfaces, but only those from the floor that occurred without any from the analysis of performance. If contact occurred after take-off, apparent stimulus or followed touching with a 100 µm diameter silver Journal of Experimental Biology

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wire were used to determine jumping performance. Some jumps occurred from the front glass wall and were thus seen from

(mg) underneath, allowing good views of angular changes of some key

1/3 leg joints. Such jumps were excluded from analyses of jumping performance, either because the propulsive legs slipped on the glass surface or because the forward movement of the insect could not be

Hind leg length (mm)/ body mass measured. All other jumps in which there was any indication that the propulsive legs slipped were also excluded. The camera pointed at the centre of a chamber. Most jumps were constrained by the shape of the chamber and were in the image plane of the camera. If the direction of the jump deviated from the image plane by more than 30 deg, then the error was obvious as changes in focus of the insect during take-off. Such videos were excluded from

Hind leg length as % of body length analysis. If the deviation was less than 30 deg, then calculations from trigonometry indicated that measures of angle or length would contain a maximum error of 10%. A further check by comparing real lengths against those recorded by the camera showed that distortions did not exceed this level in videos that were analysed. Tracks of the movements of specific body parts were made manually frame by frame with Tracker software (http://physlets.org/tracker/); auto- tracking was not used. The frame at which a particular leg lost contact with the ground was determined by playing the video backwards and forwards frame by frame. A shift in position of a tarsus indicated that the leg was no longer load-bearing and had therefore lost contact with the ground. Take-off occurred when the last leg lost contact with the ground and was designated as time (t)=0 ms. The Whole (mm) Front Middle Hind acceleration time was defined as the period from the first detectable movement of the propulsive legs until take-off. Peak velocity was calculated from the distance moved in a rolling three-point average of successive images before take-off. A point on the body that could be Tibia (mm)

indicates the number of individuals from which the measurements were taken. The ratio of leg lengths is recognized in successive frames and was close to the centre of mass N

. (estimated by balancing the insect on a pin) was used for measurements of the trajectory. The angle subtended by a line joining these positions after take-off, relative to the natural horizontal, Femur (mm) gave the trajectory angle. The body angle at take-off was defined as the

P. communis angle subtended by the longitudinal axis of the body relative to the natural horizontal. The results are based on high-speed videos of 205 jumps by 22 P. communis (10 males and 12 females) at temperatures Whole (mm) of 25–28°C. At least three jumps were analysed for each individual. Measurements are given as means±s.e.m. unless otherwise stated for an individual and as mean of means (grand means) for a particular sex. Tibia (mm) Middle leg Hind leg Ratio of leg lengths RESULTS Shape of body and legs Femaleshadameanbodymassof39.7±11.3mg(N=10) and thus were significantly heavier (t-test: t18=3.253, P=0.004) than males, Femur (mm) which had a mean mass of 26.7±1.8 mg (N=10). Females had a mean body length of 15.1±1.1 mm (N=10) that was not significantly =7) 3.5±0.3 3.8±0.3 11.5±0.9 3.9±0.4 5.3±0.6 14.1±1.1 1 1.3 1.6 98 4.3 =7) 2.6±0.3 2.8±0.3 8.5±0.7 3.2±0.3 4.3±0.3 11.6±0.8different 1 (t 1.2=1.242, 1.6P 89=0.238) from that 4.0 of males, which had a mean N N 18 body length to the tip of the abdomen of 13.2±1.1 mm (N=10). The mesothorax and metathorax formed a rigid structure supporting the middle and hind legs; the prothorax supporting the Body length (mm) front legs was separately articulated (Hasken, 1939) (Fig. 1C). The three pairs of legs moved in two parallel planes on either side of =10) 15.1±1.1 ( Panorpa communis

=10) 13.2±1.1 ( the body. A front leg of females, from the trochanter to the tarsus, N N was the shortest at 9.1±0.5 mm, a middle leg was 11.5±0.9 mm and a hind leg was the longest at 14.1±1.1 mm (N=7) (Table 1). Comparable values in males were: front leg 7.3±0.5 mm, middle leg 8.5±0.7 mm and hind leg 11.6±0.8 mm (N=7). Relative to the front legs, this gave a ratio of mean leg lengths in females of 1:1.3:1.6 (front:middle:hind) and in males of 1:1.2:1.6. In females, the length Journal of Experimental Biology FemaleMale 39.7±11.3 ( 26.7±1.8 ( Sex Body mass (mg) Table 1. Body form of Body length and mass, and lengths of the middle and hind femora and tibiae (±s.e.m.) in given relative to the front legs. of the front legs represented 61% of the body length, the middle legs

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77% and the hind legs 98% (N=7). In males (N=7), the comparable P=6×10−6), and there was also a strong difference between males −4 values were 56, 65 and 89% of body length. Front, middle and hind and females (F1,36=13.80, P=6.9×10 ), with the longer and heavier legs differed significantly in length (ANOVA, F2,36=17.05, females having longer legs than males. The lack of a significant

Panorpa communis, –19 ms –7 ms female

Front legs off ground LH RH RF RF RM RH

–18 ms –4 ms

First movement of legs

Middle legs off ground LH

LM LF –15 ms 0 ms Take-off

LH RM RF

RH LM RM

–12 ms +3 ms

LM

RH RF LM RM

–9 ms +5 ms

RH LH LM RM LH RH 5 mm RM LF

Fig. 2. Jump by a female P. communis from the horizontal and viewed from the side. Images from a high-speed video of a jump were captured at 1000 s−1 with an exposure time of 0.2 ms. The images are arranged in two columns with their timing given relative to take-off at time=0 ms. In this and Figs 1C, 3, 4A, 6 and 7, the front legs (LF, left front; RF, right front) are indicated by arrows with yellow heads, the middle legs (LM, left middle; RM, right middle) by arrows with white heads and the hind legs (LH, left hind; RH, right hind) by arrows with pink heads. The triangles in the bottom left hand corners of each image indicate a constant spatial reference point. Journal of Experimental Biology

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sex×leg interaction (F2, 36=0.32, P=0.727) indicates that the relative and hind pairs of legs, accompanied by depression of both pairs proportions of the legs remain the same in both sexes. The hind and wings. The following initial analysis is of jumping in females middle coxae of both sexes were broader than the more distal (Figs 2–5, Movies 1 and 2) and the similarity of the mechanism used segments of the legs and their articulation with the thorax allowed by males is then analysed and illustrated (Figs 6 and 7, Movie 3). Of only small forward and backward rotations. The increased length of the 205 jumps analysed, 89.8% (184 jumps) occurred without any a hind leg was due to the femur, tibia and tarsus that were, observable stimulus in the experimental chamber while just 10.2% respectively, 11, 40 and 35% longer than the equivalent parts of a (21 jumps) followed a touch to an antenna, a wing tip or a hind leg middle leg. The femora and tibia of all the legs were tubes of similar with a 100 µm diameter silver wire. The place on the insect that was diameter along their length and none were swollen to accommodate touched resulted in a forward jump and did not influence the a greater volume of muscle that could make an increased direction of a jump. contribution to jumping. In preparation for a jump, the wings started to open 20–25 ms before take-off, followed by slow upwards and forwards movements Kinematics of jumping of the middle and hind pairs of legs generated by levation of their Both male and female P. communis used the same, single strategy coxo-trochanteral joints, and flexion of their femoro-tibial joints for propelling take-off that involved fast movements of the middle (Figs 2 and 3). In different jumps by the same or a different scorpion

Fig. 3. Jump by female P. communis Panorpa communis, female –14 ms –2 ms viewed from underneath. Images of a jump RM RM from the front glass surface of the chamber First movement RH captured at 1000 s−1 with an exposure time RH of legs of 0.2 ms are arranged in two columns. Preceding take-off, the hind and middle femora are progressively depressed, the hind and middle tibiae are progressively extended RF LH and the four wings are depressed. LH

LM LF

–5 ms 0 ms RM RH Take-off

RF LH

LM LF

–4 ms +5 ms RM

RH

LH

LM

5 mm Journal of Experimental Biology

5 RESEARCH ARTICLE Journal of Experimental Biology (2019) 222, jeb205385. doi:10.1242/jeb.205385 fly, these legs did not move to a constant position in advance of jumps by the same or other individuals, the start of the wing their propulsive movements. For example, a trochanter was not movements was not tightly coupled to the movements of the legs; necessarily fully levated about its coxa and a tibia was not always the wings could begin to move a few milliseconds before the legs, or fully flexed about its femur as observed either from the side (Fig. 2, at the same time (Figs 2 and 4). The leg movements started at a mean Movie 1) or from underneath (Fig. 3, Movie 2). To quantify this of 16.1±0.9 ms (N=10 females) and 19.0±1.0 ms (N=10 males) variability, the angles adopted by a hind femur about the body and before take-off (Table 2). The time between these first propulsive the tibia about the femur were measured just before these joints leg movements and take-off defined the acceleration phase (take-off began to move in the propulsive movements of 109 jumps by 10 time) of a jump. There was no significant difference in these times males and 10 females. The mean±s.d. coxo-trochanteral (body/ between males and females (t-test: t19=−1.602, P=0.126). The rapid femur) angle was 34.1±20.1 deg (range 4–90 deg) and the mean depression and extension of these joints resulted in a progressive femoro-tibial angle was 68.4±25.9 deg (range 12−131 deg). From straightening of the two pairs of legs that raised the body from the these variable initial positions, the start of the acceleration phase of a ground and propelled it forwards (Fig. 4). In different jumps, the jump was marked by increasingly rapid movements of the hind peak angular rotation of these joints ranged from 13,000 to femora and tibiae and by movements of the same joints of the 30,000 deg s−1 (Fig. 5). In the majority of jumps, the movements of middle legs, accompanied by a depression of the wings. In different the middle and hind legs tracked each other closely (Fig. 5A), but for

Fig. 4. Tracks of the movements of the A legs, wings and head during a jump Right front tarsus by a female P. communis. (A) Tracks of positions on the x- and y-axes of particular Right middle tarsus body parts (listed with their symbols in Right hind tarsus the key) during the jump shown in Fig. 2. Head These tracked points are superimposed on the image of the insect at take-off. Right front wing tip The larger symbol on each trace indicates the position of that body part at take-off. Right front (B) The distances moved by the same wing tip body parts on the y-axis are plotted against Head time. The tarsi of the legs did not move until they lost contact with the ground. Take-off time (0 ms) is indicated by the vertical yellow line.

Right hind tarsus Right middle Right front tarsus 5 mm tarsus

B 10 Take-off

Elevate

Wings start 5 to open Depress Distance (mm)

0 –30 –20 –10 0 10 20 30

Time (ms) Journal of Experimental Biology

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Fig. 5. Changes in joint angles of the propulsive 180 A legs during a jump by a female P. communis. (A) Plot of the angular changes in the coxo- 160 Joint trochanteral and femoro-tibial joints of the hind (pink lines and filled symbols) and middle legs (black lines Middle coxo-trochanteral and open symbols) in a jump viewed from the side. 140 Middle femoro-tibial The middle legs lost contact with the ground 3 ms Hind coxo-trochanteral (vertical grey line) before take-off at time t=0 ms 120 Hind femoro-tibial (vertical yellow line). (B) A jump by another female viewed from underneath to show the angular changes of the hind leg preceding those of the 100 middle leg. The inset shows the angles measured and the symbols designating these angles in both 80 parts of the figure.

60

40

20 Middle legs Take-off off ground 0 –20 –15 –10 –5 0 5

180 B Joint angle (deg) Right middle leg Right hind leg Tarsus 160 Tibia

Femur 140 Left hind leg Tarsus

Tibia Femur 120 Left middle leg

100

80 Take-off

60 –20 –15 –10 –5 0 5 Time (ms) jumps in which the angular velocities were lower, the movements of preparation for a jump, the wings were first opened and the middle the middle legs were of lower amplitude and were less closely and hind legs were levated at the coxo-trochanteral joints and coupled to those of the hind legs (Fig. 5B). The front legs showed no flexed at the femoro-tibial joints. These movements again did not movements that were consistent with a contribution to the forward bring the middle and hind legs into a constant starting position for and upward propulsion of the body, and could leave the ground as each jump. Depression of the wings and propulsive depression and early as 7 ms before take-off, and thus could not make any extension movements of the middle and hind legs then began contribution to propulsion during the latter part of the acceleration within a few milliseconds of each other (Fig. 6, Movie 3). The phase. The middle legs were the next to lose contact with the ground joints of the hind and middle legs progressively straightened and some 4 ms before take-off (Figs 2 and 4B). The final propulsion was the wings depressed to power the forward and upward movement thus provided by the hind legs and by the wings, which completed of thebody to takeoff. Thestraighteningof thehind legs was, asin their first cycle of depression before the insect became airborne. In females, more clearly seen when viewed from underneath (Fig. 7). some jumps, the wings had begun the first elevation phase of the The middle legs did not straighten as much. The front legs were the wing beat cycle when take-off occurred (Figs 2 and 4). first to lose contact with the ground, followed by the middle legs, Males used essentially the same pattern of leg and wing and finally the hind legs provided the last contribution to movements to propel take-off (Figs 6 and 7, Movie 3). In propulsion. Journal of Experimental Biology

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Fig. 6. Jump by a male P. communis Panorpa communis, male –19 ms –3 ms from the horizontal and viewed from the side. Images were captured at 1000 s−1 with an exposure time of First movement 0.2 ms and are arranged in two columns of legs to show the propulsive movements of the hind and middle legs. Take-off occurred during wing depression.

LH RF RH RF RH RM LM RM –14 ms –2 ms

LF

–10 ms 0 ms Take-off

RF RH RM

LF +5 ms –6 ms

RF LH LM RM

LH +15 ms RH RF LM RM –5 ms

RH

5 mm

In neither females nor males were any take-offs observed in Jumping performance which leg movements alone, or wing movements alone, were the Measurements taken from the high-speed videos and calculations sole source of propulsion; all take-offs involved the combined based on these data enabled the jumping performance of the actions of legs and wings. scorpion flies to be determined (Table 2). Journal of Experimental Biology

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jumps in 12 (six males and six females) evoked by a Panorpa communis, –14 ms –6 ms male mechanical stimulus were individually compared with 12 jumps by the same insects that occurred without any detectable stimulus First movement ‘ ’ of legs RF ( spontaneous ). Neither the acceleration time to take-off (touch- ‘ ’ t RM evoked 18.1±1.2 ms, spontaneous 18.9±1.4 ms, paired -test, LF t11=0.44, P=0.670) nor the take-off velocity (mean touch-evoked −1 −1 LM 0.80±0.04 m s ,mean‘spontaneous’ 0.78±0.04 m s ,pairedt-test, t11=2.2, P=0.44) were significantly different. The mean body angle (measured as the angle of the longitudinal axis of the thorax relative to the natural horizontal) of females at RH RH take-off was 30.8±4.8 deg (N=10 insects) and of males was 35.4± LH LH 4.1 deg (N=10). There was no significant difference in this angle –12 ms –4 ms between males and females (t-test: t19=−0.818, P=0.423). The mean trajectory angle during the first 5 ms when females were airborne was 51.5±5.6 deg (N=10 insects) and in males was 57.6±4.2 deg (N=10). There was no significant difference in the trajectories of males and females (t-test: t19=−1.017, P=0.322). To determine the stability of the insect once airborne, changes in the longitudinal axis of the body in the pitch plane were measured in 63 jumps by 10 females and 10 males. Five milliseconds after take-off, the mean change of the pitch angle since take-off was 2.1±0.39 deg with no change detectable in 41 of those jumps and the maximum change being 10 deg. If this mean angular change were to continue longer into the trajectory it would lead to a spin rotation rate of –10 ms –2 ms 1.2 Hz, but this was not observed because during this time there was a smooth transition to flapping flight. The action of the four legs and RF the first downward movement of the wings thus provided the initial RM propulsion with a small pitch rotation of the body, but once the legs were clear of the ground, the subsequent flapping movements of the wings provided stability so that little energy was dissipated in rotating the body. The potential contribution of wing movement to take-off was analysed by measuring the times to take-off and the take-off velocities RH in four jumps by each of five individual scorpion flies (three females and two males) both before and after the wings had been reduced in area by 25 to 50%. This reduced the mass of the insect by a mean –8 ms 0 ms of 5.8±0.8 mg (range 3.1 to 8.4 mg). The mean time to take-off was only slightly longer when the wings were clipped (mean 19.8± Take-off 0.8 ms) compared with when the wings were intact (19.5±0.8 ms), and this difference was not significant (repeated-measures ANOVA, F1,20=0.08, P=0.792). The mean take-off velocity was slightly lower in scorpion flies with clipped wings (0.69±0.06 m s−1) compared with when they were intact (0.73±0.06 m s−1), but this difference was again not significantly different (repeated-measures ANOVA, F1,20=0.21, P=0.679). The energy and power of the jumps was calculated from the RH RH 5 mm LH LH means of the measured data. The energy required for a mean jump was 13.2 µJ in females and 7.1 µJ in males. The mean power was Fig. 7. Jump by a male P. communis viewed from underneath. Images of a 0.8 mW in females and 0.4 mW in males (Table 2). jump from the front glass surface of the chamber were captured at 1000 s−1 The mass of leg muscle that is used in jumping is assumed to be with an exposure time of 0.2 ms. They are arranged in two columns to show the 20% of total body mass because two pairs of legs are used, and thus it progressive propulsive movements of the hind legs contributing to take-off. is twice the value of the measured mass of muscle used by insects such as froghoppers (Burrows, 2007) or planthoppers (Burrows et al., The mean take-off velocity of females was 0.82±0.04 m s−1 (best 2014), which are propelled by just the hind legs. The power per 1.05 m s−1; N=10 insects) and of males was 0.73±0.03 m s−1 (best kilogram of muscle needed to generate the mean take-off velocity in 1.05 m s−1; N=10). There was no significant difference in these females was 103 W kg−1 of muscle and in males was 70 W kg−1 of mean take-off velocities between males and females (t-test: muscle. In the jumps with the fastest take-off velocities, labelled as t19=1.782, P=0.091). best jumps by an individual (Table 2), these values rose To determine whether there was a difference in either acceleration to 250 W kg−1 of muscle in females and 170 W kg−1 of muscle in time or take-off velocity between jumps that were evoked by a males. The mass of muscles involved in take-off will also be mechanical stimulus and those which occurred without any increased by any participation of the wings, so further distributing the observable stimulus, the following test was performed. Twelve power demands and lowering the figures given above. Journal of Experimental Biology

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Table 2. Comparative jumping performance of the scorpion fly P. communis Time to Take-off Body angle Power kg–1 Body mass take-off velocity Take-off angle at take-off Acceleration g force Energy Power Force muscle Symbols and formulae mtv f=v/t g=f/9.81 E=0.5mv2 =E/t =mf =power/(0.2 m) Units mg ms m s−1 deg deg m s−2 G µJ mW mN W kg−1 P. communis, female Average (N=10) 39.7±11.3 16.1±0.9 0.82±0.04 51.5±5.6 30.8±4.8 50.5 5 13.2 0.8 2 103 Best 42.5 11 1.05 66.0 17.2 95.5 10 23.4 2.1 4.1 250 P. communis, male Average (N=10) 26.7±1.8 19.0±1.0 0.73±0.03 57.6±4.2 35.4±4.1 38.6 4 7.1 0.4 1.0 70 Best 31.0 16.0 1.05 65.6 54.9 65.6 7 17.1 1.1 2.0 170 =power/(0.1 m) Boreus hyemalis, female Average (N=9) 4.2±0.37 6.6±0.17 0.7±0.04 52.2±4.55 6.2±2.33 106 11 1.1 0.16 0.46 430 Best 3.5 7.4 0.9 58 27 115 12 1.3 0.2 0.4 450 Boreus hyemalis, male Average (N=10) 2.9±0.28 6.6±0.33 0.8±0.21 39.6±2.14 14.0±2.58 121 12 0.9 0.14 0.33 430 Best 2.2 6.2 1 50 14 161 16 1.1 0.18 0.35 740 Archaeopsyllus erinacei (hedgehog flea) Average (N=10) 0.7±0.16 1.4±0.25 1.3±0.21 39±5.7 23±8.7 960 98 0.6 0.4 0.67 6000 Best 1 2 1.9 45 39 1600 160 1.8 1.5 1.6 14,000 Data in columns 2–6 are the grand means (±s.e.m.) for the measured jumping performance of 10 female and 10 male P. communis; the best performance as defined by the fastest take-off velocity of a particular individual is also given. The values in columns 7–12 on the right are calculated from the means of the measured data. N, number of individuals of each gender or species that were analysed. The comparative data on snow fleas (Boreus hyemalis) are from Burrows (2011) and on hedgehog fleas (Siphonaptera, Archaeopsyllus erinacei) from Sutton and Burrows (2011).

DISCUSSION requirements for a jump assumed that the muscles of the two pairs of The high-speed videos show that scorpion flies are able jumpers legs used in propelling the jump comprised 20% of body mass, that use a repeatable pattern of propulsive middle and hind leg extrapolating from measurements (Burrows, 2007; Burrows et al., movements and wing movements in individuals of both sexes. In 2014) made in insects that use just one pair of legs for propulsion. The preparation for a jump, the middle and hind legs were moved calculated power requirements fall well within the known maximal forwards and upwards by levation at their coxo-trochanteral and contractile limits of muscle (Askew and Marsh, 2002; Ellington, extension at their femoro-tibial joints. The starting angles of these 1985; Josephson, 1993; Weis-Fogh and Alexander, 1977). The leg joints were not always the same in each jump. In the propulsive conclusion is that an explanation of jumping performance does not acceleration phase of all jumps, the middle and hind pairs of legs need to invoke power amplification through a mechanical energy were rapidly depressed and extended whilst the wings were storage mechanism. This does not, however, rule out the possibility depressing at the point of take-off. These movements propelled an that some energy generated by the muscles may be stored and then insect in its best jumps to take-off velocities of just over 1 m s−1. contributes to the jump. The power requirements for jumping in The jumps suggest that they might serve to move the insect quickly scorpion flies are much lower than those of insects that use a away from danger posed by a potential predator such as a bird or catapult; for example, in a siphonapteran insect, the hedgehog flea, wasp, or to launch into stable flight to move to another site. the power requirements can reach 14,000 W kg−1 of muscle, almost 28 times larger (Table 2) (Sutton and Burrows, 2011). What jumping mechanism is used? Three lines of evidence indicate that the jumping method used by What is the contribution of wing movements? scorpion flies relies on direct contraction of muscles rather than a The kinematics indicate that take-off is associated with rapid catapult mechanism. First, in preparation for a jump, the joints of the propulsive movements of the middle and hind pairs of legs and the propulsive legs do not adopt a constant starting position. This indicates first cycle of depression by the four wings. Can the relative that they are not moved to a position that favours a particular ratio of contribution of the legs and the wings be assessed? A resolution of lever arms of the muscles that are moving a particular joint. Similarly, this was attempted by clipping the wings of a group of scorpion flies the legs are apparently not engaging a mechanical locking mechanism and measuring jump performance before and after this procedure. that would restrain a joint and allow energy to be stored; there is no No significant difference was found in the acceleration time or the evidence of a ‘latch’ asdiscussedbyIltonetal.(2018).Atleastoneof take-off velocity of jumps before or after this procedure, implying these requirements would need to be met if a catapult mechanism was that the wings add little to the thrust generated by the two pairs of to engage. The contrast is therefore strong between the variable legs. Reducing the area of the wing in this way will also reduce the starting positions of these leg joints of the scorpion fly, using direct volume of air that can be moved by the wings, an effect that may muscle contractions, compared with insects such as grasshoppers be offset by an approximately 9% reduction of body mass in (Bennet-Clark, 1975) or froghoppers (Burrows, 2006), which use a these particular experiments. Overall, the conclusion that can be catapult mechanism where the starting positions have always to be the drawn is that the movements of the middle and hind legs are same for loading the catapult. Second, the long duration of the the main contributors to propulsion of take-off while acceleration phase suggests that the propulsive leg movements are not the accompanying depression of the wings contributes to the the result of the recoil of a spring loaded by the preceding storage of smooth transition to flapping flight and thus stability to the energy in a catapult mechanism. Third, calculation of the power take-off trajectory. Journal of Experimental Biology

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Features in common with jumping snow fleas mantises (Mantodea, Mantidae) (Sutton et al., 2016), mirid bugs How does the mechanism used by scorpion flies compare with that (Hemiptera, Miridae) (Burrows and Dorosenko, 2017), caddis flies used by the only other mecopteran known to jump, the snow flea, (Trichoptera) (Burrows and Dorosenko, 2015b), moths (Lepidoptera) Boreus hyemalis (Burrows, 2011)? Both propel their jumps by rapid (Burrows and Dorosenko, 2015a), true flies (Diptera) (Card, 2012), a trochanteral depression and tibial extension movements of their few ants (Hymenoptera) (Baroni Urbani et al., 1994; Tautz et al., middle and hind pairs of legs. In P. communis, the hind legs often 1994) and some stick insects (Phasmida, Heteronemiidae) (Burrows moved before the middle legs whereas in snow fleas there was no and Morris, 2002), and among arachnids, a few spiders (Parry and discernible pattern as to which pair of legs moved first. The power Brown, 1959; Weihmann et al., 2010). The propulsive mechanisms in for the depression of the middle and hind trochantera is provided in the insects above generate similar jumping performances to those of both species by large trochanteral depressor muscles in the the scorpion fly; the take-off velocities all fall below 1 m s−1 while mesothorax and metathorax so that the more distal segments of acceleration times range from 6 to 100 ms. In comparison with insects the legs can be thin and light with small muscles, allowing their using a catapult mechanism, the take-off velocities are lower and the more rapid acceleration. Boreus hyemalis does not have wings that acceleration times are longer. Of the insects that use four legs to jump, can be moved so that they cannot provide further thrust to aid take- the only catapult mechanism suggested is in the snow flea off. Both scorpion flies and snow fleas achieve best take-off (B. hyemalis), which has a maximum take-off velocity of 1.3 m s−1 velocities of 1 m s−1 and the same mean velocities for all jumps of (Burrows, 2011) notably slower than most other catapult jumping 0.8 m s−1 in males and 0.7 m s−1 in females. The initial trajectory of insects. The fact that the mecopterans P. communis and B. hyemalis scorpion flies in the air propelled from a four-legged base imparted a take off with relatively slow pitch rates of their body suggests the small pitch rotation of 2 deg in the first 5 ms (400 deg s−1) after take- testable hypothesis that jumps by other insects propelled by four legs off that was then stabilized by the flapping movements of the wings will also be more stable. when airborne. In B. hyemalis without the stabilising addition of These data indicate two distinct categories of jumping insects with wing movements, the pitch rotation increases to 10−15 deg during minimal overlap and suggest responses to different evolutionary the initial 6–7 ms (1900 deg s−1) once airborne (Burrows, 2011). pressures. A faster catapult mechanism is offset by the expense of Both mecopteran examples, however, contrast with the jump of the building, maintaining and operating more specialized mechanisms wingless hedgehog flea (Siphonaptera), which is propelled only by that may also impinge on the many other functions that legs have to the two hind legs and in which rotation of the body occurs at perform. A slower jumping mechanism involving the use of two pairs 10,000 deg s−1 (Sutton and Burrows, 2011). Taken together, this is of propulsive legs runs the higher risk of predation, but this is offset further evidence that the use of four legs for jump propulsion by a more stable launch into flapping flight. The similarities revealed provides a more stable base for take-off. here between the jumping mechanism in a representative of the large Although jump performance of both insects is similar in terms family Panorpidae and the smaller family Boreidae adds a further of the take-off velocities, the acceleration time of P. communis is characteristic that can be used in the continuing discussion about the 16–19 ms and is thus almost three times longer than the 6.6 ms in phylogeny of the Mecoptera. B. hyemalis (Table 2). This may reflect the need to lift and accelerate their 10-times-greater mass, and to extend and accelerate their legs, Acknowledgements I thank Roger Northfield. Gabriel Jamie, Peter Lawrence and Pat Simpson for which are three times longer than those of the snow flea. The biggest help in collecting the insects used in this study. Steve Rogers helped with the difference between these two insects is in the calculated power statistics. I am also grateful to University of Cambridge colleagues for their requirements for jumping and the implications that follow. In snow encouragement during the experimental work and for their comments on earlier fleas, the fastest jumps require more power than could be delivered by drafts of the manuscript. direct muscle contractions. This has led to a suggested power- Competing interests amplification mechanism in each of the middle and hind legs that The author declares no competing or financial interests. could store energy generated by the contractions of their respective trochanteral depressor muscles. Supporting this suggestion is that in Funding each of these four legs is a thoracic region containing the elastic This research received no specific grant from any funding agency in the public, protein resilin (Burrows, 2011). Resilin has been associated with commercial, or not-for-profit sectors. energy storage mechanisms for jumping in diverse insect such as fleas Supplementary information (Siphonaptera) (Bennet-Clark and Lucey, 1967; Lyons et al., 2011), Supplementary information available online at grasshoppers (Orthoptera) (Burrows and Sutton, 2012) and http://jeb.biologists.org/lookup/doi/10.1242/jeb.205385.supplemental froghoppers (Hemiptera) (Burrows et al., 2008). The problem in snow fleas remains of how four catapult devices could be co-ordinated References to release their stored energy at the same time. In P. communis,the Askew, G. N. and Marsh, R. L. (2002). Muscle designed for maximum short-term power output: quail flight muscle. J. Exp. 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