Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Functional aspects of the flight apparatus

Arthropod Structure & Development xxx (2018) 1e12

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Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Functional aspects of the flight apparatus €umler, Stanislav N. Gorb, Sebastian Büsse* Fabian Ba Department of Functional Morphology and Biomechanics Institute of Zoology, Kiel University, Am Botanischen Garten 9, 24118, Kiel, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 December 2017 Received in revised form 10 April 2018 Accepted 16 April 2018 Available online xxx

Due to their unique flight mechanism including a direct flight musculature, Odonata show impressive flight skills. Several publications addressed the details of this flight apparatus like: sclerites, wings, musculature, and flight aerodynamics. However, 3D-analysis of the thorax musculature of adult dragonflies was not studied before and this paper allows for a detailed insight. We, therefore, focused on the thorax musculature of adult Anisoptera using micro-computed tomography. Herewith, we present a comparative morphological approach to identify differences within Anisoptera: Aeshnidae, Corduliidae, Gomphidae, and Libellulidae. In total, 54 muscles were identified: 16 prothoracic, 19 mesothoracic, and 19 metathoracic. Recorded differences were for example, the reduction of muscle Idlm4 and an additional muscle IIIdlm1 in Aeshna cyanea, previously described as rudimentary or missing. Muscle Iscm1, which was previously reported missing in all Odonata, was found in all investigated species. The attachment of muscle IIpcm2 in Pantala flavescens is interpreted as a probable adaption to its longdistance migration behaviour. Furthermore, we present a review of functions of the odonatan flight muscles, considering previous publications. The data herein set a basis for functional and biomechanical studies of the flight apparatus and will therefore lay the foundation for a better understanding of the odonatan flight. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Dragonflies Functional morphology Muscle setup Insect flight X-ray tomography (mCT)

1. Introduction The ability to fly has been an important evolutionary development, greatly promoting the success of winged insects (Pterygota) (Grimaldi and Engel, 2005). Within Pterygota, the Odonata (damsel- and dragonflies) are aerial key predators with outstanding flight skills enabled by their highly derived flight apparatus (Pfau, 1986; Corbet, 2004; Büsse et al., 2013). This flight apparatus with corresponding muscles, which are responsible for the wing movements, is developed differently among Pterygota (e.g. Snodgrass, 1935). Within all Pterygota e except Odonata e the mechanism supplying the main power for driving the wings is indirect; here large dorsal longitudinal muscles deform the winged thoracic segments, whereas in Odonata the musculature is connected via cap tendons to the wing base sclerites and, therefore, directly to the wings (Clark, 1940; Tannert, 1958; Hatch, 1966; Pfau, 1986; Büsse et al., 2013). This direct flight * Corresponding author. E-mail address: [email protected] (S. Büsse).

mechanism of Odonata provides impressive flight skills: Odonata can operate all 4 wings separately, allowing them to hover, glide and in some species even fly backwards (Hatch, 1966; Corbet, 2004; Grimaldi and Engel, 2005; Rüppell and Hilfert, 2013). The flight apparatus itself is located within the pterothorax (Matsuda, 1970) e fusion of the meso- and metathorax e which is tilted caudally by 45
; the prothorax is rather small (Hatch, 1966; Dathe, 2003). Pleurites of the pterothorax are enlarged in dorso-ventral direction, whereas tergites and sternites are small in comparison to other pterygote insects (Hatch, 1966). Additionally, the tergum is twistable along the longitudinal axis, supporting the ability of Odonata to operate their wing pairs separately (Hatch, 1966; Pfau, 1986; Dudley, 2002; Corbet, 2004). Several publications address various aspects of the flight apparatus of Odonata: sclerites (Chao, 1953; Russenberger, 1960; €rnschemeyer, 2007; Willkommen, 2008; Willkommen and Ho Ninomiya and Yoshizawa, 2009), musculature (Maloeuf, 1935; Clark, 1940; Asahina, 1954; Büsse et al., 2013, 2015; 2018; Büsse € rnschemeyer, 2013), wings (Wootton, 1991; Gorb, 1999; and Ho Wootton and Newman, 2008; Appel and Gorb, 2011; Rajabi et al.,

https://doi.org/10.1016/j.asd.2018.04.003 1467-8039/© 2018 Elsevier Ltd. All rights reserved.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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2016a; 2016b), aerodynamics of the odonatan flight (Pfau, 1986; Dudley, 2002; Brodsky, 1994), as well as the functional aspects of the flight musculature and sclerites (Clark, 1940; Hatch, 1966; Pfau, 1986). In total, the thoracic musculature of Odonata gained more interest recently (Büsse et al., 2013, 2015; 2018; Büsse and € rnschemeyer, 2013), showing that using modern methodology Ho can increase the knowledge even of already investigated taxa. For €rnschemeyer example, by using micro-CT technique Büsse and Ho (2013) identified nine previously unknown muscles for Anisoptera larvae, although the taxa were previously studied (Maloeuf, 1935; Snodgrass, 1954). Furthermore, the importance of consistent and high-resolution morphological data for phylogenetic analysis was shown in Büsse et al. (2018). We, therefore, focus in this study on the thoracic musculature of adult Anisoptera in a comparative morphological approach studying species of the main groups: Aeshnidae, Corduliidae, Gomphidae, and Libellulidae. We included different ecological aspects when selecting the species. Two different hunting styles are represented: perchers (e.g. Sympetrum striolatum) glean from a substrate, fly out to catch a spotted prey and ingest it while sitting again, while fliers (e.g. Aeshna cyanea) search, detect, pursuit, capture and ingest their prey in midflight (Clark, 1940; Corbet, 2004). One species among these, the flier Pantala flavescens, is a Libellulidae (all known to be perchers) that hunts in flier stile and travels over long distances (Hobson et al., 2012). Using modern techniques (e.g. mCT) in combination with traditional techniques (e.g. dissection), we aim to identify previously overlooked differences as well as confirm already known characters within Anisoptera and between the main groups of Odonata. Understanding the flight apparatus, as the functional main component of the insect flight, is crucial to understand the evolution and origin of the flight itself. Furthermore, we present an extensive review of functions of the odonatan flight muscles, based on previous works of Clark (1940), Hatch (1966) and the seminal work of Pfau (1986). The combination of these two datasets e comprehensive morphological re-investigation and review of flight muscles functionalities e sets a basis for functional and biomechanical studies of the flight apparatus and will, therefore, lay the foundation for a better understanding of the odonatan flight. 2. Material and methods Adult specimens of the following Anisoptera species of the main groups of Odonata were studied: A. cyanea (Müller, 1764) (Aeshnidae), Cordulia aenea (Linnaeus, 1758) (Corduliidae), Libellula depressa Linnaeus, 1758 (Libellulidae), Onychogomphus forcipatus (Linnaeus, 1758) (Gomphidae), P. flavescens (Fabricius, 1798) (Libellulidae), and S. striolatum (Charpentier, 1840) (Libellulidae). All regulations concerning the protection of free-living species were followed. All necessary permits for collecting Odonata in €rde”. No Germany were obtained from the “Untere Naturschutzbeho endangered or especially protected species were collected. Specimens for high resolution X-ray tomography (mCT) were fixed in an alcoholic Bouin solution (¼ Duboscq-Brasil) (Romeis, 1987), stored in 70% ethanol, dehydrated in an ascending ethanol series, and subsequently critical point dried (Balzers CPD030) prior to scanning. The scans were carried out using a SkyScan 1172 desktop micro-CT scanner (Bruker micro-CT, Kontich, Belgium) at 40 kV and 250 mA with images taken every 0.25
at a total of 180
. The generated data were reconstructed using Nrecon®, the segmentation of the data was done with Amira 6.2 (FEI SAS, France, www.vsg3d.com) and for the visualization, the open source 3D creation suite Blender (Blender Foundation, Netherlands, www. blender.org) and Illustrator CS6 (Adobe System Inc., www.adobe. com) was used.

Additionally, specimens for dissections were collected, freshly frozen and stored at 70
C and studied in a stereo microscope using micro scissors and forceps. In all following descriptions, the non-moving end of each muscle will be called point of origin and its moving end will be called point of insertion. All muscle names follow the nomenclature introduced by Friedrich and Beutel (2008) and the homologization for Odonata by Büsse et al. (2013). Anatomical structures are described using the nomenclature of Beutel et al. (2013), if necessary supplemented by Asahina (1954), Hatch (1966), Ninomiya and Yoshizawa (2009) and Büsse et al. (2013) e all used abbreviations are shown in Table A.1 in the supplementary materials. For an overview of sclerites and cuticular structures, see Figure B.1eB.4 in the supplementary materials. 3. Results Generally, conditions of morphological characters in S. striolatum (Figs. 1e5) are used as reference. For all other species, only characters that differ from S. striolatum are mentioned. In total, 54 muscles were found in the thorax of all studied species; for C. aenea, L. depressa, O. forcipatus, P. flavescens and S. striolatum: 16 in the prothorax, 19 in the mesothorax and 19 in the metathorax. The total amount is the same for A. cyanea, except that it is missing the prothoracic muscle Idlm4 and has an additional metathoracic muscle IIIdlm1. Furthermore, the muscle Iscm1 was unknown for Anisoptera, but is present in all investigated species. For an overview of the points of origin/insertion of all muscles, see Table C.1 in the supplementary materials. Additionally, mCT images of the musculature of all specimens are provided in the supplementary materials (see Figure DeH). Furthermore, a list of muscles, found in other publications can be found in the supplementary materials as well (Table I). 3.1. Musculature of the prothorax dlm e dorsal longitudinal muscles (Fig. 1) Idlm3 - M. prophragma-cervicalis Origin: Tergal apophysis 1 (TA1); broader attachment area in C. aenea. Insertion: Base of TA2. Characteristics: Runs from the prothorax to mesothorax. Idlm4 - M. cervico-occipitalis dorsalis Origin: Lateral region of the apex of TA1. Insertion: Median at the postocciput (Büsse et al., 2013); absent in A. cyanea. dvm e dorso-ventral muscles (Fig. 2) Idvm13 - M. pronoto-trochantinalis anterior Origin: Dorso-lateral area of the tergite 1, anterior to Idvm18; more median in C. aenea. Insertion: Anterior procoxal rim, median to Idvm15. Characteristics: Intersects with Idvm15. Idvm15 - M. propleuro-coxalis superior Origin: Dorso-lateral part of the tergite 1, anterior to Idvm13; more median in C. aenea. Insertion: Anterior area of the procoxal disk, lateral to Idvm13. Characteristics: Intersects with Idvm13. Idvm18 - M. pronto-coxalis lateralis Origin: Dorso-lateral part of the tergite 1, posterior to Idvm13; more median with a broader attachment area in C. aenea. Insertion: Posterior part of the procoxal rim, at the base of the mesothoracic preepisternal sclerite. pcm e pleuro-coxal muscles (Fig. 3) Ipcm8 - M. propleuro-trochanteralis Origin: Dorso-posterior part of the episternum 1.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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Fig. 1. Dorsal longitudinal and ventral longitudinal muscles of Sympetrum striolatum (A e inner layer, B e outer layer). Three-dimensional visualisation from mCT data, lateral view. At certain areas, the cuticle is made transparent to get a better look at muscles behind it.

Insertion: Tendon of the protrochanter. Characteristics: Same protrochanter tendon as Iscm6. scm e sterno-coxal muscles (Fig. 4) Iscm1 - M. profurca-coxalis anterior Origin: Lateral side of the base of profurca, anterio-ventral of Iscm6. Insertion: Anterior procoxal rim, ventral of Idvm15 and lateral to Idvm13. Characteristics: Dichotomous. Iscm2 - M. profurca-coxalis posterior Origin: Most ventral part of the profurca. Insertion: Postero-lateral procoxal rim. Iscm6 - M. profurca-trochanteralis Origin: Lateral side of the profurca. Insertion: Tendon of the protrochanter.

Characteristics: Same ventral tendon as Ipcm8. tpm e tergo-pleural muscles (Fig. 5) Itpm3 - M. pronoto-pleuralis anterior Origin: Dorso-lateral area of the tergite 1, lateral at an apophysis lateral to the TA1, anterior to Idvm15; lateral of Idvm13/15 in O. forcipatus. Insertion: Anterior part of episternum 1. Characteristics: Short muscle; comparably small in C. aenea. Itpm7 - M. protergo-cervicalis posterior Origin: Lateral region of the tergite 1, posterior to Itpm8. Insertion: Lateral of the cervix membrane (Büsse et al., 2015), median to Itpm8. Characteristics: Runs postero-median to Itpm8. Itpm8 - M. protergo-cervicalis anterior Origin: Antero-lateral region of the tergite 1, anterior to Itpm7.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

Fig. 2. Dorso-ventral muscles of Sympetrum striolatum (A e inner layer, B e middle layer, C e outer layer). Three-dimensional visualisation from mCT data, lateral view. At certain areas, the cuticle is made transparent to get a better look at muscles behind it.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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Fig. 3. Pleuro-coxal muscles of Sympetrum striolatum (A e inner layer, B e outer layer). Three-dimensional visualisation from mCT data, lateral view. At certain areas, the cuticle is made transparent to get a better look at muscles behind it.

Insertion: Lateral of the cervix membrane (Büsse et al., 2015), lateral to Itpm7. Characteristics: Runs antero-lateral to Itpm7. Itmp9 - M. protergo-preepisternalis Origin: Dorso-lateral area of the tergite 1, at the anterior part of an apophysis lateral to the TA1, anterior to Itpm3. Insertion: Base of the prothoracic preepisternal sclerite. Itpm11 - M. prosterna-coxalis sinister Origin: Lateral rim of the right procoxa. Insertion: Ventral surface of the prothoracic intestine (Büsse et al., 2015). Characteristics: Thin muscle running from the right to left side (Büsse et al., 2015); it connects more dorsal in C. aenea and L. depressa. vlm e ventro longitudinal muscles (Fig. 1) Ivlm3 - M. profurca-tentorialis

Origin: Apex of the profurca. Insertion: Cranial at the tentorial bar (Büsse et al., 2015). Characteristics: Runs into the head capsule (Büsse et al., 2015); thicker in P. flavescens and L. depressa. Ivlm7 - M. profurca-mesofurcalis Origin: Apex of the mesofurca. Insertion: Posterior part of the profurca. Characteristics: Runs from the mesothorax in the prothorax, posterior dichotomous.

3.2. Musculature of the mesothorax dlm e dorsal longitudinal muscles (Fig. 1) IIdlm1 - M. prophragma-mesophragmalis Origin: Apex of TA3.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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Fig. 4. Sterno-coxal muscles of Sympetrum striolatum (A e inner layer, B e outer layer). Three-dimensional visualisation from mCT data, lateral view. At certain areas, the cuticle is made transparent to get a better look at muscles behind it.

Insertion: Anterior edge of the base of the semi-detached scutal plate 3 (SDSP3). dvm e dorsoventral muscles (Fig. 2) IIdvm1 - M. mesonoto-sternalis Origin: Close to the point of origin of IIdvm3. Insertion: SDSP2, lateral of IIdvm3, postero-median to IIdvm4. Characteristics: Short muscle with a long cap tendon in the middle of the cap tendon of IIdvm3 (dissection only), runs lateral to IIdvm3. IIdvm3 - M. mesonoto-trochantinalis posterior Origin: Antero-median of the mesocoxal rim at the base of the mesothoracic preepisternal apodeme. Insertion: At SDSP2. Characteristics: Dorsal dichotomous prominent muscle with a big ventral cap tendon; not dichotomous in P. flavescens. IIdvm4 - M. mesonoto-coxalis anterior Origin: Antero-lateral edge of the mesocoxal rim.

Insertion: On the edge of the SDSP2 lateral to IIdvm3; median to IIdvm6 and anterior of IIdvm1 in P. flavescens. IIdvm5 - M. mesonoto-coxalis posterior Origin: At the base of the mesocoxal disk. Insertion: At the posterior region of the proximal costal plate 2 (pCP2), anterior to IIdvm6. Characteristics: Dorsal cap tendon. IIdvm6 - M. mesocoxa-subalaris Origin: Posterior area of the mesocoxal disk, posterior of IIdvm4/5. Insertion: Anterior region of the axillary plate 2 (AxP2), posterior to IIdvm4/5; at SDSP2 lateral of IIdvm4 in P. flavescens. Characteristics: Muscle gets dorsally thin and sharp with a small insertion area, big ventral attachment area. pcm e pleuro-coxal muscles (Fig. 3) IIpcm1 - M. mesanepisterno-trochantinalis

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Fig. 5. Tergo-pleural muscles of Sympetrum striolatum (A e inner layer, B e outer layer). Three-dimensional visualisation from mCT data, lateral view. At certain areas, the cuticle is made transparent to get a better look at muscles behind it.

Origin: Mesothoracic preepisternal sclerite antero-lateral to IIpcm2; longer muscle in C. aenea and L. depressa; anterior and partly surrounded by IIpcm2 in O. forcipatus, P. flavescens and C. aenea. Insertion: Inserted at the anterior region of pCP2. Characteristics: Short thin muscle with the long dorsal cap tendon, lateral to IIpcm2. IIpcm2 - M. mesobasalare-trochantinalis Origin: Mesothoracic preepisternal sclerite, mediano-lateral to IIpcm1. Insertion: Lateral to IIpcm1 at anterior region of pCP2; broader point of origin in P. flavescens. Characteristics: Most prominent muscle in the mesothorax with short dorsal cap tendon. IIpcm4 - M. mesanepisterno-coxalis posterior Origin: In the posterior region of katepisternum 2, close to the interpleural ridge and anterior of IIpcm6; closer to IIpcm2 in O. forcipatus.

Insertion: Anterior edge of the mesocoxa. Characteristics: Muscle runs close to the pleuron, dorsal attachment side is bigger than the ventral one. IIpcm6 - M. mesopleura-trochanteralis Origin: Ventral part of the interpleural ridge, between katepisternum 2 and katepimeron 2 and posterior to IIpcm4; closer to IIpcm2 in O. forcipatus. Insertion: Tendon of the mesotrochanter. Characteristics: Ventral tendon is the same as for IIscm6. scm e sterno-coxal muscles (Fig. 4) IIscm3 - M. mesofurca-coxalis medialis Origin: Lateral side of the mesofurca, posterior to IIscm6; median of IIscm6 in O. forcipatus. Insertion: Posterior edge of the mesocoxa, median to IIdvm6. Characteristics: Posterior origin to IIscm6. IIscm6 - M. mesofurca-trochanteralis Origin: Lateral side of the mesofurca, anterior to IIscm3; lateral of IIscm3 in O. forcipatus.

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Insertion: Tendon of the mesotrochanter. Characteristics: Tendon is the same as for IIpcm6. tpm e tergo-pleural muscles (Fig. 5) IItpm3 - M. mesonoto-basalaris Origin: At the base of the apophysis of the border between scutellum 2 and postnotum 2, dorsal of the attachment point of IItpm4. Insertion: At the base of an apophysis in the middle of AxP2, dorsal of the attachment point of IItpm4. Characteristics: Runs dorsal of IItpm4. IItpm4 - M. mesonoto-pleuralis anterior Origin: At the apex of the apophysis of the border between the scutellum 2 and postnotum 2, ventral of the attachment area of IItpm3. Insertion: At the apex of an apophysis in the middle of AxP2, ventral of the attachment point of IItpm3. Characteristics: Runs ventral of IItpm3. IItpm6 - M. mesonoto-pleuralis posterior Origin: Uppermost edge of the interpleural ridge 2 on the anepimeron 2 side. Insertion: Lateral on the anterior region of the SDSP2. Characteristics: Large point or origin area. IItpm7 - M. mesanepisterno-axillaris Origin: At the ventral region of the intersegmental ridge between katepimeron 2 and katepisternum 3; more anterior in O. forcipatus. Insertion: At the very posterior edge of AxP2. Characteristics: Dorsal cap tendon. IItpm8 - M. mesepimero-axillaris secundus Origin: At the ventral part of the katepimeron 2, between the interpleural ridge and the intersegmental ridge; smaller attachment area and more anterior in O. forcipatus. Insertion: At the centre region of AxP2. Characteristics: Large point of origin area, anterior of IItpm7/10, big dorsal cap tendon. IItpm10 - M. mesepimero-subalaris Origin: The ventral region of the intersegmental ridge between anepimeron 2 and anepisternum 3. Insertion: Posterior area of AxP2. Characteristics: Runs between IItpm7 (lateral) and IItpm8 (median). vlm e ventral longitudinal muscles (Fig. 1) IIvlm6 - M. mesospina-abdominosternalis Origin: Posterior end of preepisternal apodeme 3. Insertion: Very end of katepimeron 3, close to the bar between katepimeron 3 and first abdominal sternite, latero-posterior to IIIvlm2; no posterior tendon in L. depressa.

3.3. Musculature of the metathorax dlm e dorsal longitudinal muscles (supplementary material D.1) IIIdlm1 - M. mesophragma-metaphragmalis Origin: TA4. Insertion: Dorso-lateral area of the ridge between the thorax and abdomen. Characteristics: Only present in A. cyanea. dvm e dorsoventral muscles (Fig. 2) IIIdvm1 - M. metanoto-sternalis Origin: Via a long transparent tendon in the middle of the cap tendon of IIdvm3 (Dissection only). Insertion: SDSP3, lateral of IIIdvm3, postero-median to IIIdvm4. Characteristics: Long tendon, short muscle. IIIdvm3 - M. metanoto-trochantinalis posterior

Origin: Antero-median of the metacoxal rim at the base of the metathoracic preepisternal apodeme. Insertion: At SDSP3. Characteristics: Very prominent dorsal dichotomous muscle, ventral cap tendon. IIIdvm4 - M. metanoto-coxalis anterior Origin: Antero-lateral rim of the metacoxa. Insertion: On the antero-lateral region of SDSP3. Characteristics: Antero-lateral to IIIdvm3. IIIdvm5 - M. metanoto-coxalis posterior Origin: Antero-lateral part of the metacoxal disk. Insertion: At the very posterior edge of pCP3, posterior to IIIdvm4/5. Characteristics: Dorsal cap tendon. IIIdvm6 - M. metanoto-coxalis posterior Origin: Median region of the mesocoxal disk. Insertion: Anterior region of AxP3. IIIdvm8 - M. metanoto-phragmalis Origin: At the posterior edge of the thoracic abdominal apodeme. Insertion: At an apophysis at the anterior edge of the first abdominal tergite. Characteristics: Long ventral cap tendon. pcm e pleuro-coxal muscles (Fig. 3) IIIpcm1 - M. metanepisterno-trochantinalis Origin: At intersegmental ridge between the katepimeron 2 and katepisternum 3. Insertion: At the anterior edge of the SDSP3. Characteristics: Short muscle, long cap tendons on both sides, lateral to IIIpcm2. IIIpcm2 - M. metabasalare-trochantinalis Origin: With big attachment side at the metathoracic preepisternal apodeme. Insertion: At the anterior region of pCP3. Characteristics: Large dorsal cap tendon, most prominent muscle in the metathorax. IIIpcm4 - M. metanepisterno-coxalis posterior Origin: In the posterior region of the katepisternum 3, close to the intersegmental ridge 3 and anterior to IIIpcm6. Insertion: Antero-external edge of the metacoxa. IIIpcm6 - M. mesopleura-trochanteralis Origin: Posterior part of the katepisternum 3, close to the intersegmental ridge 3 and posterior to IIIpcm4. Insertion: Tendon of the metatrochanter. Characteristics: Metatrochanter tendon is the same as for IIIscm6. scm e sterno-coxal muscles (Fig. 4) IIIscm3 - M. metafurca-coxalis medialis Origin: Lateral side of metafurca, ventral of IIIscm6, broad attachment side. Insertion: Postero-internal edge of the metacoxa, median of IIIdvm6. Characteristics: Ventral origin of IIIscm6. IIIscm6 - M. metafurca-trochanteralis Origin: Lateral side of the apex of the metafurca, median of IIIscm3, broad attachment side. Insertion: Tendon of the metatrochanter. Characteristics: Same metatrochanter tendon as for IIIpcm6. tpm e tergo-pleural muscles (Fig. 5) IIItpm3 - M. metanoto-basalaris Origin: At the base of an apophysis of the border of the postnotum 3 and the first abdominal tergite, dorsal of the attachment side of IIItpm4. Insertion: At the base of an apophysis in the middle of AxP3, dorsal of the attachment side of IIItpm4.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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Characteristics: Runs dorsal of IIItpm4. IIItpm4 - M. metanoto-pleuralis anterior Origin: At the apex of an apophysis of the border of the postnotum 3 and the first abdominal tergite, ventral of the attachment side of IIItpm3. Insertion: At the apex of an apophysis in the middle of AxP3, ventral of the attachment side of IIItpm3. Characteristics: Runs ventral of IIItpm3. IIItpm6 - M. metanoto-pleuralis posterior Origin: Uppermost edge of the interpleural ridge 3 on the side of anepimeron 3. Insertion: Lateral on the anterior region of the SDSP3. IIItpm7 - M. metanepisterno-axillaris Origin: At the anterior region of the ventral carina, posteromedian of IIItpm8; more anterior (O. forcipatus). Insertion: At the very posterior edge of AxP3, medio-lateral to IIItpm10. Characteristics: Slender muscle, dorsal cap tendon. IIItpm8 - M. metapimero-axillaris secundus Origin: With a big attachment side at the ventral part of the katepimeron 3; smaller attachment area in O. forcipatus. Insertion: With a short cap tendon at the centre region of the AxP3. Characteristics: Big muscle with the short dorsal cap tendon. IIItpm10 - M. metepimero-subalaris Origin: At an apodeme between the katepimeron 3 and poststernum 3. Insertion: At the posterior edge of AxP3. Characteristics: Slender muscle, dorsal tendon. vlm e ventral longitudinal muscles (Fig. 1) IIIvlm2 - M. mesofurca-abdominalis Origin: Poststernal apodeme. Insertion: Bar at the first abdominal segment. 4. Discussion 4.1. Muscles A total number of 54 muscles in the entire thorax could be confirmed for all investigated species. In nearly all species the same muscles are present, except for A. cyanea; here the prothoracic muscle Idlm4 is missing, but an additional metathoracic muscle IIIdlm1 is present. In all investigated species, we found muscle Iscm1 that was previously reported to be absent in Anisoptera (Maloeuf, € rnschemeyer, 2013; Büsse et al., 2018). The 1935; Büsse and Ho muscle Iscm1 controls the leg movement and was probably overlooked in earlier investigations, because of its small size and the closeness to the muscle Iscm6. It has the same points of attachment at the base of the profurca (origin) and the procoxal rim (insertion) as the muscles described by Friedrich and Beutel (2008); therefore, they are hypothesized to be homologous. It seems that this character state is exclusive to Anisoptera and has not been described for Odonata until now (Maloeuf, 1935; Clark, 1940; Asahina, 1954; Hatch, 1966; € rnschemeyer, 2013). Büsse et al., 2013, 2015; 2018; Büsse and Ho The muscle Idvm13 has been previously declared as absent in €rnschemeyer (2013), though it Anisoptera larvae by Büsse and Ho was present in all investigated adult specimens. Maloeuf (1935) described a muscle in Anisoptera larvae (muscle 13) that may be homologous to Idvm15 in Anisoptera larvae described by Büsse and €rnschemeyer (2013). In adult Anisoptera, the larval muscle Ho Idvm15 splits up, forming Maloeuf (1935) muscle 13A that can be homologized with Idvm13 and 13B that can be homologized with € rnschemeyer, 2013). The Idvm15 (Maloeuf, 1935; Büsse and Ho muscle Idvm13 is described as absent in larvae and adult species of Zygoptera and Epiophlebia (Asahina, 1954; Büsse et al., 2013, 2015;

9

2018), which makes it unique for adult Anisoptera. The muscle Idvm15 can be found in both Epiophlebia and Zygoptera, and it shows a dichotomy in the studied larvae (Büsse et al., 2018, Fig. 3). Additionally, muscle II/IIItpm3 is present in all investigated Anisoptera species, although it was previously described as exclu€rnschemeyer, sively present in all odonatan larvae (Büsse and Ho 2013; Büsse et al., 2015, 2018). The attachment points among the studied species differ in 14 muscles within the pterothorax (Muscles: IIdvm3, IIdvm4, IIdvm6, IIpcm1/2, IIpcm4/6, IIscm3/6, IItpm7/8, IIvlm6, IIItpm7/8). Muscle IIdvm3 appears not to be dichotomous in P. flavescens. Additionally, both muscle II/IIIdvm4 and II/IIIdvm6 have different points of insertion in P. flavescens in comparison to the other species. While the insertion of II/IIIdvm4 is only slightly shifted in medial direction, inserting lateral to the muscle II/ IIIdvm3 and anterior to II/IIIdvm1, the insertion of the muscle II/ IIIdvm6 appears to be completely different. It inserts at the SDSP2/ 3 lateral of the muscle II/IIIdvm4. Therefore, its function is presumably purely elevating rather than supinating elevating (as discussed later). In this case, no other muscle is left to supinate and elevate the wing at the same time (working together with II/ IIIdvm5). The muscle IIpcm1 appears to be comparably larger in L. depressa and C. aenea than in S. striolatum, which might indicate an additional need for a better protection (Clark, 1940) against downward deflection (as discussed later) in both species; caused for example in extreme situations like territorial fights (Corbet, 2004). Furthermore, the point of origin of muscle IIpcm1 is antero-lateral to muscle IIpcm2 in S. striolatum, L. depressa and A. cyanea; in O. forcipatus, P. flavescens and C. aenea, however, IIpcm1 is partly surrounded by muscle IIpcm2 from the lateral and median side this should not have any significant consequences for the function of muscle IIpcm1. In S. striolatum, both muscles (II/IIIpcm1 and II/ IIIpcm2) have the same points of origin and insertion as in larvae and adult species of Epiophlebia and Zygoptera (Asahina, 1954; Büsse et al., 2013, 2015; 2018). In P. flavescens, the point of origin of the muscle IIpcm2 is comparably larger than in any other investigated species, indicating that P. flavescens could probably generate a greater force for the wing depression resulting in a faster acceleration (and speed of flight). Additionally, P. flavescens is known to migrate over great distances (e.g., from North India to the Maldives or East Afrika) for breeding (Anderson, 2009; Hobson et al., 2012), which might be the most reasonable explanation for this particular muscle arrangement. The point of origin of the muscle IIscm3/6 is slightly shifted caudally in O. forcipatus compared to all other species. The muscle IIscm3 is located median of muscle IIscm6 instead of running posterior to it. In S. striolatum, both muscles have the same points of origin and insertion as in larvae and adult species of Epiophlebia and Zygoptera (Asahina, 1954; Büsse et al., 2013, 2015; 2018). In the metathorax of O. forcipatus, two differences in comparison to all other species could be observed. The origin of the muscle IIItpm7 is located comparably further anterior and the point of origin of the muscle IIItpm8 appears to be smaller. The muscle II/IIIdvm1 has already been described in adult Anisoptera (Clark, 1940; Hatch, 1966; Pfau, 1986) and was found in all investigated specimens, although it was in most cases not traceable in the mCT scans and could only be found by dissection. The muscle could be found in adult Zygoptera and Epiophlebia as well (Asahina, 1954; Büsse et al., 2013). Finally, there is an additional metathoracic muscle found in A. cyanea, IIIdlm1 (discussed later). It was described by Pfau (1986) as absent or rudimentary in all adult Anisoptera and only present in larvae and adult species of Epiophlebia and Zygoptera (Asahina, 1954; Büsse et al., 2013, 2015; 2018) e but is present in larvae € rnschemeyer, 2013). (Büsse and Ho

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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4.2. Sclerites In Odonata, the wing base sclerites are formed to serve as important attachment structures for muscles (cf. Ninomiya and Yoshizawa, 2009). In general, all 4 tergal apophyses (pin shaped structures, formed from everted cuticle) e located in the dorsal area of the thorax, at the tergum e serve as attachment structure for dorsal longitudinal muscles (supplementary material B). TA1 - 3 can be found in all specimens, whilst TA4 is only existing in A. cyanea and rudimentary in all other specimens. Additionally, all animals have two dorsal semi-detached scutal plates (plate-like structures, formed from everted cuticle) in the pterothorax (supplementary material B), connected to the notum. SDSP2 is located in the mesothorax, is 8-shaped and flexibly connected to the notum by a pin-like structure in the middle of the dorsal side of the SDSP. It serves as attachment structure for various dorsoventral muscles and muscle tpm6. The SDSP3 is more strongly connected to the tergum at the anterior and posterior part without the pin like structure in the middle and is of plate-like shape. This prevents it from being tilted, likely resulting in a better force transmission. This is true for all specimens except A. cyanea, where the SDSP3 is more flexibly connected and of similar shape as SDSP2. Those two differences might be crucial for the functioning of A. cyanea's additional muscle IIIdlm1, serving as a steering muscle to swing the hindwing in anterior direction. The present TA4 serves as an attachment point for muscle IIIdlm1, while the more flexible SDSP3 provides enough flexibility to swing the hindwing in anterior direction. Also, the katepisternum in O. forcipatus is comparably smaller than in the other species (supplementary material B). It is located ventral of the anepisternum and between the katepimeron (caudal) and the mesothoracic preepisternal sclerite (cranial). 4.3. Review of the odonatan flight muscle function The flight apparatus and its musculature are one of the unique features of Odonata, making them aerial key predators among insects (Corbet, 2004). The muscles can functionally be subdivided in active phasic and supporting tonic muscles and further grouped in terms of their function for the flight apparatus, serving one or different purposes e depressors/elevators and or steering muscles (Hatch, 1966; Pfau, 1986). In general, all tonic muscles (Muscles: II/IIIpcm1, II/IIIdvm1, II/IIItpm6, II/IIItpm10; Fig. 6E) function as supporters for phasic muscles and are very important for the wings movement along its longitudinal axis (Clark, 1940; Hatch, 1966; Pfau, 1986). The muscle II/IIIpcm1 applies a steady downward force to the anterior part of the pCP2/3, to prevent the wing from an upward deflection (Hatch, 1966). The muscle II/IIIdvm1 applies a steady downward force to the SDSP2/3 and prevents the wing from a downward deflection (Clark, 1940; Hatch, 1966). Due to the size difference between these two muscles and their associated phasic muscles, the phasic contraction of muscles II/IIIpcm1 and II/IIIdvm1 is hypothesized to be ineffective for the elevation/depression of the wing (Pfau, 1986). Therefore, their tonic contraction probably provides a further antagonistic function e the muscle II/IIIpcm1 counteracts the elevating movement of the muscle II/IIIdvm3 and II/IIIdvm1 counteracts the depressing movement of the muscle II/IIIpcm2 (Pfau, 1986). This could enable the separate control of speed and amplitude of the wing (Pfau, 1986). The tonic contracting muscle II/IIItpm6 is special because it affects the wing movement in both the depression and elevating phase. The tergal bridge keeps the wing bases of both sides at the same distance and, additionally, the distance between the connection points of the wing base with the tergal bridge (Fig. 7 A,

TB) and the pleura (Fig. 7A, P) remains constant as well. There is a dorsal membranous gap between the episterna of the mesothorax and, while elevating and depressing the wings through phasic muscles, the pleura of the pterothorax is oscillating in an opposite direction of the force direction of the muscle II/IIItpm6 (Pfau, 1986). By pulling the interpleural ridge and, therefore, the pleura in median direction (Fig. 7B and C), the muscle II/IIItpm6 functions alternately as a synergist and antagonist, depending in which position the wing is (Pfau, 1986). It serves as a spring with an adjustable drag. At the first part of each movement phase (elevation or depression), the phasic muscle has to overcome the resistance of the muscle II/IIItpm6 (this antagonist effect is shown in Fig. 7A) and after a certain point is exceeded, II/IIItpm6 becomes the synergist (Fig. 7B and C). Additionally, Hatch (1966) described the muscle II/IIItpm10 to apply a steady downward force to the posterior area of AxP2/3 to provide wing supination. Pfau (1986) extended this assumption, that the muscle mainly supinates the wing due to its low contracting power and movement in opposite direction of the elevating musculature, potentially decelerating the upward movement with the properties of an adjustable spring. We agree with Pfau (1986), that the muscle IIdlm1 most likely helps to move the forewings in an anterior direction by pulling at the TA3. The SDSP2 is thereby lifted caudally, while the forewing is swinging anteriorly at the end of the downstroke. The muscle IIIdlm1 in A. cyanea serves a similar purpose: it pulls at the TA4 to lower the SDSP3 anteriorly and move the hindwing in a posterior direction (Pfau, 1986). The muscle IIIdlm1 is just rudimentary or reduced in most Anisoptera (Pfau, 1986), but present in Zygoptera and Epiophlebia (Asahina, 1954; Pfau, 1986; Büsse et al., 2013, 2015; 2018). Therefore, in A. cyanea, the SDSP3 is shaped like the SDSP2 and connected to the wing bases in the same way, whilst the SDSP3 in all other investigated Anisoptera is connected more strongly to the wing bases, in order to support a better force transmission from muscles to the wings and to make the metathorax a purely power segment for the wing beat (Pfau, 1986). Additionally, there is another muscle in Zygoptera and Epiophlebia, antagonizing the movement of II/IIIdlm1. The muscle II/IIItpm2 might function as an antagonist, bringing the SDSP2/3 back into its original position (Asahina, 1954; Pfau, 1986; Büsse et al., 2013). The following phasic muscles can be divided in elevators (Muscle: II/IIIdvm3, II/IIIdvm4, II/IIIdvm5, II/IIIdvm6 and II/IIItpm3 and II/IIItpm4; Fig. 6A) and depressors (Muscle: II/IIIpcm2, II/IIItpm7, II/IIItpm8; Fig. 6B) (Clark, 1940; Hatch, 1966; Pfau, 1986). For the downstroke: The muscle II/IIItpm8 mainly depresses the wing by pulling at the centre of AxP2/3 and supinating it at the same time (Hatch, 1966; Pfau, 1986). Pfau (1986) describes muscle II/IIIpcm2 as a pure depressing muscle by applying a force to the anterior part of the pCP2/3, but the explanation by Hatch (1966), that the muscle also pronates the wing, seems reasonable. The muscle II/IIItpm7 depresses the posterior end of the AxP2/3, to supinate the wing as a possible antagonist to the muscle II/IIIdvm5 (Hatch, 1966; Pfau, 1986), but helps depressing it in the downward movement of the wing as well (Pfau, 1986). For the upstroke: The muscle II/IIIdvm3 elevates the wing evenly by applying a depressing force to the SDSP2/3 (Clark, 1940; Hatch, 1966; Pfau, 1986). The muscle II/IIIdvm5 applies force to the pCP 2/3, in order to mainly pronate the wing as an antagonist to the muscle II/ IIItpm7 and elevate it in an upward movement of the wing as well. The muscle II/IIIdvm6 is connected to the anterior region of AxP2/3, supinating the wing (Hatch, 1966; Pfau, 1986) and elevating it at the same time. It probably supports the function of II/IIIdlm1 by decelerating and keeping the wing supinated at the end of the downstroke (Pfau, 1986).

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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Fig. 6. Schematic of the function of different flight muscles, anterior view. A e wing elevation, B e wing depression, C e wing pronation, D e wing supination, E e tonic muscles. Abbreviations: AxP e axillary plate, d e dorsal, dvm e dorso-ventral musculature, l e lateral, m e median, pcm e pleuro-coxal musculature, pCP e proximal costal plate, tpm e tergo-pleural musculature, v e ventral.

4.4. Conclusion In summary, besides many minor differences within the thorax of the studied adult Anisoptera, some major characteristics could be found. In all investigated species we could identify three muscles (Iscm1 and II/IIItpm3) previously undescribed for adult Anisoptera. Also, Idvm13 (recently misinterpreted as Idvm15 in larvae by Büsse €rnschemeyer, 2013) could be confirmed to be present in all and Ho species. Furthermore, one missing muscle (Idlm4) and one additional muscle (IIIdlm1) could be confirmed for A. cyanea, accompanied by a semi-detached scutal plate 3 of specialized shape. We could show the value of a re-investigation using modern techniques (e.g. mCT) with traditional techniques (e.g. dissection), for identifying previously overlooked differences as well as for confirming already known characters. Although differences could be

found in A. cyanea and P. flavescens in comparison to all other species, no significant differences were discovered regarding different hunting stiles (flier, percher). Our results are confirmatory to all remarks regarding the evolution and phylogeny of Odonata recently made by Büsse et al. (2018). All 19 synapomorphies for the sister group relationship of Odonata [Epiprocta þ Zygoptera] (cf. Misof et al., 2014), as well as the 4 synapomorphies for Epiprocta [Epiophlebia þ Anisoptera] (cf. Bybee et al., 2008; Blanke et al., 2012, 2015; Misof et al., 2014) itself could be confirmed (Büsse et al., 2018). In more detail, the synapomorphies for Anisoptera: absence of muscle Iscm4, presence of muscle IIscm2, absence of muscle IIscm4, position of muscle IItpm8 anterior of muscle IItpm10, position of muscle IIIpcm1 posterior of muscle IIIpcm2, insertion of muscle IIIpcm4 at the antero-external edge of the metacoxa, origin of muscle IIIscm6

Fig. 7. Antagonistic and synergistic functions of the muscles II/IIItpm6. A e wing initial, B e wing depression, C e wing elevation. Black arrows show the direction of the force produced by the muscles; white arrow shows the direction, in which the cuticle is deformed by flight musculature; grey arrows show the resulting movement direction of the wing. Abbreviations: d e dorsal, dvm3 e dorso ventral muscle 3, l e lateral, m e median, P e pleura, SDSP e semi-detached-scutal plate, TB e tergal bridge, tpm6 e tergo pleural muscle 6, tpm8 e tergo-pleural muscle 8, v e ventral.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003

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at the latero-external face of the metafurca, position of muscle IIItpm7 median of muscle IIItpm10 (Büsse et al., 2018) could be confirmed and the presence of muscle Iscm1, Idvm13 and II/IIItpm3 could be added as new synapomorphies for Anisoptera. The review of functions of the odonatan flight muscles, based on previous works of Clark (1940), Hatch (1966) and the seminal work of Pfau (1986) e latter only available in German e, will help to understand the functionality of this highly complex character system. This all together will lay a basis for functional and biomechanical studies of the flight apparatus, for example for muscular manipulation experiments. Here our study can help considerably to detect single muscles of interest and identify the functional effects of a manipulation. Acknowledgements We are grateful for the support by the members of the Functional Morphology and Biomechanics Group at Kiel University, especially € hnsen. SB is directly supported through the DFG to E. Appel and A. Ko grant BU3169/1-1. The project is partly financed through the DFG €te” grant INST 257/405-1 FUGG. Many thanks “Forschungsgroßgera to J. Ware (Rutgers University, Newark, USA), F. Suhling (Technische €t Braunschweig, Braunschweig, Germany) and B. Kunz Univesita (Langenburg, Germany) for providing specimens. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.asd.2018.04.003. References Anderson, R.C., 2009. Do dragonflies migrate across the western Indian Ocean? J. Trop. Ecol. 25 (4), 347e358. Appel, E., Gorb, S.N., 2011. Resilin-bearing wing vein joints in the dragonfly Epiophlebia superstes. Bioinspiration Biomimetics 6, 046006. https://doi.org/ 10.1088/1748-3182/6/4/046006. Asahina, S., 1954. A morphological study of a relic dragonfly Epiophlebia superstes Selys (Odonata, Anisozygoptera). Japan Society for the Promotion of Science, Tokio. Beutel, R.G., Friedrich, F., Yang, X.-K., Ge, S.-Q., 2013. Insect Morphology and Phylogeny: A Textbook for Students of Entomology. De Gruyter, Berlin. Blanke, A., Wipfler, B., Letsch, H., Koch, M., Beckmann, F., Beutel, R., Misof, B., 2012. Revival of Palaeoptera head characters support a monophyletic origin of Odonata and Ephemeroptera (Insecta). Cladistics 28, 560e581. Blanke, A., Büsse, S., Machida, R., 2015. Coding characters from different life stages for phylogenetic reconstruction: a case study on dragonfly adults and larvae, including a description of the larval head anatomy of Epiophlebia superstes (Odonata: Epiophlebiidae). Zool. J. Linn. Soc. 174, 718e732. Brodsky, A.K., 1994. The Evolution of the Insect Flight. Oxford University Press, Oxford, p. 229. €rnschemeyer, T., 2013. The thorax musculature of Anisoptera (Insecta: Büsse, S., Ho Odonata) nymphs and its evolutionary relevance. BMC Evol. Biol. 13, 237. https://doi.org/10.1186/1471-2148-13-237. € rnschemeyer, T., 2013. Homologization of the flight muscuBüsse, S., Genet, C., Ho lature of Zygoptera (Insecta: Odonata) and Neoptera (Insecta). PLoS One 8, 1e16. € rnschemeyer, T., 2015. The thorax morphology of EpioBüsse, S., Helmker, B., Ho phlebia (Insecta: Odonata) nymphs e including remarks on ontogenesis and evolution. Sci. Rep. 5, 12835.

€rnschemeyer, T., Bybee, S.M., 2018. The phylogenetic Büsse, S., Heckmann, S., Ho relevance of thoracic musculature: a case study including a description of the thorax anatomy of Zygoptera (Insecta: Odonata) larvae. Syst. Entomol. 43, 31e42. https://doi.org/10.1111/syen.12246. Bybee, S.M., Ogden, T.H., Branham, M.A., Whiting, M.F., 2008. Molecules, morphology and fossils: a comprehensive approach to odonate phylogeny and the evolution of the odonate wing. Cladistics 23, 1e38. Chao, H., 1953. The external morphology of the dragonfly Onychogomphus ardens needham. Smithsonian Misc. Collect. 122 (6), 1e56. Clark, H.W., 1940. The adult musculature of the anisopterous dragonfly thorax (Odonata, Anisoptera). J. Morphol. 67 (3), 523e565. Corbet, P.S., 2004. Dragonflies Behavior and Ecology of Odonata. Revised Edition. Harley Books (B.H. & A. Harley Ltd), Colchester, United Kingdom. Dathe, H.H., 2003. Lehrbuch der Speziellen Zoologie Band I: Wirbellose Tiere e 5. Teil: Insecta, second ed. Spektrum Akademischer Verlag Heidelberg, Berlin. Dudley, R., 2002. The Biomechanics of Insect Flight: Form, Function, Evolution. Princeton University Press, New Jersey. Friedrich, F., Beutel, R.G., 2008. The thorax of Zorotypus (Hexapoda, Zoraptera) and a new nomenclature for the musculature of Neoptera. Arthropod Struct. Dev. 37, 29e54. Gorb, S.N., 1999. Serial elastic elements in the damselfly wing: mobile vein joints contain resilin. Naturwissenschaften 86, 552e555. https://doi.org/10.1007/ s001140050674. Grimaldi, D., Engel, M.S., 2005. Evolution of the Insects. University Press, Cambridge. Hatch, G., 1966. Structure and mechanics of the dragonfly pterothorax. Ann. Entomol. Soc. Am. 59 (4), 702e714. Hobson, K.A., Anderson, R.C., Soto, D.X., Wassenaar, L.I., 2012. Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern indian subcontinent. PLoS One 7 (12). https://doi.org/10.1371/ journal.pone.0052594. Maloeuf, N.S.R., 1935. The postembryonic history of the somatic musculature of the dragonfly thorax. J. Morphol. 58 (1), 87e115. Matsuda, R., 1970. Morphology and evolution of the insect thorax. Mem. Entomol. Soc. Can. 76, 1e431. Misof, B., Liu, S., Meusemann, K., et al., 2014. Phylogenomics resolves the timing and pattern of insect evolution. Science 346, 763e767. Ninomiya, T., Yoshizawa, K., 2009. A revised interpretation of the wing base structure in Odonata. Syst. Entomol. 34, 334e345. Pfau, H.K., 1986. Untersuchungen zur Rekonstruktion, Funktion und Evolution des Flugapparates der Libellen (Insecta, Odonata). Tijdschr Entomol. 129, 35e123. Rajabi, H., Shafiei, A., Darvizeh, A., Dirks, J.-H., Gorb, S.N., 2016a. Effect of microstructure on the mechanical and damping behaviour of dragonfly wing veins. Royal Soc. Open Sci. 3 https://doi.org/10.1098/rsos.160006. Rajabi, H., Shafiei, A., Darvizeh, A., Gorb, S.N., 2016b. Resilin microjoints: a smart design strategy to avoid failure in dragonfly wings. Sci. Rep. 6 https://doi.org/ 10.1038/srep39039. Romeis, B., 1987. Mikroskopische Technik. Urban und Schwarzenberg, München. Rüppell, G., Hilfert, D., 2013. The flight of the relict dragonfly Epiophlebia superstes (Selys) in comparison with that of the modern Odonata (Anisozygoptera: Epiophelebiidae). Odonatologica 1993 (22), 295e309. Russenberger, H.M., 1960. Bau und Wirkungsweise des Flugapparates von Libellen. Mittl. Naturforschenden Ges. Schaffhausen 27, 1e88. Tannert, W., 1958. Die Flügelgelenkung bei Odonaten. Deutsche Entomolomologische Zeitschrift 5, 394e455. Willkommen, J., 2008. The tergal and pleural wing base sclerites e homologous within the basal branches of Pterygota? Int. J. Freshw. Entomol. 31, 443e457. €rnschemeyer, T., 2007. Erratum to ‘‘The homology of wing base Willkommen, J., Ho sclerites and flight muscles in Ephemeroptera and Neoptera and the morphology of the pterothorax of Habroleptoides confusa (Insecta: Ephemeroptera: leptophlebiidae)’’. Arthropod Struct. Dev. 36, 253e269. Wootton, R.J., 1991. The functional morphology of the wings of Odonata. Adv. Odonatol. 5, 153e169. Wootton, R.J., Newman, D.J.S., 2008. Evolution, diversification and mechanics of dragonfly wings. In: Cordoba-Aguilar, A. (Ed.), Dragonflies and Damselflies. Model Organisms in Ecological and Evolutionary Research, Chapter 20. Oxford University Press, New York, pp. 261e274.

€umler, F., et al., Comparative morphology of the thorax musculature of adult Anisoptera (Insecta: Odonata): Please cite this article in press as: Ba Functional aspects of the flight apparatus, Arthropod Structure & Development (2018), https://doi.org/10.1016/j.asd.2018.04.003