title: A Beetle Flight Muscle Displays Leg Muscle Microstructure.
abstract: In contrast to major flight muscles in the Mecynorrhina torquata beetle, the third axillary (3Ax) muscle is a minor flight muscle that uniquely displays a powerful mechanical function despite its considerably small volume, ∼1/50 that of a major flight muscle. The 3Ax muscle contracts relatively slowly, and in flight strongly pulls the beating wing to attenuate the stroke amplitude. This attenuation leads to left-right turning in flight or wing folding to cease flying. What enables this small muscle to be so powerful? To explore this question, we examined the microstructure of the 3Ax muscle using synchrotron x-ray diffraction, optical microscopy, and immunoblotting analysis. We found that the 3Ax muscle has long (∼5 μm) myofilaments and that the ratio of thick (myosin) filaments to thin (actin) filaments is 1:5 or 1:6. These characteristics are not observed in the major flight muscles, which have shorter myofilaments (∼3.5 μm) with a smaller ratio (1:3), and instead are more typical of a leg muscle. Furthermore, the flight-muscle-specific troponin isoform, TnH, is not expressed in the 3Ax muscle. Since such a microstructure is suitable for generating large tension, the 3Ax muscle is appropriately designed to pull the wing strongly despite its small volume.
authors.affiliation: Department of Physics, Graduate School of Advanced Science and Engineering, Waseda University, Tokyo, Japan.
authors.forename: Toshiki
authors.initials: T
authors.lastname: Shimomura
authors.affiliation: Research and Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Iwamoto
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Republic of Singapore.
authors.forename: Tat Thang
authors.initials: TT
authors.lastname: Vo Doan
authors.affiliation: Department of Physics, Faculty of Science and Engineering, Waseda University, Tokyo, Japan.
authors.forename: Shin'ichi
authors.initials: S
authors.lastname: Ishiwata
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Republic of Singapore. Electronic address: hirosato@ntu.edu.sg.
authors.forename: Hirotaka
authors.initials: H
authors.lastname: Sato
authors.affiliation: WASEDA Bioscience Research Institute in Singapore, Singapore, Republic of Singapore; Comprehensive Research Organization, Waseda University, Tokyo, Japan; PRESTO, Japan Science and Technology Agency, Saitama, Japan. Electronic address: suzu_mado@aoni.waseda.jp.
authors.forename: Madoka
authors.initials: M
authors.lastname: Suzuki
mesh_terms: D000818:Animals; D001517:Coleoptera / Q000033:anatomy & histology* / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005426:Flight, Animal / Q000502:physiology; D015151:Immunoblotting; D019476:Insect Proteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008858:Microscopy, Phase-Contrast; D009132:Muscles; D009210:Myofibrils / Q000378:metabolism / Q000648:ultrastructure*; D061848:Optical Imaging; D014336:Troponin / Q000378:metabolism; D014921:Wings, Animal / Q000502:physiology; D014961:X-Ray Diffraction
chemical_list: D019476:Insect Proteins; D014336:Troponin