Biomechanical basis of wing and haltere coordination in flies Tanvi Deora, Amit Kumar Singh, and Sanjay P. Sane1 National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bangalore 560065, India Edited by M. A. R. Koehl, University of California, Berkeley, CA, and approved December 16, 2014 (received for review June 30, 2014) The spectacular success and diversification of insects rests critically unclear, especially in its ability to mediate rapid wing movement. on two major evolutionary adaptations. First, the evolution of flight, Moreover, faster wing movements require rapid sensory motor which enhanced the ability of insects to colonize novel ecological integration by the insect nervous system. The hind wings of habitats, evade predators, or hunt prey; and second, the miniatur- Diptera have evolved into a pair of mechanosensory halteres that ization of their body size, which profoundly influenced all aspects of detect gyroscopic forces during flight (10–13). The rapid feed- their biology from development to behavior. However, miniaturi- back from halteres is essential for flies to sense and control self- zation imposes steep demands on the flight system because smaller rotations during complex aerobatic maneuvers (13–15). In the insects must flap their wings at higher frequencies to generate majority of flies, the bilateral wings move in-phase, whereas sufficient aerodynamic forces to stay aloft; it also poses challenges halteres move antiphase relative to the wings. This relative co- to the sensorimotor system because precise control of wing ordination between wings and halteres is extremely precise even kinematics and body trajectories requires fast sensory feedback. at frequencies far exceeding 100 Hz. How do wings and halteres These tradeoffs are best studied in Dipteran flies in which rapid maintain precise coordination at such rapid frequencies? There mechanosensory feedback to wing motor system is provided by are two principal hypotheses to address this question. First, as halteres, reduced hind wings that evolved into gyroscopic sensors. suggested by Pringle (13) in his pioneering studies on halteres, Halteres oscillate at the same frequency as and precisely antiphase the wings and halteres, although driven by independent set of to the wings; they detect body rotations during flight, thus myogenic muscles, may be mechanically coupled. Second, be- providing feedback that is essential for controlling wing motion cause haltere sensory feedback influences wing motor neurons during aerial maneuvers. Although tight phase synchrony between (14), it may also be required to drive the precise coordination of halteres and wings is essential for providing proper timing cues, the wing and haltere motion. To address these questions, it is nec- mechanisms underlying this coordination are not well understood. essary to understand the contribution of thoracic mechanics and Here, we identify specific mechanical linkages within the thorax its role in modulating wing kinematics through the wing hinge. Here, we show that the biomechanics of the thorax and wing that passively mediate both wing–wing and wing–haltere phase hinge is essential for wing–wing and wing–haltere coordination, synchronization. We demonstrate that the wing hinge must possess as well as in mediating the independent control of each wing. a clutch system that enables flies to independently engage or dis- engage each wing from the mechanically linked thorax. In concert Results with a previously described gearbox located within the wing hinge, To address these questions, it is first necessary to clearly visualize the clutch system enables independent control of each wing. These the moving halteres in rapidly flapping insect. Hence, we studied biomechanical features are essential for flight control in flies. these questions in the soldier fly, Hermetia illucens, because their naturally white halteres could be easily visualized during flight insect thorax | halteres | insect wings | wing hinge | wing clutch (SI Materials and Methods, Fly-Rearing Procedure). Under teth- ered and untethered conditions, soldier flies synchronously flap rom giant Atlas moths (wingspan ∼30 cm) to microscopic their two wings in-phase, whereas their halteres move antiphase Fwasps (wingspan ∼400 μm) (1), flying insects span nearly three orders of magnitude in body size. Smaller insects typically Significance flap their wings at frequencies that often exceed 100 Hz, thereby limiting the ability of their nervous system to exercise stroke-to- Insect wing movements must be both precise and fast. This stroke nervous control (2). The insect musculoskeletal system has requirement is especially challenging in smaller insects whose evolved several adaptations that enable high wing-beat frequen- flapping frequencies exceed 100 Hz, because the nervous sys- cies. Key among these adaptations is the evolution of specialized tem cannot exercise stroke-by-stroke control at such rates. In myogenic or asynchronous flight muscles in combination with the flies, the hind wings have evolved into halteres, gyroscopic indirect flight muscle (IFM) architecture in insects of the order sense organs that oscillate exactly antiphase to wings. We Diptera, Coleoptera, some Hymenoptera, and Hemiptera (Fig. show that wing–wing and wing–haltere coordination at high 1A). Asynchronous muscles are stretch activated, which means frequencies is mediated by passive biomechanical linkages in that they are primarily activated by externally imposed stretches thorax. This system requires a clutch mechanism in the wing due to thoracic deformation (3), although periodic neural stimu- hinge to independently engage each wing with the vibrating lation is required to maintain the calcium levels for muscle con- thorax. Once the wings are engaged, the gearbox modulates tractility; thus, cycle-by-cycle activation of their motor neurons is the amplitude of each wing. Thus, the force transmission not necessary in these muscles (4). Contraction of dorsoventral mechanism from thorax to wings in flies bears remarkable EVOLUTION muscles causes extension of the antagonistic dorsolongitudinal similarity to automobile transmission systems. muscles and vice versa, thereby setting up resonant oscillations of the thoracic cavity, which are then translated via a complex wing Author contributions: T.D. and S.P.S. designed research; T.D. and A.K.S. performed re- hinge into large-amplitude wing movements (3, 5–8). The subtler search; T.D., A.K.S., and S.P.S. analyzed data; and T.D. and S.P.S. wrote the paper. alterations in wing kinematics are actuated by separate sets of The authors declare no conflict of interest. steering muscles controlled by direct input from motor neurons This article is a PNAS Direct Submission. within the thoracic ganglia (4, 9). 1To whom correspondence should be addressed. Email: [email protected]. The kinematic changes are mediated via a complex wing hinge This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. (4), but the function and composition of the hinge remains quite 1073/pnas.1412279112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1412279112 PNAS | February 3, 2015 | vol. 112 | no. 5 | 1481–1486 Downloaded by guest on September 29, 2021 A Anterior Instead, these experiments show that wings and halteres are co- ordinated by mechanical linkages embedded within the thorax (13). Dorsal Scutum Lateral Anterior Dorso-ventral muscles Dorso-longitudinal muscles Wing–Wing Coordination Is Mediated via Linkages Within the Scutellum Scutellar lever arm Scutellum. The Dipteran thorax is subdivided into two parts: Haltere a large anterior scutum, followed by a posterior small scutellum, Sub-epimeral Ridge which is the reduced hind thorax (Fig. 1A). To identify the Haltere muscle linkages within the thorax, we made systematic lesions on the dorsal surface of the scutum or scutellum in live, tethered flies to disrupt strain transfer between wings, and filmed the wing and haltere motion (SI Materials and Methods, Wing–Wing and Wing– – Wing Haltere Coordination Experiments). The wing wing coordination data from these experiments were compared with the control B C right wing right haltere group in which there is near-exact phase synchrony of the flapping left wing right wing 1 wings (Fig. 1C). In a scutum-lesioned group (Materials and Meth- 0.4 ods), we made systematic surgical cuts along the scutum while 0.5 0 keeping the scutellum intact (Fig. S1). If the linkage system is lo- 0 cated within the scutum, the coordination between wings should be -0.4 -0.5 disrupted. However, the wings flapped in-phase similar to controls, -1 suggesting that the linkage was not in the scutum (Fig. 2A). In Wing position (rad) -0.8 140 160 180 200 220 240 contrast, wing–wing coordination in the scutellum-lesioned group Wing/Haltere position (rad) 70 80 90 100 11 0 120 130 Time (msec) Time(msec) of flies was significantly disrupted (Fig. 2B and Movies S4 and S5) (17) but was restored in scutellum-reattached flies, in which the slit D Wing-Wing E Wing-Haltere scutellum was glued by an adhesive (Fig. 2C). Thus, the bilateral 90 90 wings in flies are mechanically linked via the scutellum. Although 120 120 60 60 the wing–wing coordination in scutellum-lesioned flies was se- 150 30 150 30 verely impaired, their halteres maintained antiphase coordina- tion to ipsilateral wings (Fig. 2D) similar to control flies (Fig. 1E), 180 0 180 0 but haltere–haltere coordination was disrupted (Fig. 2E). Thus, 210 330 210 330 240 300 240 300 270 270 A B C Scutellum Scutellum Fig. 1. Wings and halteres are precisely coordinated at high frequencies ≥100 Sham lesioned lesioned reattached Hz. (A) Diagram of Dipteran thorax in lateral and dorsal view. (B) The right (gray) and the left (black) wings move in phase with each other. (C)Thewing (black) and the ipsilateral haltere (gray) move antiphase to each other. (D) Wing-Wing Wing-Wing Wing-Wing – 90 90 90 Vector strength representation of control data for wing wing phase (teth- 120 60 120 60 120 60 ered flies; n = 6; mean phase angle ϕ = 5.63°, vector length r = 0.9965; P < 150 30 150 30 150 30 0.001; nonparametric Moore’s test for uniformity).