A dragonfly wing consists of membranes and both longitudinal and cross veins.We observed the microstructure cross-section at several locations in the dragonfly wing using environmental scanning electron microscopy(ESEM).The organic nature of the junction between the vein and the membrane was clearly identifiable.The membrane was divided into two layers,the upper epidermis and the lower epidermis.These layers extend around the sandwich structure vein,and combine with the adjacent membrane at a symmetrical location along the vein.Thus,we defined this as an organic junction between the vein and the membranes. The organic junction is able to form a tight corrugation angle,which dramatically increases both the warping rigidity and the strength of the wing,but not the torsional rigidity.The torsional deformation is primarily controlled by the microstructure of the longitudinal veins,and is based on the relative rotation angle between the epidermal layer and the inner layer of the vein that forms the zigzag section.
In this paper, the natural structures of a dragonfly wing, including the corrugation of the chordwise cross-section, the sandwich mierostructure veins, and the junctions between the vein and the membrane, have been investigated with experimental observations, and the morphological parameters of these structural features are measured. The experimental result indicates that the corrugated angle among the longitudinal veins ranges from 80° to 150°, and the sandwiched microstructure vein mainly consists of chitin and protein layers. Meanwhile, different finite element models, which include models I and I＊ for the planar forewings, models II and II＊ for the corrugated forewings, and a submodel with solid veins and membranes, are created to investigate the effects of these structural features on the natural frequency/modal, the dynamical behaviors of the flapping flight, and the deformation mechanism of the forewings. The numerical results indicate that the corrugated forewing has a more reasonable natural frequency/modal, and the first order up-down flapping frequency of the corrugated wing is closer to the experimental result （about 27.00 Hz）, which is significantly larger than that of the planar forewing （10.94 Hz）. For the dynamical responses, the corrugated forewing has a larger torsional angle than the planar forewing, but a lower flapping angle. In addition, the sandwich microstructure veins can induce larger amplitudes of torsion deformation, because of the decreasing stiffness of the whole forewing. For the submodel of the forewing, the average stress of the chitin layer is much larger than that of the protein layer in the longitudinal veins. These simulative methods assist us to explain the flapping flight mechanism of the dragonfly and to design a micro aerial vehicle by automatically adjusting the corrugated behavior of the wing.
