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Introduction
Dragonflies fly in a highly maneuverable manner with their four flapping wings. They are capable of forward flight, backward flight, hovering, reversing direction instantaneously and accelerating rapidly. During flight, the beating wings accelerate the surrounding air to produce sufficient lift and propulsion [1,2] . Many previous researches focused on the aerodynamic performance of the flapping wings. The high-lift generation [3-7] , flight efficiency [8-11] and stability [12,13] of the wings were comprehensively investigated. Both the motion of a wing and its three-dimensional shape contribute to the aerodynamics in flight. The insect wings are mainly passive structures and detailed studies of the structural features are necessary in determining the dynamic responses in the interaction with air. In addition, the understanding of aerodynamics of insect wings in turn facilitates the design of flapping-wing micro air vehicles.
Accounting for only 1-2% of the total body mass, the highly corrugated wings possess a high load-bearing capacity and structure stabilizing function. The pleats vary both along the spanwise direction and chordwise direction, increasing the flexural rigidity throughout the wing [14-18] . Combes and Daniel [19,20] studied the relationship between the venation pattern and the wing flexibility through finite element method (FEM) by calculating the displacement along the wing in response to a point force. Kesel et al. [21] developed several finite element models to study the expenditure of material in constructing the dragonfly wing. The effects of sandwich structure of veins on mechanical properties of dragonfly wing were studied by Wang et al. [22] . Rajabi et al. [23] investigated the microstructural and morphological aspects of dragonfly wings and determined the natural frequencies and mode shapes by FEM.
In almost all the numerical researches, the veins of dragonfly wing were considered as empty tubular structures with nothing inside. In our opinion, however, it is unreasonable, because the blood is not included in these researches. The biologist Arnold [24] introduced the blood circulation in some insect wings systematically and indicated that the veins of dragonfly wings are generally pathways for the blood, the tracheae as well as the nerves. Several distinct flow patterns were distinguished, involving longitudinal veins, cross-veins, and ambient veins. Based on this, the pattern of blood flowing in the right forewing of a C.servilia Drury is...