Unusual Phase Relationships between the Forewings and Hindwings in Flying Dragonflies

Flying insects can generally be divided into two groups: 'primitive' orders with forewings and hindwings that move independently (for example, Odonata, Orthoptera, Isoptera) and more 'advanced' orders with wings that are functionally one pair, with the foreand hindwings in contact so as to function as one wing (for example, Hymenoptera, Lepidoptera, Homoptera), or with only one pair of wings that functions primarily as a lifting surface (for example, Diptera, Strepsiptera). Some insects in more primitive orders, such as locusts, maintain a fixed phase relationship between their forewings and hindwings with the hindwings leading slightly (Chadwick, 1953; Weis-Fogh, 1956; Wilson, 1968). Dragonflies normally beat their forewings and hindwings out of phase, with the hindwings about a half stroke ahead of the forewings, i.e. in antiphase (Chadwick, 1940; Neville, 1960; Alexander, 1982). Many textbooks state that the antiphase relationship is more efficient (Chadwick, 1953, p. 582; Wigglesworth, 1972, p. 161; Romoser, 1973, p. 162). Wilson (1968), however, suggested that individual dragonflies may exhibit large variations in this phase relationship. In this study, I have made high-speed cine films of dragonflies flying in a wind tunnel; the dragonflies were free to turn about their vertical and longitudinal axes. From these films, I assessed the forewing-hindwing phase relationship in straight flight, turning flight, and flight requiring larger than normal aerodynamic forces. The techniques were as previously described (Alexander, 1982, and in preparation) and may be summarized as follows. Field-caught dragonflies were flown in an opensection wind tunnel. Most were flown on a tethering system that allowed them to yaw (turn about their dorsal-ventral axis) or roll (turn about their anterior-posterior axis), but a few were flown without a tether. The tether was attached to the posterio-ventral surface of the thorax so as to minimize interference with the wing articulations. Several dragonfly species were used, primarily Libellula luctuosa and Celithemis elisa. The animals were filmed in flight with a LOCAM II (Redlake Corporation) 16mm, high-speed cine camera at film speeds of 400 to SSOframess". Each film sequence consisted of the dragonfly performing a turn or a continuous series of turns, and a sequence ended with the dragonfly leaving the field of view of the camera or stopping its flapping. The films were analysed by projecting them at a low frame rate with an L-W International Mark V film analyser. For each sequence, I recorded the