Free Flight of Aphids in the Laboratory

Much has been learned of the kinematics and aerodynamics, the metabolism and even the sensory and nervous physiology of insect flight from observations on tethered specimens, preferably in a wind tunnel or on a roundabout to provide a relative wind simulating that created by free flight (Pringle, 1957). Such devices were developed to a high pitch of refinement by Weis-Fogh (1956) for locusts and have proved invaluable for studying what he called 'the basic flight process itself' in a number of insects. Cockbain (1961a) mounted aphids (Aphis fabae Scop.) on pins in a wind tunnel and showed that they resembled locusts (Weis-Fogh, 1952a) in consuming carbohydrate initially but fat mainly, during a long flight, in accordance with Weis-Fogh's (19526) theoretical expectation for migrant insects in general. Tethered flight, however, whether in still air ('tethered flapping': Weis-Fogh, 1956) or in an appropriate relative wind, does not provide an adequate approximation to free flight when some other aspects of it are under study. Thus Cockbain (1961 b) noted that his fixed aphids continued 'flying' for considerably longer than free aphids were estimated to fly in the field. When flight movements are more or less forced, by deprivation of tarsal contact and other devices, then fuel can be studied because it has become the limiting factor, but the factors limiting free flights naturally cannot be studied. Again, when the purpose is to study not the flight process in itself but the interaction between flight and alternative behaviour patterns, the use of tethered insects may distort the results owing to abnormal sensory inputs both during the fixed flapping and when the insects are transferred from the suspension to some test situation. Moericke (1941,1955), Miiller & Unger (1951,1952) and Miiller (1953) deduced from general observations in cages and in the field that a long flight must intervene before young winged adult aphids will respond to a host plant by settling down, feeding and larvipositing upon it. Correspondingly, most specimens of Aphis fabae and other species alighting on host plants, and virtually all alighting on non-hosts, in the field, have been observed to take off again very promptly (references in Kennedy, Booth & Kershaw, 1959, p. 438). B. Johnson (1958) remarked that it was very unlikely that such field alighters had been air-borne for less than 30 sec.; nevertheless, he found that 20—30 sec. of tethered flapping were enough to make A. fabae respond well when placed on leaves of both hosts and non-hosts. He obtained a similar effect from brief flights by aphids that were not tethered but mechanically interfered with in various ways. It seemed that the interference may have encouraged premature settling in some unknown manner.