Biology and physics of locust flight. I. Basic principles in insect flight. A critical review

Abstract

Natural flapping flight is a complicated type of locomotion; it involves a large number of parameters and demands a higher rate of energy consumption than any other known achievement within the animal kingdom. The extensive literature provides a diffuse and sometimes misleading picture of the mechanics and energetics of flight. The purpose of this critical review is twofold; first, an attempt is made to reveal how far contemporary knowledge has advanced towards a quantitative understanding of the basic physical events which result in flight; secondly, the conceptual analysis and the conclusions arrived at serve as a general introduction to a detailed experimental study of the flight performance of the desert locust ( Schistocerca gregaria Forskal). The results of this study will be published as a series of mutually dependent papers which are referred to as parts, the present being part I; their content is indicated by the general title ‘Biology and physics of locust flight’. The abstracts at the end of the major sections can be read as a continuum and independent of the remaining text. Most qualitative principles in insect flight are fairly well understood, although some authors have claimed that the observed movements did not permit flight according to ordinary aerodynamic principles. The data upon which these calculations are based were, however, found to be inadequate. In general, if one attempts to interpret the observed performance in quantitative terms, a conceptual analysis of the necessary knowledge shows that both the conditions for observation and the measured parameters must be defined more rigorously than has hitherto been the case. A theoretical treatment, partly based upon the experimental evidence in the succeeding parts, reveals that the energy account of a wing stroke comprises at least three independent terms, namely,an aerodynamic term due to the wind forces on the wings, an inertial term due to the acceleration of the wing mass, and an elastic term caused by the elastic deformations of the thorax. Finally, extra-muscular damping within the thorax may be involved but it does not seem to be important compared with the other quantities. The mechanical energy and the forces which the muscles must provide can be estimated by the torque about the wing hinge to which these terms give rise. However, in order to summate the torque contributions their detailed variations during the stroke must be known, and since all of them are extremely sensitive to alterations in the stroke cycle, no existing theory can deal with the energetics of the flight as a whole. One is therefore forced to investigate the wing stroke empirically. An experimental study must take place under rigorously controlled aerodynamic and biological conditions and must include the simultaneous variation in time and space of a considerable number of kinematic and dynamic parameters whose nature is briefly outlined. However, an experimental approach would be much hampered if one had to take unusual aerodynamic (inertal) forces into account. Fortunately, an analysis of existing aerodynamic theories on flapping flight reveals that there is little to be said against and much in favour of considering natural flight as being based upon conventional aerodynamic principles, even in the case of small insects like mosquitoes. At least, unconventional aerodynamics need not be assumed in order quantitatively to explain observed performances, although some authors (cf. Osborne 1951) have reached the opposite conclusion. The assumptions made in the most complete theories (Holst & Küchemann 1941 ; Walker 1925 , 1927 ; Osborne 1951) are discussed in detail and the basic equations have been homogenized to fit the rest of the text. Various theoretical deductions considered of general interest are tabulated (e.g. influence of distance from fulcrum, induced wind) and the theories are tested by inserting values from insect types whose performances are best known. The calculations also gave the order of magnitude of the power necessary to overcome the aerodynamic forces ( = aerodynamic power). This external power certainly claims a considerable fraction of the total metabolic rate. A short review (§6) of the regulation of flight also emphasizes the importance of considering both aerodynamic, inertial and elastic terms as significant simultaneous factors in the energy account and thus in the regulation of the power output; the special mechanisms concerned with the stability in flight fall outside the scope of the paper, however, and will be discussed in part IV .