Biology and Physics of locust flight II. Flight performance of the desert locust ( Schistocerca gregaria )

Abstract

The main purpose is to analyze how a number of wing-stroke parameters are related to the lift (average vertical force) and thrust (average horizontal force) produced by the insect under well defined aerodynamic conditions. The locust was suspended from a complicated balance and flew against a uniform horizontal wind from an open-jet wind tunnel. The wind speed was automatically adjusted to the preferred flying speed (air speed), i.e. the speed at which the thrust equals the extra-to-wing drag . The lift was measured as the apparent reduction in weight; it is given as a percentage of the weight which the individual would have if it had flown for about one hour, was full-grown and well fed but, if a female, with undeveloped eggs ( = basic weight). This figure is the relative lift, and it is used because the actual weight changes much with age, feeding, sexual development, etc., while the dimensions of the flight motor remain constant. The angle between the wind and the long body axis is the body angle and was chosen by the observer or by the insect itself. Most experiments took place at 30° C (constant temperature room), but series were run at the upper and lower limits for flight, including experiments with small flocks of locusts suspended from a roundabout. The rate of evaporation of water from the thorax was kept constant. In a large number of individuals sustained steady-state flight was studied; at regular intervals a set of simultaneous readings were taken consisting of the lift, the speed, the body angle, the stroke frequency, the extreme angular positions of the wings, and of the inclination to the vertical of the stroke planes. In addition, the angular movements of the entire wings relative to the body were estimated from slow-motion films. The results are seen in §§4 to 7. The frequency distribution of the relative lift has its maximum about 100 %, showing that, in this respect, the flight comes near to free flight. It varied from 35 to 175 %, i.e. about five times. During continuous horizontal flight the flying speed was 3•5+ 0•1 m/s and may increase to 4•2 m/s in free flight. At larger lifts (climbing) the steady-state speed could reach 4•5 m/s. During the first minutes the speed was often 4•5 to 5•0 m/s, the maximum observed being 5•5 m/s. No locust lifted its own weight at speeds less than 2•5 m/s. The power necessary to overcome the extra-to-wing drag only corresponds to 1 to 3 % of the total metabolic rate. The effect of altering the body angle is fundamentally different from that of altering the pitch of an aircraft; the lift is controlled and kept constant by the locust and proved to be independent of alterations in the body angle amounting to as much as 20°. This is the basis for the technique and for the treatment of the results. In spite of the large variations in lift, the following stroke parameters varied little or not at all: the stroke angles , the stroke-plane angles , the middle position of the wings , and the time course of the angular movement of the entire wing, y = y(t). The latter function deviates considerably from a simple harmonic oscillation. According to figure II, 20, the average points are determined with an accuracy of better than + 1 %, permitting graphical differentiation. The stroke frequency was rather constant but increased with the reflexly controlled lift, contrary to Chadwick’s experiments on Drosophila , and decreased with increasing size, according to Sotavalta’s findings in other insects. The maximal changes were small, however, amounting to 8 % (lift) and 15 % (size) respectively. The flight performance and the stroke parameters were independent of changes in air temperature (no radiant heat) within 25 to 35° G, although the pterothorax is subjected to similar changes. Sustained flight does not take place below 25° C and above 35° G, but short performances were observed between 22 and 24° C as well as above 37° C. The great variation in lift could not be explained by changes in the measured stroke parameters, and by analogy with a variable-pitch propeller, it must be caused by differences in wing twisting 0(r,t). It was also found that lift and thrust varied in a more intricate way than in a simple actuator disk. The regularity of the stroke and its independence of temperature makes it possible to define a standard stroke , making it easy to compare a given performance with the normal.