Flight and flight control by the antennae in the Small Tortoiseshell (Aglais urticae L., Lepidoptera)

Abstract

1. In the Small Tortoiseshell (Aglais urticae L.), flying tethered on a flight balance in front of a wind tunnel (Fig. 1), different kinematic and aerodynamic flight variables were recorded under ‘closed loop’ conditions, i.e., when the butterfly's drag was compensated by its thrust. The wings are moved synchronously and nearly in a vertical plane (Fig. 2). Both ‘flight speed’ (in relation to the air) and lift depend on body angle (Fig. 3). Wing-beat frequency, wing-stroke angle, lift and flight speed do not vary significantly with flight duration in normal insects. Amputation of one flagellum does not influence this normal flight behavior. If both flagella are cut off, these variables remain independent of flight duration, but wing-beat frequency, wing-stroke angle and flight speed are increased, and lift is decreased relative to normal (Fig. 4). 2. Flight variables were also measured under ‘open loop’ conditions, i.e., air speed of the wind tunnel was changed stepwise between 0 and 2.5 m/s. In normal animals, wing-beat frequency and lift increase with increasing air speed, whereas wing-stroke angle and horizontal force (= thrust — drag) decrease simultaneously (Fig. 5). After cutting off the flagella, wing-beat frequency, wing-stroke angle and horizontal force increase with respect to normal, and lift decreases. In normalAglais, the lift is positively correlated with wing-beat frequency, but negatively correlated with wing-stroke angle and horizontal force (Fig. 6). 3. The antennal angle during flight is about 43 ° and independent of air speed up to 2.0 m/s (Fig. 7). Under normal flight conditions, the passive antennal deflection is below 0.2 ° (Fig. 8).