Travelling air vortex rings as potential communication signals in a cricket

Abstract

1. Crickets of the non-stridulating speciesPhaeophilacris spectrum (Phalangopsidae) generate travelling air vortex rings, which might serve as a hitherto unknown method of non-acoustic communication involving air particle movements. 2. The males produce a series of forward wing flicks during courtship and single wing flicks during aggressive encounters (Figs. 1, 2). In simulation experiments with moving wing pairs (Fig. 3) travelling vortex rings were visualized by using TiCl4 smoke or tracer particles in the air flow field (Fig. 4). 3. The vortex rings have an initial diameter of about 2 cm, increasing to 5 cm, and travel a maximum distance of 15 cm, depending on wing size and wing flick duration. Both wing size and flick duration determine the Reynolds number (Fig. 6). 4. Generally the velocity of propagation of vortex rings (from an initial velocity of about 40 cm/s) decays exponentially with distance (Fig. 5B). 5. The internal structure of the vortex rings, as visualized with tracer particles (Fig. 8), reveals a typical rotational velocity field. Particle velocities approach 15 cm/s (Fig. 9). 6. The physical basis of vortex ring generation by wing flicks and their possible communication role are discussed.