Abo Bibliothek: Guest
TSFP DL Home Archive Vorstand

THE AIR FLOW GENERATED BY A FLYING PREY INSECT AROUND A WANDERING SPIDER AND ITS MOTION SENSING HAIR SENSILLA

Christian Klopsch
Department of Neurobiology and Cognition Research University of Vienna Althanstr.14, A-1090 Wien, Austria

Joseph A. C. Humphrey
Department of Mechanical and Aerospace Engineering University of Virginia Charlottesville, VA 22904, USA

Friedrich G. Barth
Department of Neurobiology and Cognition Research University of Vienna Althanstr.14, A-1090 Wien, Austria

Abstrakt

Spiders use cuticular filiform hairs (trichobothria) to detect air motions generated by flying prey and predators. The wandering spider Cupiennius salei has about 900 such hairs on its pedipalps and legs. Their length and base diameters range from 100 µm to 1400 µm and 5 µm to 10 µm, respectively, and hairs of different length are arranged in characteristic clusters. Using an optically transparent test section for digital particle image velocimetry measurements, we analyzed the flow field generated by a flying prey (Calliphora erythrocephala) around Cupiennius salei. The humming fly was placed in biologically important positions with respect to the spider. The flow fields ahead of the fly showed velocities detectable by the trichobothria from a distance of at least 13.2 cm. Since their frequency content differs significantly from that of background flow they may well be used by the spider as signals indicating the fly's approach. Flow patterns change with the fly in different positions. The closer the fly was to the spider the more the absolute values for peak velocity and turbulence intensity increased. The peak velocity and the turbulence intensity above the fourth walking leg (WL4) changed from 5.11 mm/s and 29 % with the fly approaching the spider and 10 cm away from its center to 13.84 mm/s and 97 % (only 5 cm away). As the trichobothria do not respond to constant air velocity but to its fluctuations (Barth and Holler 1999), the rms values and the turbulence intensity are of major interest. The increasing turbulence intensity may well indicate to the spider that a fly is approaching. When comparing the peak velocities and the turbulence intensity right above the tarsi of each walking leg the highest values were measured above the walking leg closest to the fly (see WL4). The values decreased for the walking legs further away. Thus above WL1 and with the fly approaching the spider from behind and 5 cm away from its center the values for peak velocity and turbulence intensity were only 2.68 mm/s and 63 %. Considering the differences at the eight legs of the spider these may well be used as indicators of the azimuthal direction towards the prey.