Fig. 2. Types of seismic signals. Top panels (i) show body positions, with numbers (1-5) illustrating movements of the forelegs and abdomen. Middle panels show (ii) the position of one of the forelegs (mm above the substrate) and (iii) the oscillograms of the seismic signals. Bottom panels (iv) show the frequency characteristics of the seismic signals. Panels ii-iv are shown in the same time scale, with numbers (1-5) corresponding to the body movements illustrated in panel i. (A) Thump signal. Front legs come down (1-2), contact the substrate and quickly move back up (2-3). Shortly afterwards the abdomen is pulled back and released, and the abdomen `rings' at 58.3 Hz (4-5). Thumps are broadband signals with peak frequencies at 203 Hz and 1203 Hz. Production of signal corresponds with the percussive contact of the front legs against the substrate (1-2) and movements of the abdomen (4-5). (B) Scrape signal. Abdomen moves up (1-2) and shortly afterwards the front legs come down (2-3). Scrapes occur in groups with a frequency of 5.7 Hz. Scrapes are broadband signals with peak frequencies at 230 Hz and 550 Hz. Production of seismic signal corresponds to movements of the abdomen. (C) Buzz signal. Front legs come down (1-2) as the abdomen oscillates at 65 Hz (1-2). This signal has a fundamental frequency at 65 Hz with several harmonic frequencies (130 Hz, 195 Hz and 260 Hz). Production of seismic signal corresponds with movements of the front legs and abdomen.