Fig. 4. Heartbeat reversals in D. hydei (female 6). The pulse sequences of three traces are sampled at 2000 Hz and the different mean levels of transmission are equalized by use of a high-pass filter of 0.5 Hz. (A) Survey of two series of heartbeat periods. (B,C) Shorter time periods with individual heart pulses. Detail from the boxes marked in A. The metachronous delay of the pulse wave is recorded by three sensor elements along the abdominal segment 3 (green), inter-segmentally (black) and segment 2 (red). (B) Anterograde pulses show typically a double peak (arrowheads) or even further peaks (asterisk). (C) Retrograde pulses show only one peak. The peak of highest transmission corresponds to the moment when the heart is maximally distended by the presystolic wave. To illustrate the metachrony of the pulse wave, the maximum peaks are connected (dotted lines) and the resulting intervals are projected as a bar on the x-axis. Green anterograde, red retrograde.