Electrocardiograms (ECGs) and electromyograms (EMGs) from the dorsal musculature of the sucking stomach of tarantulas (Grammastola cala) were recorded simultaneously from unrestrained animals during ingestion. Resting heart rates averaged 33.6±1.9 beats min−1, which increased significantly to an average of 51.6.±3.8 beats min−1 during ingestion. This is not significantly different from the average sucking stomach rate of 53.6±3.9 cyclesmin−1. Activity of the sucking stomach constrictor muscles started with a phase shift of 0.64±0.007 cycles relative to dilator activity. In some cases the stomach dilators were active almost in synchrony with heart systole (phase shift of 0.10±0.009 relative to ECG), and the constrictors were active later in the cycle, during diastole, with a phase shift of 0.71±0.007 relative to the ECG. These data suggest that, during ingestion, fluid flows in the anterior aorta and midgut are negatively coupled as the fluids traverse the pedicel. Although such coupling was observed in about half the traces, other results were obtained that showed no evidence of coupling. Possible mechanisms for coupling and the sequence of events in the pedicel during ingestion are discussed.
Note: Present address: Department of Geology, University of Manchester, Manchester M13 9PL, UK.