Fig. 2. The effect of NO on the cycle frequency of the digging rhythm. (Ai) The bath application of 20 mmol l^–1 L-arginine caused a statistically significant increase in the oviposition cycle frequency (t=2.44, ^*P<0.05, mean ± s.e.m. from eight animals). (Aii) An example showing the effect of a 10 min bath application of 20 mmol l^–1 L-arginine on the ovipositor valve opener muscles. The number of cycles of muscle activity increased from a control value of 5 to 6 after L-arginine application within the same time period. After a 30 min wash with locust saline, the number of cycles of opener muscle activity returned to the control value. (Bi) The effect of the NO donor PAPANONOate (PNONO) on the cycle frequency. A 10 min bath application of 0.2 mmol l^–1 PAPANONOate caused a statistically significant increase in the cycle frequency (t=–2.82, P<0.01) (^*=P<0.05). Mean ± s.e.m. of five animals. (Bii) An example showing a 10 min bath application of 0.2 mmol l^–1 PAPANONOate resulted in a significant increase in the number of cycles of muscle activity within the same time period. A 30 min wash with locust saline resulted in the digging rhythm partially returning to its original cycle frequency.