Fig. 6. Electromyogram (EMG) recordings of all four sonic muscles during the production of a series of vibration pulses. (A) Lobsters most often produced body vibrations by using the paired remotor and promotor muscles of only one antenna (i.e. left or right) and often alternated between sides. Every lobster tested used both sides in alternation, although in some individuals one side was favored and used most often. (B) Lobsters also used simultaneous contractions of the sonic muscles of both antennae to produce signals. In some cases, both sets of muscles were used during the entire vibration, or, as seen in B, one set was used for only part of the event. Vibrations produced by both sets of muscles were not significantly longer in duration or different in frequency than vibrations produced by one set. Scale: A and B, 20 mV for the top trace (accelerometer), 16 µV for the EMG traces; time scale=200 ms.