The sound produced by a to- OR fro- movement of the stridulatory apparatus.
Broughton (1963) took the view that this term should only apply to either of the components of a diplosyllable, preferring the term haplosyllable for syllables where either the to or fro motion was silent. Later authors (e.g. Ragge & Reynolds, 1998) often replace haplosyllable with hemisyllable.
"The sound produced by one unidirectional movement (opening, closing, upward or downward) of the fore wings or hind legs." 
- . The Songs of the Grasshoppers and Crickets of Western Europe. Colchester, Essex: Harley Books; 1998.
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