As discussed by Hemp et al. (2010), the call of Acrometopini is often complex, making it difficult to apply the traditional terminology of syllables and echemes to describe it. For this reason, it is preferable to use the term ‘element’ for a more or less fixed sequence of sound events, without attempting to distinguish the sections of the call are the products of full closing or opening of the stridulatory structures.
H. serrifera males call from elevated perches and every few minutes fly to another perch, usually a tall herbaceous plant, and start a new bout of calling. The call, lasting on average 1.6–3.0 seconds, is divided into two sections, each with a different frequency spectrum (Fig. 52G). The first section is a uniform train of similar, short syllables produced at the rate of 60–80/sec; mean syllable duration is 0.0094 s (SD=0.001325, n=75); the peak fre- quency of this section of the call is 4.7–9.9 kHz (Figs. 52H, I). The first section, which lasts approximately 0.1–1.0 s, is immediately followed by a more complex section with multiple elements of various duration; some elements resemble long echemes with similar peak frequency as the first section, while others are short clicks (mean duration 0.011 s) with the peak frequency of 12.9–14.0 kHz (Figs. 52H, I). [1]