Bioacoustics of Hexacentrus unicolor

Redescription. Male: Body medium sized and slender. Antennae with alternate short dark bands apically. Fastigium of vertex triangular, narrow; laterally compressed with a median longitudinal sulcus; apex rounded in lateral view; base of fastigium 1.88 times as wide as scapus; fastigium of vertex separated from the fastigium of frons by a deep furrow. Eyes globular. Pronotum saddle-shaped, dorsal region longer than wide and expanded towards posterior region, slightly depressed behind first transverse sulcus; with three transverse sulci, second and third ones restricted to disc; anterior dorsal margin slightly concave, posterior margin broadly rounded with a slight median excision. An hour glass like band covers the whole pronotum; median carina faintly visible. Pronotal disk flat, expanded, broadly rounded and highly rugose on posterior region; lateral lobes of pronotum inclined; longer than high; humeral sinus absent. Prosternum armed with two long spines; mesosternum and metasternum armed with a pair of short, thick sternite. Thoracic auditory spiracle large, oval, slightly hidden under the lateral lobe of pronotum with a tubercle at mid- anterior edge. Tegmina (narrow in males) longer than hind wings. Tegmen reaching beyond the middle of hind tibia when straight; apex rounded. Stridulatory file with 46 teeth; moderately sulcate; teeth are almost of same length throughout the file except at distal end where it becomes narrow gradually, and abruptly narrow towards proximal end with few teeth. Legs: genicular lobes of fore femora armed with a single spine on inner side and unarmed on outer side; mid genicular lobe armed with a single spine on both inner and outer side; hind genicular lobe armed with bispinose on both side. Fore coxae armed with a forward outwardly projecting spine. All femora dorsally unarmed. Fore femora ventrally armed with 4 to 5 minute spines with small spines each on inner margin and outer margin. Mid femora ventrally armed with small spines on inner margin; 2–3 minute spines with more minute spines on outer margin. Hind femora ventrally armed with 7–8 medium spines each on inner margin and outer margin. Tympanum on fore tibiae bilaterally closed, tympanal slits facing dorsally with a small, elongated pit below it. Fore tibiae ventrally armed with 6 long, movable, outwardly bowed opposing spines decreasing in length from basally to apically each on inner margin and outer margin. Mid tibiae dorsally armed with 2 small spines each on inner margin and outer margin. Mid tibiae ventrally armed with 6 long, movable, outwardly bowed opposing spines decreasing in length from basally to apically each on inner and outer margin. Hind tibiae dorsally armed with 40–42 spines each on inner margin each on inner and outer margin. Hind tibiae ventrally armed with 10–12 spines increasing in length and density from basally to apically on inner margin, 12–14 spines increasing in length and density from basally to apically on outer margin. Single dorsal pair of spurs and two ventral pairs (inner small and outer large) of spurs present on the apical region of hind tibiae. Ninth tergite feebly excised. Tenth abdominal tergite with deep excision and. Supra-anal plate triangular with basal groove; as long as broad. Subgenital plate long with median longitudinal furrow apically, lateral ridges well developed, apical margin with rounded excision; styles diverging basally, apex slightly incurved. Cerci robust with a small tubercle on internal side, then abruptly narrowing to a long digitiform apical appendage with decurved apex. Female: Tenth abdominal tergite with longitudinal median incision. Supra-anal plate triangular with dorsal groove, apex truncate; Paraprocts with triangular projection at apex. Subgenital plate basally broad, apex with weak median excision. Cerci conical, thick, apex with smooth conical appendage. Ovipositor broad at base; apex narrow; slightly upcurved. Morphometry (length in mm)

The song of Hexacentrus unicolor consists of short chirps (approximately 50—100ms long) presented in a rhythm of about 2Hz (1.6-3.3 Hz). One chirp consists of a crescendoing group of pulses, which follow one another with a frequency of 320-415 Hz. The variation is principally due to different ambient temperatures (22.5-28.5°C). The frequency spectrum shows two narrow maxima at 11 and 33 kHz and two submaxima at 22 and 44 kHz. Data known from studies of other bushcrickets (e.g. Busnel, Busnel & Dumortier, 1956) suggest that during one opening and closing sequence one chirp, comprising a group of pulses, is produced. However, recordings of the wing movements show that this is not the case in this species: with each cycle of movement only one pulse is created. Accordingly the frequency of the wing movements is very high (320-415 Hz, see above). The course of the stridulatory movement is shown in Fig. 2. Fig. 2A shows a long recording of the song, Fig. 2B gives a more detailed account of the generation of a chirp. Some time before the generation of sound, the elytra are moved with very high frequency; only after a short period of 'introductory swinging' does the crescendoing tone start. The pulses always emerge during the second half of the closing movement of the wings (Fig. 2C). At the end of a pulse group the wings remain in an opened position for some time. During the successive closure of the wings an isolated sound pulse is created. Throughout the song the animal is positioned in such a manner that its slightly opened elytra render the alae visible (see fig. 6 of Heller, 1983). The amplitude of the movement is very small and only a vibration is perceptible with the naked eye. An identical wing vibration can be recorded before the song. At this stage the movement is soundless, with almost the same rhythm, frequency and amplitude. The only difference from singing - except for the missing sound production - is that in most cases the elytra spring back without delay to the neutral position at the end of a chirp. The frequency of the wing beat during flight is, as expected, far lower: 22-23 Hz (also measured opto-electronically on two females at 26 °C).