Hetrodini Brunner von Wattenwyl, 1878

Heller, Klaus-Gerhard, Chobanov, Dragan, Warchałowska-Śliwa, Elżbieta & Hemp, Claudia, 2022, Review of song patterns and sound production in armoured ground crickets (Orthoptera: Tettigoniidae: Hetrodini) with karyological data and taxonomic notes, Zootaxa 5120 (4), pp. 451-481 : 453-460

Hetrodini Brunner von Wattenwyl, 1878 stat. nov

The 14 genera of Hetrodini have been divided into five groups (subtribes): Acanthoplina Ebner & Beier, 1964 , Anepisceptina Schmidt, 1998 , Enyaliopsina Weidner, 1955 , Eugastrina Karsch, 1887 and Hetrodina Brunner von Wattenwyl, 1878 . Despite some reservations, we follow this conventional classification.

Surprisingly, the internal genitalic organs of the males did not attract much attention (except in Enyaliopsis ; Glenn 1991) in taxonomic studies. This may be partly due to the fact that many studies were made before the importance of this character was known, and the titillators are difficult to prepare, but partly also because in some genera these structures do not seem to be sclerotized. In Eugaster and Eugasteroides we could not find distinct sclerotized elements, whereas, in other genera they are quite large (see Specific part).

Bioacoustics

Male tegmina. As in almost all tettigonioids, Hetrodini produce their songs by rubbing the stridulatory file on the lower side of the left tegmen against the scraper, the amplified edge of the right tegmen. In all species studied, left and right tegmen were quite similar in shape ( Fig. 2 ). They do not show structures adapted for sound propagation or amplification like e.g., glossy mirror cells. Species differ in the size of the tegmina and in details of the venation but are in general quite uniform ( Fig. 3 ).

Stridulatory files. In all specimens studied by us and by others ( Grzeschik 1969) the left tegmen was nearly always placed above the right tegmen as it is typical for tettigonioids. In Eugaster only 4 out of 237 males had a reversed position, but also these males were able to sing ( Grzeschik 1969). The left file carried also more and stronger teeth and was longer than that of the right tegmen ( Grzeschik 1969, Glenn 1991). The tooth number in the left file varied between 20 and 65, the length of the file between 1.4 and 4.3 mm, and the intervals between the teeth in the middle of the file between 32 and 136 µm ( Table 1 ; see Fig. 4 for examples). The intervals between the teeth were distinctly larger in the two Acanthoplus species than in all other investigated species.

Amplitude pattern. All Hetrodini species studied until now produce non-resonant songs with each syllable consisting of a series of distinct impulses ( Fig. 7 , 8 , 10 ; see also e.g., figures in Kowalski & Lakes-Harlan 2013). However, in recordings containing many echoes these impulses can often not be recognised as separated units. According to the grouping of the syllables, the songs can be subdivided into three categories. Some species produce very long (sometimes lasting more than one hour), uninterrupted sequences of syllables (trilling species; Fig. 5A ). Other species have also songs containing series of syllables lasting several seconds, but these sequences and their intervals are typically variable in duration. There is a more or less continuous transition from nearly trill-like sequences ( Fig. 5B 1 ) to short groupings or even single series separated by large gaps ( Fig. 5B 2 , 3 ). The third group of species differs clearly from the preceding ones showing short groups of syllables (echemes) with intervals in a similar range, both quite constant in their duration ( Fig. 5C ). The echemes contained 7 to 31 syllables depending on the species. The syllable repetition rate of the species studied here varied between 7 and 44 Hz ( Table 2 ; all song types).

In all species studied, there seems to be only one type of hemisyllable; the sound is probably produced during the closing movement of the tegmina only. Accordingly, the steep side of the teeth is directed towards the wing edge (in SEM photos of Conti & Viglianisi 2005). Opening hemisyllables are missing or are very weak ( Fig. 7I , 8G ).

Spectra. The power spectra of the male songs show one broad peak with the maximum always in the high audio range, between 9 and 19 kHz ( Fig. 6 , Table 2 ). The bandwidth 10 dB below peak ranged from 4 to 12 kHz. In interspecific comparisons, there were no obvious correlations between any morphological characteristics (body size, pronotum length, tooth number, file length, inter-tooth intervals) and peak frequency (r 2 =0.01–0.09; p=0.11–0.63) or bandwidth (r 2 =0.00–0.08; p=0.33–0.68; see Table 1 and 2 ).

Song intensity. The loudness of the song is an important parameter that determines the range of the song to conspecifics and predators, depending also on spectral characteristics of the sounds and of the ears of the receivers. It is rarely measured because the procedure requires special equipment. In Hetrodini, Kowalski & Lakes-Harlan (2010) presented data for one species, and here another three are added ( Table 3 ; all measured or calculated for a singing male in 1 m distance). Our new data may have a tendency for underestimating the loudness because the equipment was not sensitive for ultrasonic frequencies (>20 kHz). The much lower values of Acanthoplus longipes were obtained with equipment sensitive to 10 kHz only according to its datasheet.

Syntopic occurrences. Despite the large number of (sub)species described there is little information which of these forms do coexist at a certain place and how they might manage to live together. In Table 4 we present all of the surprisingly few examples of syntopic occurrences we could find together with data on body size and song characteristics of the species involved.

Chromosomes. A comparison of the karyotype of eighteen Hetrodini species/taxa revealed differences in the chromosome number (2n), the chromosome morphology (including X and Y chromosomes), the fundamental number of chromosomes arms (FN), the sex chromosome system, and C-banding pattern (C-band/C-block). The information presented applies to both new and previously published data ( Fig. 13 ; Mbata 2005, Warchałowska-Śliwa & Bugrov 2009, Grzywacz et al. 2015, Warchałowska-Śliwa et al. 2015). The examined males had from 29 to 17 chromosomes and one of two sex determination systems: the classical X0 as well as neo-XY. Males of four Hetrodini species belonging to two different subtribes ( Enyaliopsina : Enyaliopsis bloyeti ; Eugastrina : Eugaster —2 species and Eugasteroides loricatus ) have a similar karyotype characterized by 2n = 29 and acrocentric chromosomes including the X chromosome, the largest element in the set. In Hetrodes pupus (Hetrodini) , Enyaliopsis ephippiatus , Enyaliopsis spec. 2 Mpwapwa and Gymnoproctus (both Enyaliopsina ) the complement is reduced to 2n = 27 (all chromosomes acrocentric) as well as in Cosmoderus (Enyaliopsina) and Acanthoplus (Acanthoplina) to 25 with one or two bi-armed pairs of autosomes and an acrocentric or bi-armed X chromosome. The lowest chromosome number, 2n = 17 was found in Spalacomimus magnus (Eugastrina) with five bi-armed autosomes. All the abovementioned taxa show X0 sex determination system. On the other hand, a neo-XY sex chromosome mechanism was also observed in two subtribes: in the genera Enyaliopsis [ E. jennae (2n = 28) and E. carolinus (2n = 26)] and in Spalacomimus [ S. talpa and S. verruciferus (2n = 24) with different morphology of the neo-sex chromosomes]. After C-banding, chromosome regions showed quantitative variation in constitutive heterochromatin blocks among species and genera. For detailed information see Specific part.

Figures

FIGURE 1. General appearance of some Armoured Ground Bush-crickets. A Enyaliopsis spec. near ephippiatus (Mwala Hill), B Gymnoproctus rammei, female (Simbo Forest), C Spalacomimus magnus (Gulwe), D Spalacomimus spec. near verruciferus m (Lossogonoi). Photos by C. Hemp

FIGURE 2. Left and right male tegmina of A Cosmoderus femoralis (CH8627), B Enyaliopsis bloyeti (CH8853), C Eugaster guyoni (CH0656), D Spalacomimus spec. near verruciferus (CH7898). Scale 5 mm.

FIGURE 3. Left male tegmina of Acanthoplus discoidalis (CH8946), Acanthoproctus cervinus (CH8947), Hetrodes pupus (CHelb8948), Enyaliopsis bloyeti (CH8853), E. carolinus (CH8625), E. ephippiatus (CH7737), E. jennae (damaged; CH8267), Gymnoproctus rammei (CH8763), Eugasteroides loricatus (CH7352), Spalacomimus stettinenesis, S. talpa (CH7354), S. verruciferus (CH7188). Scale 5 mm.

FIGURE 4. Stridulatory files on the underside of the left tegmen (wing articulation at the right) of A Acanthoplus discoidalis (CH8946), B Cosmoderus femoralis (CH8627), C Eugasteroides loricatus (CH7352), D Spalacomimus talpa (CH7354). Scale 1 mm.

FIGURE 5. Oscillograms of song types in Hetrodini. Song consisting A of long trills (Cosmoderus femoralis CH8627), B of broken trills (including transitions to long echemes; B1 Spalacomimus talpa, B2 Enyaliopsis jennae, B3 Enyaliopsis ephippiatus) or C of regularly repeated echemes (C1 Spalacomimus spec. near verruciferus, C2 Eugaster guyoni). 1-min-sections.

FIGURE 6. Power spectra of male calling songs in Hetrodini species

FIGURE 7. Oscillograms of the calling songs in the genera Acanthoplus, Gymnoproctus, Acanthoproctus and Hetrodes (figures of Acanthoplus, Acanthoproctus and Hetrodes based on figures and data from literature; see text). A Acanthoplus discoidalis, B–C Acanthoplus longipes, D Gymnoproctus rammei, E Gymnoproctus sculpturatus, F Gymnoproctus spec., G Acanthoproctus cervinus, H Acanthoproctus diadematus, I Hetrodes pupus. Left column overview (5-s-section), right column detail (250-mssection).

FIGURE 8. Oscillograms of the calling songs in the genera Cosmoderus and Enyaliopsis (figures of E. iaculator based on figures and data from literature; see text). A Cosmoderus femoratus, B Enyaliopsis bloyeti. C Enyaliopsis carolinus, D Enyaliopsis ephippiatus, song type A, E Enyaliopsis spec. 1 (near ephippiatus), song type B, F Enyaliopsis iaculator, G Enyaliopsis jennae, H Enyaliopsis spec. 2 Mpwapwa. Left column overview (5-s-section), right column detail (250-ms-section).

FIGURE 10. Oscillograms of the calling songs in the genera Eugaster, Eugasteroides and Spalacomimus (figures of S. liberianus based on figures and data from literature; see text). A Eugaster guyoni, B Eugaster spinulosa, C Eugasteroides loricatus, D Spalacomimus liberianus, E Spalacomimus magnus, F Spalacomimus stettinensis, G Spalacomimus talpa, H Spalacomimus verruciferus, I Spalacomimus spec. near verruciferus.

FIGURE 13. Chromosome characters in the genera of Hetrodini: the diploid number (2n), followed by the fundamental number (FN) of chromosome arms (in brackets) and the sex determination system. At the left a specimen of Eugasteroides loricatus (photo by C. Hemp).