The genus Dicranostomus belongs to the very few Orthoptera with elongated mandibular processes, here called tusks. However, it is also one of the least studied genera from whose two species only one female and two males have been known so far. We present additional material from both species and sexes that confirms that the males have the relatively longest (2–2.8 times pronotal length) tusks of all Orthoptera. Surprisingly, the females of both species differ in this character: females of D. monoceros have tusks and those of D. nitidus do not. Based on a comparison with other species, we hypothesize that the species use holes that males can defend and use to monopolize the females.

Eucocconotini, mandible, Peru, South America, taxonomy, weapon

In many species of animals, males possess elaborate structures used mainly in intraspecific fights over access to females (Emlen 2008). Among mammals, well-known examples include antlers in deers, horns in bovids, and tusks in elephants. In many, but not all, of these species, the weapons are also found in a reduced version in females. In insects, impressive examples of such structures are observed, e.g., in Lucanidae (stag beetles; see Emlen 2008 for other species).

Such weapons are relatively rare among Orthoptera. There are some species where the mandibles are enlarged only in males (see review in Field and Deans 2001, Gorochov 2012), but only in very few species are these special structures found to be used (or, with good reasons, assumed to be used) in the context of antagonistic intraspecific contacts. Males with tusks are found in five species of the family Anostostomatidae [see Field and Deans 2001; in the New Zealand tusked wetas, a monophyletic group of three species in two genera (Trewick and Morgan-Richards 2004) and in two South African species, in Libanasidus vittatus (Kirby, 1899) and in Libanasa capicola (Péringuey, 1916)]. In the superfamily Tettigonioidea, there are two genera with tusks. Three species of the Neotropical genus Listroscelis Serville, 1831 (Listrocelidinae) have one asymmetrical tusk on the left mandible only (Fialho et al. 2014), while the two known species of the Neotropical genus Dicranostomus Dohrn, 1888 (Pseudophyllinae) have a tusk on both mandibles. Dicranostomus nitidus Brunner von Wattenwyl, 1895, so far known only from two males, has the relatively longest tusks of all Orthoptera, ranging from 2 to 2.7 times the pronotal length (Brunner von Wattenwyl 1895, Gurney 1950). The second species was, until recently, known from a single female that, interestingly, also bears tusks, but which are distinctly shorter than that of male D. nitidus. Having obtained some more specimens of this fascinating genus, including the missing sexes of both species, we herein revise the genus and provide a review of all available data.

All specimens were dried, pinned, and photographed using a CANON EOS 1200D. Photos of cerci and mirrors were taken using a dissecting microscope (OLYMPUS SZ Binocular Stereo Zoom Microscope) and a digital camera (SONY Cyber-shot DSC-P120). The inter-tooth distances were measured as the mean between every eleven teeth (ten intervals), starting at the anal end, using ADOBE PHOTOSHOP Elements 6.

The specimens, collected between 2013 and 2018, were obtained from a trader with an export license (http://gerfor.regionloreto.gob.pe/dublincore/biblioteca/descargar/4269/4500000100020_7.PDF).

Depositories.—

CH Private collection of K.-G. Heller;

C_Helb Private collection of M. Helb;

ISNB Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium;

MZPW Polish Academy of Science, Museum of the Institute of Zoology Warszawa, Poland.

Taxonomy

Family Tettigoniidae Krauss, 1902

Subfamily Pseudophyllinae Burmeister, 1838

Tribe Eucocconotini Beier, 1960

The genus Dicranostomus belongs to the tribe Eucocconotini, which is part of the supertribe Pleminiiti Brunner von Wattenwyl, 1895 (Braun 2015) (or the subfamily Pleminiinae; Gorochov 2012). Within this tribe, the genus is most similar to Gnathoclita Haan, 1843 (see Gorochov 2012), with both genera having the “dorsal surface of anterior tibiae granular or with distinctive tubercles” (Cadena-Castañeda and Monzón-Sierra 2014). According to the key provided by these authors, males of the two genera are easily separable by their mouth parts: Dicranostomus males have tusks and Gnathoclita males have enlarged mandibles. The females, however, differ only in the presence of dorsal spine(s) on the middle tibia in Dicranostomus. Only D. monoceros females have tusks like their males. Possibly, both genera can also be separated by the presence of an elongated (either horizontally or vertically) fastigium frontis in Dicranostomus. Such a structure is not described nor figured for any Gnathoclita (s.str.; sensu Gorochov 2018) species [see G. izerskyi Gorochov, 2018, G. peruviana Carl, 1921 (Gorochov 2014), G. laevifrons Beier, 1960, G. sodalis Brunner von Wattenwyl, 1895, and G. vorax (Stoll, 1813) (Beier 1960)], although data for females are sparse.

Dicranostomus and Gnathoclita are also similar in the shape of their mirrors (compare Fig. 4 to figs 20, 21, 27, 28 in Gorochov 2018, and to fig. 7D in Hugel 2019) and—to a limited extent—in their stridulatory files. In tooth number, Dicranostomus (130 teeth) is situated between the two known Gnathoclita species (G. vorax c.101 teeth (Hugel 2019), G. sodalis 217 teeth (Montealegre-Z and Morris 1999)). The inter-tooth distances in both species (G. vorax 30 µm; G. sodalis 10 µm) correlate negatively with the carrier frequencies of their resonant songs (G. vorax, 8.8 kHz; G. sodalis, 16 kHz). From these data, it can be assumed that Dicranostomus males also sing in this audio range (but perhaps with each species at a different peak frequency) and that they can be located with unaided ears.

Unfortunately, no data are available concerning the function of the most distinctive structure of Dicranostomus: the tusks. These tusks are the longest found among Orthoptera in comparison to the male body size (measured relative to pronotal length) and are clearly longer than in the well-known tusked wetas (Table 2). In Dicranostomus, the tusks do not show any indication of being stridulatory structures, as documented for some wetas (Field 2001). However, although there are no observations of the use of the tusks, there are three lines of evidence that all point in the same direction. In his review about animal weapons, Emlen (2008) writes in the context of resource-defense or female-defense mating systems: “In a surprising diversity of taxa, these critical resources were burrows or tunnels where females lay eggs, and the especially defensible nature of burrows may have played an important role in favoring the evolutionary enlargement of weapons in these cases.” In line with these conclusions, the three New Zealand orthopteran species with long tusks (see Table 1) all inhabit burrows in the ground or holes in trees (see Trewick and Morgan-Richards 2014 for a review). The tusked king cricket Libanasidus vittatus digs holes every night but may sometimes return to previously used holes (Bateman and Toms 1998). Also supporting these ideas, new behavioral observations in the genus Gnathoclita, sister to Dicranostomus (e.g., Gorochov 2012), have shown that G. vorax also inhabits the hollow dead stems of plants, with the males displaying “a form of mate guarding” (Hugel 2019). Thus, in our opinion, it is a plausible hypothesis that Dicranostomus inhabits holes, probably in plant material, and that the males defend these safe places against rivals using their tusks. Holes in plants are also used by other acoustically active species like frogs (e.g., the tree hole frog Metaphrynella sundana; Lardner & bin Lakim 2002). In Disceratus Scudder, 1869, a related genus, its low acoustically determined population density is used as an argument against male to male combat (Braun 2016). Thus, other reasons for the tusks should not be excluded. Completely unknown, however, is the function of the female tusks in D. monoceros. Females of D. nitidus do not possess tusks, but they have a strongly elongated fastigium frontis, as found in both sexes of D. monoceros and in D. nitidus males. Gwynne (2001) speculated on the probability of female-to-female interactions in D. monoceros, and certainly the access, possession, and defense of holes may be important for females as well. Unfortunately, data to confirm or deny this are missing.

We are grateful to Holger Braun, Sigfrid Ingrisch, Ming Kai Tan, and an anonymous referee for helpful comments on the manuscript. The Orthopterists’ Society provided free publication of this paper. Duplication of information presented in Cigliano et al. 2021 on request of the main editor.