Dromochorus

Duran, Daniel P., Herrmann, David P., Roman, Stephen J., Gwiazdowski, Rodger A., Drummond, Jennifer A., Hood, Glen R. & Egan, Scott P., 2019, Cryptic diversity in the North American Dromochorus tiger beetles (Coleoptera: Carabidae: Cicindelinae): a congruence-based method for species discovery, Zoological Journal of the Linnean Society 186, pp. 250-285 : 263-265

GENUS DROMOCHORUS GUÉRIN- MÉNEVILLE, 1845

Type species

Dromochorus pilatei Guérin-Méneville, 1845 . By monotypy.

Taxonomic history

Dromochorus was described by Guérin-Méneville (1845) as a new genus, believed to be most closely related to ‘ Apteroessa and especially Dromica ’. This perceived relatedness was due to the remarkable similarity in gestalt, although several morphological differences were identified, especially with respect to the mouthparts, such as the maxillary and labial palpi, and labrum. Subsequent authors have not shared his view about the systematic placement of the genus within the larger tiger beetle clade. In his revision of the Cicindelinae, Horn (1908) placed Dromochorus in the subtribe Cicindelina, and Dromica in a separate subtribe, Prothymina ( Horn 1910). More recent molecular phylogenies have supported Horn’s (1908) treatment, finding Dromochorus to be nested within a clade of other Nearctic Cicindelina ( Vogler & Welsh, 1997; Barraclough & Vogler, 2002; Pons et al., 2004; Vogler et al., 2005), and Dromica to be more related to Prothyma ( Galián, Hogan & Vogler, 2002) . Any similarities between Dromochorus and Dromica must, therefore, be the result of convergent evolution owing to their similar habitats and microhabitats.

The Dromochorus species have sometimes been treated as a distinct genus ( Guérin-Méneville, 1845; Sallé, 1877; Fleutiaux, 1892; Casey, 1897; Rivalier, 1954, 1963, 1971; Johnson, 1991; Wiesner, 1992; Erwin & Pearson, 2008; Bousquet, 2012; Pearson et al., 2015); or viewed to be a synonym of Cicindela (e.g. LeConte, 1875; Leng, 1902; Harris, 1911; Horn, 1915; Harris & Leng, 1916; Cazier, 1954; Arnett, 1963; Willis, 1968; Bousquet & Larochelle, 1993; Arnett & Thomas, 2000), with some North American workers recognizing the taxon as a subgenus ( Boyd, 1982; Freitag, 1999). Despite the variety of treatments, justifications for different taxonomic ranks were almost never given.

Recent phylogenies have given support to many of the named clades within the Cicindela sensu lato ( Pons et al., 2004; Vogler et al., 2005; Gough et al., in review), and these generally correspond to groups recognized in Rivalier’s revision (1954, 1963, 1971), including the Dromochorus . Based on the aforementioned research, and our own mtDNA tree ( Fig. 2 ), Dromochorus was recovered as a well-supported monophyletic lineage, and sister to another clade of Nearctic tiger beetles. As such, we recognize the group as a distinct genus. Although Dromochorus may appear nested within Cylindera in our tree, this latter group is polyphyletic ( Gough et al., 2018), as previously believed by many workers, and the sister clade to Dromochorus is being named as a new genus.

Diagnosis

Dromochorus are separable from all other North American tiger beetle genera by the following combination of characters, present in the adult stage. Beetles are flightless and lack flight wings, but the elytra are not fused. The elytra are oval shaped, and completely lacking pale maculations. The dorsum is dark, usually black or brown, but may also have a frosted blue, violet, grey or green sheen. The legs and tarsi are clothed in decumbent setae.

In subsequent species descriptions, the most salient or diagnostic characters are indicated in bold. Rarely are any of these characters diagnostic by themselves, but the combination of these characters may be.

Distribution

Dromochorus are geographically restricted to the south-central United States and adjacent Mexico ( Fig. 3 ). Texas is the centre of diversity, and all eight species are found there, at least in part of their ranges. Records from Mexico are few and imprecise. Historically, D. belfragei was the only species recorded from Mexico, although it would appear as if these populations belong to D. chaparralensis sp. nov., described in this treatment. In general, Dromochorus are found at low elevations; apparently, they do not occur in montane environments, such as the Ouachita Mountains of Arkansas and Oklahoma, which border a section of the eastern range of D. pruininus . Most Dromochorus are found below 500 m, with only the westernmost populations of D. belfragei occurring at elevations up to 1000 m.

Ecology/natural history

Dromochorus appear to have a 2-year life cycle, based on observations of D. pruininus (Herrmann & Duran, unpublished). Adult beetles are terrestrial predators of small invertebrates, and are generally believed to be crepuscular or active in the late afternoon, but we have found them at all hours of the day in more shaded microhabitats or when significant cloud cover is present. Dromochorus are extremely fast runners and may evade capture by darting into dense grasses, or in some species, hiding in cracks in the earth, especially D. belfragei and D. pruininus . Larvae are poorly known, and were only recently described ( Spomer, Nabity & Brust, 2008).

There are remarkably few specimens of these beetles in major museum collections, even though several species occur near major cities and universities in Texas and Oklahoma. Many species records are based on one or a few specimens, and as such, Dromochorus were reported to be rare, or at low densities. This is likely a consequence of their atypical natural history compared to other diurnal North American tiger beetles, and the fact that their habitats are not as commonly visited by collectors. We have found that Dromochorus can be remarkably abundant in the appropriate habitat during the ideal time of year, rivalling or exceeding densities observed in some common riparian tiger beetle species. These observations are unlikely to represent unusual population explosions, as we have witnessed similarly large numbers of beetles every year in areas that we visited each year between 2012 and 2016.

KEY TO THE GENUS DROMOCHORUS

1a. Labial palps at least partly yellow to dark amber, with contrasting darker apical segment ( Fig. 7A, B ) ......................................................................................................................................................2

1b. Labial palps consistently dark brown to black throughout ( Fig. 7C ) ..........................................................4

2a. Elytral surface smooth and finely frosted in texture, without any pitting or subsutural foveae. Dorsum with strong blue to violet reflections throughout. Labial palps yellow, with contrasting darker apical seg- ment. Kansas and western Missouri, south to Central Texas .................................................. D. pruininus

2b. Elytra surface dull, textured with fine to deep pitting throughout. Distinct shallow pits running along elytra suture (subsutural foveae, Fig. 8A ) may be present ..........................................................................3

3a. Dorsum dark brown with prominent shallow subsutural foveae and irregular pitting throughout ely- tral surface. Foveae and smaller pits with metallic green reflections. Irregular green marbling may be present on elytra, head and pronotum. Labial palps yellow, with contrasting darker apical segment. Louisiana to East Texas.................................................................................................................... D. pilatei

3b. Dorsum black with shallow to deep pitting on elytral surface. Some metallic blue reflections may be pre- sent, especially on the supraorbital region of the head and humeral area of the elytra. Subsutural foveae may be present. Labial palps yellow to dark amber, with apical segment darkest. Oklahoma and Texas panhandle, south to East Central Texas...................................................................................... D. belfragei

4a. Elytral surface rough, with shallow to deep pitting. Subsutural foveae present ( Fig. 8A ), often with metallic green or blue reflections in pits. ‘Hill Country’ region of Central Texas........... D. knisleyi sp. nov.

4b. Elytral surface smooth, often with velvety or frosted texture. No subsutural foveae present ( Fig. 8B ) ..........................................................................................................................................................5

5a. Pronotum glabrous or with few scattered long thin erect setae irregularly placed, rarely concentrated along margins.................................................................................................................................................6

5b. Pronotum with sparse to regular white decumbent setae, mostly in lateral third ....................................7

6a. Dorsum finely velvety black, with strong violet, blue or green reflections, especially along margins. Male labrum entirely dark or very nearly so. Body form gracile. South Texas, coastal areas south of Corpus Christi and inland to vicinity of Dimmit County ................................................................. D. velutinigrens

6b. Elytra dark ash-grey, sometimes with frosty blue reflections. South Texas in mesquite chaparral forest. Known only from Bexar, Frio and Atascosa Counties .................................................... D. minimus sp. nov.

7a. Elytra dull black, may have bluish reflections especially near margins. South Texas to Mexico, in mes- quite chaparral. Known from Dimmit, LaSalle, and Webb Counties in Texas, and the state of Tamaulipas, Mexico..................................................................................................................... D. chaparralensis sp. nov.

7b. Elytra finely velvety black, may have a faint dark blue sheen. Found in coastal prairie habitat near Gulf of Mexico ...................................................................................................................... D. welderensis sp. nov.

Figures

Figure 2A. Mitochondrial DNA (mtDNA) cytochrome oxidase b (Cytb) Bayesian genealogy of all members of the genus Dromochorus. Outgroups include North American representatives of Cylindera and Cicindelidia. On the left is a reduced version of the full tree indicating clades shown, in detail, in each figure. Colours are arbitrarily chosen to show distinctions among the taxa, and directly correspond to populations shown in Figure 3. Only posterior support values above 60 are displayed, and the scale bar at bottom shows a branch length representing nucleotide substitutions per site. Taxon names include the unique specimen number (see also Supporting Information, Table S1), along with the state and collection locality; genomic material from specimens marked with a ~ were also used in the multilocus analysis (Figure 4), and holotypes of species described in this work are marked with an ‘H’. The longitudinal bars on the right identify specimens and clades historically considered to be a single species.

Figure 3. Distributionof sampled Dromochorus populations.Colours correspondto those used in Figure 1. Green = D.pilatei, pink = D. welderensis sp. nov., sky blue = D. velutinigrens, red = D. chaparralensis sp. nov., blue = D. pruininus, lime green = D. knisleyi sp. nov., orange = D. minimus sp. nov., black = D. belfragei.

Figure 7. Labial palps of: A, D. pruininus; B, D. belfragei; C, D. knisleyi sp. nov. A and B illustrate labial palps with a contrasting darker apical segment, and C illustrates labial palps that are consistently dark throughout.

Figure 8. A, Elytra showing distinct punctures (foveae) running parallel to the suture of the elytra (D. knisleyi sp. nov.). B, lack of distinct subsutural foveae (D. welderensis sp. nov.).