Canadian Journal of Arthropod Identification

Soldier flies of the subfamily Pachygastrinae of Canada (Diptera, Stratiomyidae)

CJAI 42 -- December 4, 2020

Linley M. Sherin

| Abstract | Introduction | Materials & Methods | Checklist | Species Key | Results | Acknowledgments | References | PDF | Cite | Appendix |

The Stratiomyidae, or soldier flies, are a large, cosmopolitan family (~2850 spp.) of lower brachyceran flies that are diverse in both size and morphology (Hauser et al. 2017; James 1981).  There are 114 species recorded from Canada. However, a recent review of Canadian Diptera predicted an additional 5-10 species remain to be recorded or described (Savage et al. 2019). The Canadian stratiomyid fauna includes species in seven of the twelve subfamilies: Antissinae, Stratiomyinae, Beridinae, Sarginae, Nemotelinae, Clitellariinae and Pachygastrinae (McFadden 1972, Woodley 2001). I here review the taxonomy of the 11 Canadian species of Pachygastrinae.

Pachygastrinae are a predominantly tropical subfamily with a global distribution. Possibly due in part to their small size, they have poor representation in collections when compared with larger stratiomyids. Pachygastrinae are morphologically distinguished from other Canadian Stratiomyidae in having five visible abdominal tergites (tergites 6 and 7 reduced), wing with m-cu crossvein connected to vein M3+4, and in having only three veins arising from the discal medial cell (Fig. 1; James 1981; Hauser et al. 2017). This subfamily was previously thought to be paraphyletic (Nagatomi and Iwata 1978; Woodley 2001), however the only molecular study of Stratiomyid phylogeny (Brammer and von Dohlen 2007) found evidence of a monophyletic Pachygastrinae. Monophyly was also supported in a 2010 morphological analysis (Brammer and von Dohlen 2010), though pachygastrine representation was limited.

Adults of the Nearctic Region are small (2.0-3.6 mm), predominantly black flies, often with silvery vestiture (Kraft and Cook 1961; Woodley 2001). Though sexual dimorphism is prevalent in soldier flies, Pachygastrinae often display only subtle variation between sexes (Curran 1927). Adults of many species form swarms, and the Nearctic species Gowdeyana punctifera Malloch has been recorded in swarms of up to 50 individuals (Woodley 2001). For unknown reasons, many species are collected from windows (Hull 1930) and few were collected prior to the common use of Malaise traps (Woodley 2001). Adults appear attracted to dead wood or fallen trees where females oviposit (Woodley 2001).

Relatively little is known about the habitat associations and feeding habits of larval Pachygastrinae (Woodley 2001). Larvae are universally brown to white, flattened dorso-ventrally and vary in length from 4.0-7.2 mm (Kraft and Cook 1961). Pachygastrine larvae are terrestrial and have been found under the bark and in tree holes of both deciduous and coniferous trees (Kraft and Cook 1961; McFadden 1967). They appear to feed on either the sap or microorganisms that occur on the moist wood of wounded and dying trees (McFadden 1967) and use cylindrical brush-like mouthparts to sweep food into their oral cavity (Cook 1953; Kraft and Cook 1961; McFadden 1967). It has been suggested previously that the larvae are predatory (Malloch 1917; Johannsen 1922; James 1981). However, McFadden (1967) and Woodley (2001) considered it doubtful given their common appearance in large numbers with McFadden (1967) recording over 100 larvae existing under the bark of a single log. The Canadian species Berkshiria albistylum Johnson has also been reared from a large number of larvae located under the bark of a fallen poplar tree (Marshall, S.A., pers. comm.).

Previous work addressing the Canadian pachygastrine fauna has been limited and largely attributable to a small number of dedicated dipterists. Kraft and Cook (1961) provided a revision of the Pachygastrinae of America north of Mexico, including keys to 21 species, 13 of which were newly described in that paper. Following Kraft and Cook, James (1965) provided a supplementary revision and key to the genera of the southwestern United States and parts of Mexico, adding two species to the Nearctic fauna (one new species, Zabrachia yuccae James, and one new Nearctic record, Cosmariomyia pallidipennis (Williston)). James (1981) provided illustrated adult and larval keys to the subfamilies and genera of Nearctic Stratiomyidae within the Manual of Nearctic Diptera, referencing 24 pachygastrine species in its generic treatment. Most recently, Woodley (2001, 2011) provided a catalogue of the global Stratiomyidae. This catalogue included 25 Nearctic pachygastrine species in nine genera, adding the species Artemita nana (Bellardi) to the Nearctic fauna. Of these 25 species, Woodley reported seven species from Canada (Berkshiria albistylum Johnson, Eidalimus fuscus (Kraft & Cook), Neopachygaster maculicornis (Hine), N. occidentalis Kraft & Cook, N. reniformis Hull, Zabrachia plicata (Kraft & Cook) and Z. polita (Coquillett)) and another five species were recorded from the adjacent USA.

Despite past attempts to generate a larval key to species of Nearctic Pachygastrinae, no complete key currently exists. Kraft and Cook (1961) provided a larval key to genera and larval keys to species for three genera (Eidalimus Kertesz, Gowdeyana Curran (as Eupachygaster Kertesz) and Zabrachia Coquillett). Most recently, McFadden (1967) provided a larval key to the genera of Stratiomyidae in America north of Mexico and incomplete larval keys to species.

Current pachygastrine adult keys are also inadequately illustrated and none present an up-to-date review of the Canadian fauna; the last Nearctic revision (Kraft and Cook 1961) included only five of the 11 Canadian species. I here key the seven Canadian genera including nine of the 11 Canadian species, illustrating adult diagnostic characters with photographs, adding new characters and highlighting taxa that need formal revision. Four of the species included here are new Canadian records (Table 1). By providing an updated and interactive key to the Canadian fauna, I hope to encourage the future study of the Pachygastrinae in Canada and bring attention to the need for a global revision of the subfamily. I also hope that this key serves as a tool for amateur naturalists to discover these beautiful and elusive flies.

Figure 1. Wing of Pachygaster pulchra Loew, edited to highlight characteristic wing venation of the subfamily Pachygastrinae in red; m-cu crossvein connected to M3+4, and only three veins arising from cell dm (M1, M2 apical and M4). Terminology follows Cumming and Wood (2017). Abbreviations: dm – discal medial cell; M – medial vein or media; M1+2 – fused first and second branch of medial vein; M1 – first branch of medial vein; M2 – second branch of medial vein; M3+4 – fused third and fourth branch of medial vein; M3 – third branch of medial vein; M4 – fourth branch of medial vein; m-cu – medial-cubital crossvein, m-m – medial crossvein.