Omorgus

Omorgus suberosus, copyright Hectonichus.

Belongs within: Trogidae.

Omorgus is a genus of keratin beetles found in warmer regions of the world characterised by an elongate scape and a straight or reflexed clypeus.

Omorgus: a beetle with a taste for hair
Published 17 March 2016
A group of Omorgus clambering over what looks like a scat, copyright Stephen Cresswell.

I still remember my first Omorgus. Pretty much as soon as I saw it in the pitfall trap, I knew that this was a different type of beetle from any I’d seen before. Large, knobbly, robust… it looked a picture of glorious ugliness. Which only made it all the more frustrating that, somewhere in the process of making it into the trap, this particular specimen appeared to have somehow lost its head. Without the ability to look it in the face, I might never know what I’d found.

It wasn’t until later in the lab that I discovered my mistake: my beetle wasn’t headless at all! Instead, the head was retracted back, hidden beneath the expanse of the pronotum (the dorsal shield of the first thoracic segment). And so I became acquainted with my first keratin beetle.

A similar Omorgus to the one I found, O. bachorum, to give some idea how I missed the head. Copyright Clare McLellan.

Omorgus is one of the handful of genera of keratin beetles, a group of relatives of the scarabs known as the Trogidae or Troginae (there has been some inconsistency as to whether trogids are treated as their own family or as a subfamily of the main scarab family Scarabaeidae). They have robust forelegs with large femora, and striate elytra that are often covered with tubercles and/or setae. Trogids vary in size from about half a centimetre in length up to three centimetres. They get their name of ‘keratin beetles’ from their unique diet: both as adults and larvae, trogids feed primarily on keratin such as animal hair. They are most commonly scavengers, feeding at animal carcasses (often arriving late in the process, taking the parts of the animal rejected as indigestible by other scavengers). However, they also feed on other animal foods such as insect larvae, eggs or guano, and some appear to be specialist associates of bird nests or animal burrows (Scholtz 1986b). An Australian flightless species Omorgus rotundulus was found to have a gut full of other arthropods, particularly ants and termites, in quantities that lead to the suggestion that it might be an active predator rather than a scavenger (Houston et al. 2010).

Omorgus chowing down on a dead lizard, copyright William Archer.

Earlier authors commonly treated all trogids as belonging to a single genus Trox, but more recent authors have recognised four or five genera in the family. Omorgus includes about 150 species (Strümpher et al. 2014) found mostly in arid regions. The most obvious feature separating Omorgus species from other trogids is that the pedicel (the second segment of the antennae) is attached to the scape (the first segment) subapically rather than apically. In all species but one, the scutellum (the little thoracic shield visible between the bases of the elytra) is hastate (shaped a bit like a spear-head, with a constricted base broadening out further down) rather than a more simple oval as in other trogids. The exception, T. batesi, is a South American species that is placed in its own subgenus Haroldomorgus. The remaining species are divided between two subgenera Omorgus sensu stricto and Afromorgus, distinguished by features of the male genitalia (Scholtz 1986b). Afromorgus is found in Africa and Asia whereas the type subgenus contains the Australian and other American species.

Most trogids are fully capable of flight (many are attracted to lights at night) but, as alluded to above, a handful of species are flightless. In flightless species, the elytra become fused together into a sold carapace. The impression I get from scanning the literature is that flightlessness in trogids may not be so much a matter of conserving energy as it is of conserving water. For animals living on a dry diet in a dry habitat, such adaptations are only to be expected.

Systematics of Omorgus
Omorgus Erichson 1847 [incl. Lagopelas Burmeister 1876]J02
| i. s.: O. scabrosusJ02
|--O. (Omorgus)SS14 [incl. Chesas Burmeister 1876SS14, J02, Megalotrox de Borre 1886CW92]
| | i. s.: O. (O.) alatus (Macleay 1888) [=Trox alatus]CW92
| | O. (O.) alius (Scholtz 1986) [=Trox alius]CW92
| | O. (O.) amictus (Haaf 1954) [=Trox amictus]CW92
| | O. (O.) aphanocephalus (Scholtz 1986) [=Trox (O.) aphanocephalus]CW92
| | O. (O.) augustae (Blackburn 1892) [=Trox augustae]CW92
| | O. (O.) australasiae (Erichson 1842) [=Trox australasiae]CW92
| | O. (O.) brucki (Harold 1872) [=Trox brucki]CW92
| | O. (O.) carinicollis (Scholtz 1986) [=Trox (Omorgus) carinicollis]CW92
| | O. (O.) crotchi (Harold 1871)CW92 (see below for synonymy)
| | O. (O.) curvipes (Harold 1872) [=Trox curvipes]CW92
| | O. (O.) dilaticollis (Macleay 1888) [=Trox dilaticollis]CW92
| | O. (O.) dohrni (Harold 1871) [=Trox dohrni]CW92
| | O. (O.) elderi (Blackburn 1892) [=Trox elderi]CW92
| | O. (O.) elongatus (Haaf 1954) [=Trox elongatus]CW92
| | O. (O.) euclensis (Blackburn 1892)CW92 [=Trox euclensisCW92, T. litigiosus euclensisS86]
| | O. (O.) eyrensis (Blackburn 1904)CW92 [=Trox eyrensisCW92, T. litigiosus eyrensisS86]
| | O. (O.) gigas (Harold 1872) [=Trox gigas, T. (*Megalotrox) gigas; incl. T. castelnaui Lansberge 1887]CW92
| | O. (O.) granuliceps (Haaf 1954) [=Trox granuliceps]CW92
| | O. (O.) howdenorum (Scholtz 1986) [=Trox (O.) howdenorum]CW92
| | O. (O.) insignicollis (Blackburn 1896) [=Trox insignicollis]CW92
| | O. (O.) mariae (Scholtz 1986) [=Trox (O.) mariae]CW92
| | O. (O.) mariettae (Scholtz 1986) [=Trox (O.) mariettae]CW92
| | O. (O.) marshalli (Haaf 1957) [=Trox marshalli]CW92
| | O. (O.) mentitor (Blackburn 1896)CW92 [=Trox mentitorCW92, T. litigiosus mentitorS86]
| | O. (O.) mollis (Arrow 1927) [=Trox mollis]S86
| | O. (O.) monteithi (Scholtz 1986) [=Trox (O.) monteithi]CW92
| | O. (O.) nigroscobinus (Scholtz 1986) [=Trox (O.) nigroscobinus]CW92
| | O. (O.) nodicollis (Macleay 1888)CW92 [=Trox nodicollisCW92, T. fenestratus nodicollisS86]
| | O. (O.) ovalis (Haaf 1957) [=Trox ovalis]CW92
| | O. (O.) parvicollis (Scholtz 1986) [=Trox (O.) parvicollis]CW92
| | O. (O.) pellosomus (Scholtz 1986) [=Trox (O.) pellosomus]CW92
| | O. (O.) perhispidus (Blackburn 1904) [=Trox perhispidus]CW92
| | O. (O.) quadridens (Blackburn 1892) [=Trox quadridens]CW92
| | O. (O.) quadrinodosus (Haaf 1954) [=Trox quadrinodosus]CW92
| | O. (O.) regalis (Haaf 1954) [=Trox regalis]CW92
| | O. (O.) rotundulus (Haaf 1957) [=Trox rotundulus]CW92
| | O. (O.) salebrosus (Macleay 1872) [=Trox salebrosus; incl. T. vitreomaculatus Macleay 1888]CW92
| | O. (O.) semicostatus (Macleay 1871) [=Trox semicostatus; incl. T. asperrimus Macleay 1888]CW92
| | O. (O.) setosipennis (Blackburn 1904) [=Trox setosipennis]CW92
| | O. (O.) stellatus (Harold 1872) [=Trox stellatus]CW92
| | O. (O.) strzeleckensis (Blackburn 1895) [=Trox strzeleckensis]CW92
| | O. (O.) tasmanicus (Blackburn 1904) [=Trox tasmanicus]CW92
| | O. (O.) villosus (Haaf 1954) [=Trox villosus]CW92
| |--+--*O. (O.) suberosus (Fabricius 1775)SS14, CW92 (see below for synonymy)
| | `--+--O. (O.) monachusSS14
| | `--O. (O.) tessulatusSS14
| `--+--+--O. (O.) costatus (Wiedemann 1823)SS14, S86 (see below for synonymy)
| | `--O. (O.) demarzi (Haaf 1958)SS14, CW92 [=Trox demarziCW92]
| `--+--+--O. (O.) borreiSS14
| | `--+--O. (O.) pastillarius [=*Chesas pastillarius]SS14
| | `--O. (O.) spatulatusSS14
| `--+--+--O. (O.) candidus (Harold 1872)SS14, CW92 [=Trox candidusCW92]
| | `--O. (O.) trilobus (Haaf 1954)SS14, CW92 [=Trox candidus trilobusCW92]
| `--+--O. (O.) squamosus (Macleay 1871)SS14, CW92 (see below for synonymy)
| `--+--O. (O.) alternans (Macleay 1826)SS14, CW92 (see below for synonymy)
| `--+--O. (O.) subcarinatus (Macleay 1864)SS14, CW92 (see below for synonymy)
| `--O. (O.) tatei (Blackburn 1892)SS14, S86 [=Trox tateiS86]
`--O. (Afromorgus)SS14
|--+--O. (A.) paulianiSS14
| `--+--O. (*A.) squalidusSS14
| `--O. (A.) melancholicusSS14
`--+--+--O. (A.) asperulatusSS14
| `--O. (A.) freyiSS14
`--+--O. (A.) gemmatusSS14
`--O. (A.) radulaSS14

Omorgus (Omorgus) alternans (Macleay 1826)SS14, CW92 [=Trox alternansCW92; incl. T. australasiae Germar 1848 non Erichson 1842CW92, T. litigiosus Harold 1872CW92]

Omorgus (Omorgus) costatus (Wiedemann 1823)SS14, CW92 [=Trox costatusCW92; incl. T. montalbanensis Schultze 1915S86, T. regularis Harold 1868S86, T. velutinus Blackburn 1892CW92]

Omorgus (Omorgus) crotchi (Harold 1871)CW92 [=Trox crotchiCW92; incl. T. asperatus Macleay 1888CW92, T. eremita Blackburn 1892CW92, T. crotchi eremitaS86]

Omorgus (Omorgus) squamosus (Macleay 1871)SS14, CW92 [=Trox squamosusCW92; incl. T. semmelinki Lansberge 1887S86, T. speculifer Heller 1914S86]

Omorgus (Omorgus) subcarinatus (Macleay 1864)SS14, CW92 [=Trox subcarinatusCW92; incl. T. fenestratus Harold 1872CW92]

*Omorgus (Omorgus) suberosus (Fabricius 1775)SS14, CW92 [=Trox suberosusCW92; incl. T. cancellatusFS90, T. crenatusFS90, T. gibbusFS90, T. manilensis Schultze 1915S86, T. muricatusFS90, T. murinusFS90, T. nobilis Woll. 1867FS90, T. novaecaledoniae Balthasar 1966S86, T. ovatusFS90, T. punctatusFS90, T. tricolor Blackburn 1904CW92]

*Type species of generic name indicated

References

[CW92] Cassis, G., & T. A. Weir. 1992. Trogidae. In: Houston, W. W. K. (ed.) Zoological Catalogue of Australia vol. 9. Coleoptera: Scarabaeoidea pp. 30–40. AGPS Press: Canberra.

[FS90] Fleutiaux, E., & A. Sallé. 1890. Liste des coléoptères de la Guadeloupe et descriptions d’espèces nouvelles. Annales de la Société Entomologique de France, 6e série 9: 351–484.

Houston, T. F., J. Zhang & B. P. Hanich. 2010. Diet of the flightless trogid beetle Omorgus rotundulus (Haaf) (Coleoptera: Trogidae) in the Little Sandy Desert of Western Australia. Australian Entomologist 36 (4): 207–212.

[J02] Jameson, M. L. 2002. Trogidae MacLeay 1819. In: Arnett, R. H., Jr, M. C. Thomas, P. E. Skelley & J. H. Frank (eds) American Beetles vol. 2. Polyphaga: Scarabaeoidea through Curculionoidea pp. 17–19. CRC Press.

[S86] Scholtz, C. H. 1986a. Revision of the genus Trox Fabricius (Coleoptera: Trogidae) of the Australasian region. Australian Journal of Zoology, Supplementary Series 125: 1–99.

Scholtz, C. H. 1986b. Phylogeny and systematics of the Trogidae (Coleoptera: Scarabaeoidea). Systematic Entomology 11: 355–363.

[SS14] Strümpher, W. P., C. L. Sole, M. H. Villet & C. H. Scholtz. 2014. Phylogeny of the family Trogidae (Coleoptera: Scarabaeoidea) inferred from mitochondrial and nuclear ribosomal DNA sequence data. Systematic Entomology 39: 548–562.

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