Austrotritia

Austrotritia lebronneci, copyright R. Penttinen.

Belongs within: Ptyctima.

Austrotritia is a genus of euphthiracaroid box mites found primarily in tropical Asia and Oceania (Subías 2004).

Austrotritia: Jack-in-the-box mites
Published 7 May 2020

We just keep coming back to the oribatids, don’t we?

In an earlier post, I introduced you to Oribotritia, one of the genera of box mites. These, you may recall, are the armoured mites that have evolved the ability to curl the front of the body under themselves and tuck back their legs to form a solid case (in the Oribotritiidae, that mechanical defense is supplemented by the production of a defensive chemical, chrysomelidial, from glands in the cuticle—Shimizu et al. 2012). In the earlier post, I also gave you a quick overview of the families of what are known as the ‘true’ box mites. Today’s post is for another component of the family Oribotritiidae, the genus Austrotritia.

Austrotritia accounts for nearly twenty species of box mite, the great majority of which are found in Australasia and southern and eastern Asia (Liu et al. 2009). Outliers are A. engelbrechti in South Africa, A. herenessica in the Canary Islands and, most unexpected of all, A. finlandica in Finland. Austrotritia differs from all other oribotritiids except the small Bornean genus Terratritia in lacking any division between the genital and aggenital plates on the underside of the body. The distinction between Austrotritia and Terratritia perhaps requires reassessment: Niedbała (2000) distinguished them by the presence of five-segmented palps and a single pair of exobothridial setae in Austrotritia versus three-segmented palps and two pairs of exobothridial setae in Terratritia (the bothridia are the structures bearing large sensory setae on the prodorsum of the mite; exobothridial setae are thus setae sitting alongside the bothridia). However, Liu & Zhang (2014) redescribed the widespread species Austrotritia lebronneci as having three-segmented palps but only a single pair of exobothridial setae. Note that classification of oribatids has mostly been conducted from a diagnostic rather than a phylogenetic perspective; it would not surprise me if Terratritia turned out to be a derived subgroup of Austrotritia.

Schematic of jump performance by Indotritia cf. heterotrichia from Wauthy et al. (1998); the solid line represents observed jumps, the dashed lines modelled jumps. Line drawings represent (a) body posture when beginning jump, (b) rotation during jump, and (c) enclosed posture after jumping.

As well as the aforementioned defenses standard for box mites, Austrotritia and the related genus Indotritia stand out from other oribotritiid genera in that at least some species have the ability to jump. The mechanics of jumping were described for a species of Indotritia by Wauthy et al. (1998) who recorded the mites jumping nearly a centimetre in height over a distance of just under an inch (for perspective, the mite itself is about half a millimetre in length). Jumping was preceded by compressing the notogaster while raising the ventral plates under the opisthosoma, together with lowering the prosoma and bringing the legs together under the body. Small hooks at the end of femur of the first pair of legs were used to catch ridges on the side of the prodorsum in order to hold the body compression. The force for the jump was presumably supplied by the release of the hydraulic compression of the body fluids when the legs disengaged from the prodorsum, propelling the mite backwards while the body rolled forwards: essentially, the mite would star-jump away. The mite would curl up after jumping to lie in an enclosed state.

Whether all Austrotritia species are jumpers is not entirely certain. The femoral hooks that seem to play a significant role in jumping have not been described in all species. However, it is not clear if this lack of observation represents an actual absence or whether this minute feature has simply been overlooked. I also wonder whether the aforementioned fusion of the ventral plates in Austrotritia is related to their jumping abilities (Indotritia species also have the genital and aggenital plates fused anteriorly though they retain a degree of separation at the rear of the plates; non-jumping Oribotritia have the plates entirely separated). As always, there’s still a lot we could potentially find out.

Systematics of Austrotritia

Characters (from Balogh & Balogh 1992): Aspis with one pair of exobothridial setae present. Palpi four- or five-segmented. Posterior median apodeme absent. Genu IV with solenidion and seta. Genital-aggenital suture absent, the two plates completely fused; genital setae in paraxial position, in longitudinal row; ano-genital cleft present; interlocking triangle absent.

<==Austrotritia Sellnick 1959 [incl. Macarotritia Pérez-Íñigo 1986]S04
    |--*A. quadricarinata Sellnick 1959S04
    |--A. bifurcata Niedbała 2000N06
    |--A. bryani Jacot 1928N96
    |--A. carinata Niedbała 2000S04
    |--A. dentata Aoki 1980S04
    |--A. gibba Bayoumi & Mahunka 1979S04
    |--A. glabrata Niedbała 2000S04
    |--A. herenessica (Pérez-Íñigo 1986)S04 [=Indotritia (*Macarotritia) herenessicaBB92]
    |--A. kinabaluensis Ramsay & Sheals 1969S04
    |--A. lanceolata Aoki 1988S04
    |--A. lebronneci (Jacot 1935) [=Indotritia lebronneci]N06
    |    |--A. l. lebronneciS04
    |    |--A. l. crassiori Jacot 1934S04
    |    |--A. l. flagelloides Jacot 1934S04
    |    `--A. l. tahitiana (Jacot 1934) [=Indotritia lebronneci tahitiana]S04
    |--A. ramsayi Mahunka 1991S04
    |--A. robusta Niedbała 1997N06
    |--A. saraburiensis Aoki 1965 (see below for synonymy)S04
    `--A. unicarinata Aoki 1980S04

Austrotritia saraburiensis Aoki 1965 [=A. lebronneci saraburiensis; incl. A. ishigakiensis Aoki 1980, A. optabilis Niedbała 1991, A. shealsi Mahunka 1987]S04

*Type species of generic name indicated

References

[BB92] Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World vol. 1. Hungarian Natural History Museum: Budapest.

Liu, D., J. Chen & G. Qiao. 2009. Review of Austrotritia (Acari: Oribatida: Oribotritiidae), with descriptions of two new species from China. Zootaxa 2144: 54–64.

Liu, D., & Z.-Q. Zhang. 2014. Redescription of Austrotritia lebronneci (Oribotritiidae) and descriptions of two new species of Euphthiracaridae (Acari, Oribatida) from Australian region. International Journal of Acarology 40 (1): 43–51.

[N96] Niedbala, W. 1996. Origin of euptyctimous mites (Oribatida) in Hawaii. In: Mitchell, R., D. J. Horn, G. R. Needham & W. C. Welbourn (eds) Acarology IX vol. 1. Proceedings pp. 613–614. Ohio Biological Survey: Columbus (Ohio).

Niedbała, W. 2000. The ptyctimous mites fauna of the Oriental and Australian regions and their centre of origin (Acari: Oribatida). Polskie Towarzystwo Taksonomiczne: Wrocław (Poland).

[N06] Niedbała, W. 2006. Supplement to the knowledge of ptyctimous mites (Acari: Oribatida) from Australian region. Annales Zoologici 56 (Suppl. 1): 99–156.

Shimizu, N., R. Yakumaru, T. Sakata, S. Shimano & Y. Kuwahara. 2012. The absolute configuration of chrysomelidial: a widely distributed defensive component among oribotritiid mites (Acari: Oribatida). Journal of Chemical Ecology 38: 29–35.

[S04] Subías, L. S. 2004. Listado sistemático, sinonímico y biogeográfico de los ácaros oribátidos (Acariformes, Oribatida) del mundo (1758–2002). Graellsia 60 (número extraordinario): 3–305.

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