Atropacarus

The little guy pictured above (photo copyright Scott Justis) is a representative of the box mite genus Atropacarus, members of which can be found in most parts of the world. Atropacarus is a genus of the Phthiracaroidea, a group of box mites characterised by the plates on the underside of body being relatively wide, in contrast to the narrow ventral plates of its sister group, the Euphthiracaroidea (members of which have featured on this site before: here and here). The difference in configuration of these plates reflects a difference in the way that the body is contracted to allow legs and prosoma to be withdrawn beneath the protective cover of the notogaster. In euphthiracaroids, the sides of the notogaster are contracted inwards; in phthiracaroids, the ventral plates of the body are lifted upwards (Schmelzle et al. 2015).

The classification of phthiracaroids is subject to conflict with two main systems in the recent literature. In one, championed by the Polish acarologist Wojciech Niedbała, the phthiracaroids are divided between two families with Atropacarus in the Steganacaridae. Species of Atropacarus have the surface of the notogaster extensively covered with dimples. The dorsal seta on the tibia of the fourth leg is short and closely associated with a solenidion (a type of specialised sensory hair). The setae of the genital plate are arranged in a more or less straight row along the inner margin of the plate with the fifth and sixth setae further apart than the fourth and fifth (Niedbała 1986). Niedbała divides Atropacarus between two subgenera. In Atropacarus sensu stricto, there are sixteen or more pairs of setae on the notogaster and the second adanal seta is moved inwards on the ano-adanal plate to form a more or less straight line with the anal setae. In Hoplophorella, there are fifteen pairs of setae on the notogaster and the second adanal seta is distinctly laterally placed relative to the anal setae.

The super-hairy Atropacarus niedbalai, from Liu & Zhang (2013). Scale bar = 100 µm.

In the competing system, used for instance by Subías (2019), Atropacarus and Hoplophorella are treated as distinct genera and each is in turn divided into subgenera by the number of setae on the ano-adanal plate. To a certain extent, of course, the question of whether to treat Atropacarus and Hoplophorella as genera or subgenera is arbitrary. Nevertheless, this arguably cosmetic distinction does relate to an underlying difference in theory. The classification of phthiracaroids used by Subías (2019) is a largely diagnostic one, inspired by a desire to facilitate specimen identifications. Niedbała's classification, in contrast, is intended to reflect phylogenetic relationships. Simple setal counts may be convenient when composing keys but one might question its overall phylogenetic significance. Neotrichy (increases in setal count by multiplication of the original setae) is not uncommon in phthiracaroids, particularly on the notogaster. Setal counts may vary between individuals of a single species and overall neotrichy reaches an extreme in the New Zealand species Atropacarus niedbalai. In this species, the basic count of fifteen or sixteen pairs of notogastral setae has been increased to 109 or 115 pairs, with further neotrichy on the prodorsum and ventral plates (Liu & Zhang 2013). Subías (2019) defends his choice of classification by arguing that Niedbała's key features are often difficult to discern. I sympathise with the difficulty but, as a wise man once said, species are under no obligation to evolve with regard to the convenience of taxonomists.

REFERENCES

Liu, D., & Z.-Q. Zhang. 2013. Atropacarus (Atropacarus) niedbalai sp. nov., an extreme case of neotrichy in oribatid mites (Acari: Oribatida: Phthiracaridae). International Journal of Acarology 39 (6): 507–512.

Niedbała, W. 1986. Système des Phthiracaroidea (Oribatida, Euptyctima). Acarologia 27 (1): 61–84.

Schmelzle, S., R. A. Norton & M. Heethoff. 2015. Mechanics of the ptychoid defense mechanism in Ptyctima (Acari, Oribatida): one problem, two solutions. Zoologischer Anzeiger 2015: 27–40.

Subías, L. S. 2019. Nuevas adiciones al listado mundial de ácaros oribátidos (Acari, Oribatida) (14a actualización). Revista Ibérica de Aracnología 34: 76–80.