Caloppiidae

The concept of ranks in taxonomy is ultimately an arbitrary one. There is no real definition of what constitutes an 'order', a 'family' or a 'subfamily'. What determines the rank that a given taxon is recognised at is a combination of tradition, convenience, and the taxon's relationships to other recognised taxa. As such, the question of whether a given classification is overly 'split' or 'lumped' is a meaningless one and arguing the point is a complete waste of time. That said, the classification of the 'higher' oribatid mites is massively oversplit.

A big part of the reason why oribatid classification seems such a mess, with large numbers of small families containing only a handful of genera and/or species apiece, can be attributed to simple ignorance. We simply do not have a good handle on how many oribatid taxa are related to each other and as a result we find ourselves with a great many orphan taxa still hunting for a good home. The Caloppiidae may be regarded as one such taxon.

Dorsal view of Luissubiasia microporosa, from Ermilov (2016). Scale bar = 100 µm; labels with 'A' indicate areae porosae.

Caloppiids are a pantropical group of about thirty species of poronotic oribatids (the group of oribatids exhibiting the octotaxic system, an arrangement of glandular openings on the notogaster), with three genera recognised in the family by Ermilov (2016): Zetorchella, Brassiella and Luissubiasia. Zetorchella, which includes the majority of the family's species, is also pantropical in distribution. Brassiella is known from the Indo-Pacific region and Liussubiasia is known from a single species from Cuba. Past authors have often referred to Zetorchella and the Caloppiidae by the names Chaunoproctus and Chaunoproctidae, respectively, but as the name Chaunoproctus had already had dibs called on it before the mite was named (by a bird, the now-extinct Bonin grosbeak Chaunoproctus ferreorostris), their respective most senior synonyms have to take over. Caloppiids are more or less egg-shaped in dorsal view. They lack the distinct pteromorphs of most other poronotics though they may have quadrangular projections in the humeral region (the 'shoulders'). The integument is usually heavily sculpted and foveate. The legs end in three claws apiece. The most characteristic feature of the group is that the openings of the octotaxic system on the notogaster, of which five pairs are present, are extremely small. The octotaxic system can take two forms, recessed saccules or porose patches. Those of caloppiids have usually been described as saccules but Ermilov (2016) states that, at least in some species, they are very small porose areas.

Going by their overall appearance, caloppiids are classified within the superfamily Oripodoidea. However, one of the most characteristic features of the Oripodoidea as an evolutionary group is that their nymphs have notogastral setae borne on individual off-centred sclerites (oribatid nymphs often look very different from their adults and are often more soft-bodied). At this point in time, we simply do not know what the nymphs of caloppiids look like so we cannot say whether they possess this crucial feature. Conversely, with their lack of pteromorphs, caloppiids bear a distinct similarity to the more diverse oripodoid family Oribatulidae. The two families have mostly been separated on the basis of caloppiids supposedly having an octotaxic system of saccules rather than porose areas, a distinction that I've already noted may not hold up. There's also something of an open question whether the distinction between saccules and porose areas is really as significant as it has been thought in the past. So, at present, we can't say with confidence whether caloppiids are true oripodoids... or whether they are not only oripodoids but don't even warrant recognition as a distinct family from oribatulids.

REFERENCE

Ermilov, S. G. 2016. Luissubiasia microporosa gen. nov., sp. nov. (Acari, Oribatida, Caloppiidae) from Cuba. International Journal of Acarology 42 (2): 127–134.