Eniochthonius

Norton, R. A. & Behan-Pelletier, V. M., 2007, Eniochthonius Mahunkai Sp. N. (Acari: Oribatida: Eniochthoniidae), From North American Peatlands, With A Redescription Of Eniochthonius And A Key To North American Species, Acta Zoologica Academiae Scientiarum Hungaricae 53 (4), pp. 295-333 : 316-320

ENIOCHTHONIUS – REDESCRIPTION AND NOMENCLATURE

Redescription

The diagnosis of Eniochthonius by WEIGMANN (2006) is sufficient for identification, but traits of the genus have never been described in detail. As with his diagnosis, the following description serves for both the genus and family, since Eniochthoniidae includes only the type genus. It is based on our studies of: the three species we discuss above, including all their instars (except we have not seen the larva of E. crosbyi ); a paratype adult of E. sumatranus ; and literature descriptions of E. paludicolus and E. fukushimaensis . Not all relevant characters are yet known for the latter two species.

Adult. With characters of Hypochthonioidea ( NORTON 1984, 2001; see Note 3). Thelytokous, males unknown. Integument with light sclerotization and epicuticular chambers mineralized with birefringent whewellite. Prodorsum fully articulated with proterosomal coxisternum; bothridium with three chambers, inner one spiculate, without porose pouch. Supracoxal seta (eI) spiniform, with minute distal fork or subterminal barb. Notogaster with transverse sulcus at level of setal row d and tectiform (telescoping, type-L) transverse scissure posterior to row e; pronotaspis bordered laterally by elongated suprapleural plate, bearing setae h 2 and h 3; pleural region continuous with pygidium, not isolated as pleuraspis. Pleural carina present dorsal to legs III, IV; anterior half of carina bears taenidium, which leaves carina to curve ventrad around leg IV; elongated porose area present within anterior part of taenidium. Normal five pairs of notogastral lyrifissures present; ia slit-like, at edge of taenidium, others cupular in form; ip far anterior in pleural region. Coxisternum with apodeme 3 short, apodeme 4 not evident. Articulations with leg trochanters sunken, no soft cuticle exposed, approaching ball-socket formation. Genital plate unusually large, pentagonal as pair, subdivided transversely by having narrow band with unsclerotized procuticle but mineralized epicuticle; with 10 pairs of setae in two longitudinal rows (3 lateral, 7 medial). Aggenital plate divided at same level as genital plate: posterior part narrow, elongated; anterior part large, triangular, with or without secondary oblique scissure isolating small middle plate; anterior plate with postpedal fossa aligned with curving notogastral taenidium; single aggenital seta on anterior, posterior or middle plate (if subdivided). Ovipositor very short, with coronal setae. Anal and adanal plates distinct and fully articulated, with two and three pairs of setae, respectively; anal plates anteriorly with adjacent pair of horn-like apodemes. Paired plicature plate present, bearing lyrifissure ips. Legs monodactylous; articulation of segments approaching ball-socket formation, no soft cuticle exposed. Setation (legs I–IV, including famulus but not solenidia) of trochanters (0–0–2–2), femora (3–5–3–3), genua (5–3–3–3), tibiae (5–4–3–3), tarsi (18–15–13–13) or (18–16–14–13); iteral seta it’ absent from tarsi I and IV, present or absent on II and III. Tarsus I with seta m”. Primilateral pair present on tarsus I, absent from III and IV; tarsus II with pl’ but not pl”. Fundamental seta bx’ present on femur II. Famulus forked, one branch thick, terminating bluntly or with slight knob-like swelling, other narrower, but with various form. Seta d of tibiae I and II minute, coupled to respective solenidion σ, in adjacent but separate alveoli; seta d of tibia III and of all genua independent of respective solenidia. Solenidial formula (legs I–IV) of genua (2–1–1–1), tibiae (1–1–1–0), tarsi (3–1–0–0). Subcapitulum anarthric but with groove in labiogenal position; gena with two or three setae; medial cilia of adoral seta or 2 numerous (more than 10), those of pair meeting to form sieve; postpalpal seta distally forked, inserted at nearly right angle to vertical post-like tubercle. Palp setation 0–2–1–3–11, plus solenidion ω.

Immatures. Cuticle less sclerotized than adult, but equally mineralized. Bothridium with two chambers, inner one spiculate. Larva with two simple (type-E) transverse scissures, posterior to setal rows d and e, respectively; nymphs without anterior scissure, posterior scissure of unique form, with pronotaspis deflected sharply ventrad posterior to row e and linear, hinge-like articulation at its base. Suprapleural plate absent from larva; present in nymphs but shorter than that of adult, bears setae h 2 and h 3. Larva with single pair of small porose areas near seta c 3; nymphs with two pairs, second on weak pleural carina. Epimeral setation: La (3–1–2), Pn (3–1–2–1), Dn (3–1–3 –3), Tn (3–1–3–4); 1c scaliform in larva. Genital setation (La-Tn): 0–1–3–7; aggenital seta formed in Tn or adult. La with adjacent pair of preanal spines, lost in later instars. Inguinal seta h 4 and p 4 present in La or not; if present, lost in later instars. Without paraproctal atrichosy or setal regressions; paraproctal setation 4–3–2–2 or 3–3–2–2 (La-Tn). Seta d of leg genua minute, subspiniform in La, normal in later instars. Protonymphal leg setation 0–0–0–0–7; unpaired fastigial seta on anterior face. Subcapitulum with adoral seta or 3 formed in Pn; genal seta m 2 (if present) formed in Dn. Palp femoral seta v” (inf) formed in Dn, tarsal seta cmp formed in Pn.

Nomenclatural problems

Since the original proposal of Eniochthonius by GRANDJEAN (1933), authors have been divided on using that genus name in preference to Hypochthoniella BERLESE, 1910 . Confusion stems from possible misidentification of the type species of the latter genus, as well as several subsequent proposals. The most relevant points are briefly described here in chronological order.

1. Hypochthonius pallidulus C. KOCH, 1835 was proposed based on material from Regensberg , Germany. There is no type specimen, and both the description and illustration are rudimentary .

2. NICOLET (1855) was first to doubt that KOCH’ s (1835) specimen was an adult mite. He considered both H. rufulus and H. pallidulus as “larvae” (used in the sense of immature) of Leiosoma ovata C. KOCH (= Adoristes ovata (C. KOCH )) . For at least H. rufulus he was clearly incorrect.

3. MICHAEL (1888) did not follow NICOLET’ s (1855) assertion and applied the name Hypochthonius pallidulus to a mite that, from his clear description and illustrations, is recognizable with certainty as the common and widespread species herein called Eniochthonius minutissimus (see below). He did not discuss his interpretation of KOCH’ s original description, but we suppose that he focused on the rather diamond-shaped notogaster and two dark bands running transversely across it in KOCH’ s illustration. As described above, Eniochthonius species have two such bands in either transmitted or reflected light, one representing the thickened sulcus at setal row d on the pronotaspis and the other representing the anterior margin of the pygidium. MICHAEL incorrectly interpreted each of these as a “cut” (scissure), separating the “abdomen” (notogaster) into three parts. No other species of Eniochthonius is known to occur in Europe.

4. Hypochthonius minutissimus BERLESE, 1904 was proposed based on material from the Boboli Gardens in Florence.

5. In a brief footnote, BERLESE (1910) proposed Hypocthoniella (sic; a diminutive of Hypochthonius ) as a subgenus of Hypochthonius C. KOCH. He included only Hypochthonius pallidulus , which became the type by monotypy; his species H. minutissimus was not mentioned. He wrote that Hypocthoniella was distinguishable from the nominate subgenus in having an “abdomen” divided into three parts (only two in Hypochthonius sensu stricto). BERLESE neither described nor illustrated his concept of H. pallidula , but this trait clearly indicates that he shared the interpretation of MICHAEL (1888). Hypocthoniella was subsequently elevated to genus rank and emended to Hypochthoniella ( SELLNICK 1928; see also HAMMEN 1959).

6. Apparently unaware of BERLESE’ s (1910) paper, EWING (1917) proposed the genus Arthrochthonius , also with Hypochthonius pallidulus KOCH as type species.

7. In a footnote, GRANDJEAN (1933) stated unequivocally that KOCH’ s Hypochthonius pallidulus was a nymph of Hypochthonius rufulus . He gave no reasons for this interpretation, and we have found no illuminating statements in his subsequent publications. In his opinion such an interpretation had two consequences: 1) Hypochthoniella and Arthrochthonius were then junior synonyms of Hypochthonius ; and 2) there was no named genus to accommodate the common mite described by MICHAEL (1888) as H. pallidulus . Therefore, he proposed the genus Eniochthonius ; the etymology was not explained, but the prefix may derive from aenigma (L.) or ainigma (Gr.) since the situation was certainly enigmatic. Unfortunately, GRANDJEAN chose not to provide a new epithet for the type species, and used an inappropriate attribution: “ Eniochthonius pallidulus ( MICHAEL, 1888) .” He did not discuss the various misinterpretations of morphology in the literature, but his excellent paper was the first to disclose the unique body structure of this genus of mites.

8. HAMMEN (1952) recognized the impropriety of GRANDJEAN’ s (1933) assignment of the species name pallidulus to MICHAEL (1888) and proposed the name Eniochthonius grandjeani for this common species.

9. In an important work on BERLESE’ s oribatid mites, HAMMEN (1959) examined type specimens of Hypochthonius minutissimus BERLESE. He agreed with BERLESE’ s handwritten indication that this was conspecific with Hypochthonius pallidulus, sensu MICHAEL (1888) , and therefore with Eniochthonius grandjeani . He proposed the combination Eniochthonius minutissimus (BERLESE) as a senior synonym of E. grandjeani . However, he also maintained that the presence of labeled material in the BERLESE collection made the concept of Hypochthoniella unmistakable and that the name was therefore valid. He implied that the choice between generic names was simply a preference.

10. MARSHALL (1968) included the first use of the combination Hypochthoniella minutissima (BERLESE) of which we are aware.

11. MARSHALL et al. (1987) pointed out that the International Rules of Zoological Nomenclature require cases involving the misidentification of type species to be submitted to the International Commission on Zoological Nomenclature (ICZN) for ruling. They assumed that GRANDJEAN (1933) was correct, and concluded that, pending such a ruling, Hypochthoniella was invalid and Eniochthonius should be applied to the genus.

Both genus names continue to be used in the literature and the ICZN has not been petitioned for a decision. However, when authors have used Hypochthoniella in recent decades (e.g. PÉREZ-IÑIGO 1969, GHILAROV & KRIVOLUTSKY 1975, BALOGH & MAHUNKA 1983, SERGIENKO 1994, SUBÍAS 2004) the type species invariably has been given as H. minutissimus (BERLESE) . This is clearly inappropriate, since the use of Hypochthoniella assumes that MICHAEL’ s (1888) interpretation of Hypochthonius pallidulus was correct, making H. minutissimus a junior synonym. The name should be either Hypochthoniella pallidula (C. KOCH) , or Eniochthonius minutissimus (BERLESE) , depending on whether the respective opinion of MICHAEL (1888) or GRANDJEAN (1933) is believed.

In the absence of concrete statements, it is difficult to understand GRANDJEAN’ s (1933) certainty that KOCH’ s mite was a nymph of the common species Hypochthonius rufulus and not an adult of Eniochthonius minutissimus . KOCH’ s figure seems synthetic. It has a posteriorly tapered notogaster with anterior width equal to that of the prodorsum and two dark transverse bands, all of which suggest E. minutissimus ; by contrast, H. rufulus nymphs have a notogaster that is broad posteriorly, is noticeably wider than the prodorsum anteriorly, and has a single transverse band. However, the long notogastral setae and long interlamellar seta in KOCH’ s figure are consistent with H. rufulus , but not E. minutissimus . We see no clear choice and reiterate that this possible misidentification of type can only be formally judged by the ICZN.