Tarsonemini, Canestrini & Fanzago, 1877

Lofego, Antonio Carlos, Cavalcante, Ana Cristina Cerqueira, Demite, Peterson Rodrigo, Rezende, José Marcos, Ochoa, Ronald & Moraes, Gilberto José De, 2019, Reinstatement of Metatarsonemus Attiah (Acari: Tarsonemidae), with description of a new species, redefinition of the genus and a key to the world species, Zootaxa 4711 (2), pp. 307-329 : 310-311

Tribe: Tarsonemini Canestrini & Fanzago, 1877

Genus: Metatarsonemus Attiah, stat. rev.

Metatarsonemus Attiah, 1970: 188; Lofego et al., 2005: 7.

Tarsonemus Lindquist, 1985: 1279 ; 1986: 333.

Type species: Metatarsonemus simplicissimus Attiah , by original designation.

Diagnosis: The apomorphic characters of genus are found in females, and are basically two: a) the presence of a set of fissures on the venter of the idiosoma: a pair near the base of seta 1a (present or absent); a pair laterad of the bases of setae 1a and 2a, sometimes with posterior end overlapping apodemes 2 (always present); a pair near the base of seta 3b, sometimes overlapping apodemes 4 (present or absent); an unpaired fissure at the posterior end of the poststernal apodeme (always present); and b) a pair of linear ridges flanking the tegula. Other characteristics that in conjunction also characterize the genus are as follows. Both sexes: gnathosomal capsule subcircular, pharynx wide and fusiform. Females: dorsal idiosomal setae on plates D and EF (setae d, e and f) thick and barbed; dorsal seta sc2 inserted at same level or anteriad insertion of bothridial sc1; anterolateral ends of apodemes 1 forming prominent angular projections next to posterolateral corners of gnathosoma. Males: anterior margins of plates III and IV fused and concave. Solenidion ω on tarsus II longer than that of tarsus I. Apodemes 5 present or absent. Leg IV robust, without a flange on femorogenu and with separate tibia and tarsus.

Gnathosoma. BOTH SEXES. Capsule subcircular, not beaklike anteriorly; palpcoxal setae not discernible. Dorsal gnathosomal setae smooth with acute apex. Palpi directed anteriorly, short and robust, no more than 2x longer than wide, each with two minute setae and two or three inconspicuous processes on distal extremity. Cheliceral stylets short, with basal levers hardly discernible. Pharynx wide (about 0.3x as wide as capsule), fusiform, muscular, with lightly sclerotized walls; without conspicuous paired gland-like structures posteriorly .

Idiosoma: FEMALE. Dorsal shield smooth. except for tiny punctation. Dorsal idiosomal setae acuminate, smooth or serrate, and not elongate. Prodorsal shield extending over gnathosoma, anterolateral margins slightly emarginated at level of stigmata. Stigmata on anterolateral margins of prodorsal shield, midlength between v1 and scapular setae; main tracheal trunks with unsclerotized atria, and without sclerotized or divided postatrial structures. Pits v2 located mediad oblique alignment of v1 and sc2, near the latter; seta sc2 inserted approximately at midlevel of prodorsal shield. Seta sc1 capitate, covered by prodorsal shield. Posterior margins of tergites not conspicuously concave. Setae c1 closer together than to setae c2. Venter with apodemes 1 fused to each other and with prosternal apodeme to form Y-shaped structure.Anterolateral ends of apodemes 1 forming prominent angular projections close to posterolateral corners of gnathosoma. Apodemes 2 not fused with prosternal apodeme. The latter well sclerotized up to level slightly posteriad medial ends of apodemes 2, then widening and less conspicuous until reaching sejugal apodeme, which is continuous and slightly arched. Apodemes 3 extending from anterior end of trochanter III to about longitudinal level of insertion of 3a. Apodemes 4 extending convergently from insertion of 3b to central region of poststernal apodeme. Poststernal apodeme bifurcate anteriorly. Bases of legs IV separated by interval 2–3 times diameter of trochanter IV. Coxisternal plates with a variable number of fissures on venter of idiosoma: a pair near base of seta 1a (present or absent); a pair laterad bases of 1a and 2a, sometimes with posterior end overlapping apodemes 2 (always present); a pair near base of seta 3b, sometimes overlapping apodemes 4 (present or absent); an unpaired fissure at posterior end of poststernal apodeme (always present). Anterior margin of coxisternal plates III straight. Tegula moderately long to short, about as long as or shorter than its basal width, rounded apically. Posterior extensions of coxisternal plates IV approaching, touching or barely overlapping each other beneath tegula. With a pair of linear ridges flanking tegula.

MALE. Dorsal idiosomal setae not elongate. Prodorsum with four pairs of setae, sc2 inserted somewhat laterad in relation to oblique longitudinal alignment with other three pairs (v1, v2, and sc1). Distance c1-c1 slightly shorter than distance c2-c2. Prosternal apodeme interrupted medially near medial end of apodemes 2. Sejugal apodeme continuous or interrupted. Coxisternal plates III with two pairs of setae. Coxisternal plates IV without setae, flanked by plates III laterally but not anteriorly; anterior margins of coxisternal plates III–IV united in a unique concave line. Apodemes 4 connected to apodemes 3 anteriorly on each side; both sides connected medially where joining poststernal apodeme. Poststernal apodeme splitting into apodemes 5 on posterior end or without apodemes 5.

Legs: Female and male with ambulacrum of leg I with a single, unciform claw; ambulacra of legs II and III with empodia well developed and symmetrically paired claws. Femora I and II of females with ventral flanges. Tibiotarsus I on female slightly bent in region of fusion, resembling an elbow. Excluding trochanter, legs I and III of females and males with similar length, slightly longer than leg II; no segment distinctly elongate. Femorogenu apparently immovably connected to tibia, though fused with it (except in M. caissara ). Leg III of adult male not enlarged, femur and genu separated, articulating with each other. Leg IV of female with femorogenu about 1.5 to 3.0 times as long as tibiotarsus, each with two setae. Leg IV of male with femorogenu not constricted near base, lacking flanges and with three setae, of which v’G or l”G is longest, but not longer than segment; tibia distinct from tarsus, with rodlike solenidion moderately long, but shorter than tibia, and with a tactile seta elongate but shorter than femorogenu IV; tarsus with three setae and with well-developed and robust terminal unciform claw. Number of tactile setae (solenidia in parentheses) on femur, genu, tibia and tarsus respectively: female – leg I, 4-4-6(1 or 2)+8(1); leg II, 3-3-4-5 or 6 (1); leg III, 1+3-4-5; male – leg I, 4-4-6(1 or 2)+9 or 10(1); leg II, 3-3-4-5 or 6 (1); leg III, 1+3-4-4 or 5. In both sexes legs I-III with femoral and genual setation complete; tibial sensory cluster of leg I complete or incomplete (sometimes lacking φ1); tarsus I with seta pl’ present and pl’’ absent; tarsus II with or without spine-like pl’’; subunguinal seta s of tibiotarsus I and unguinal seta u’ of tarsi II and III thorn-like. Female with femoral seta l’ on leg I barbed and thickened. Male tarsus I with one or two minute setae ft’-ft”.

Remarks. The genus Tarsonemus does not present any derived character state, as observed in many other genera in Tarsonemidae . Therefore the inclusion of species under this genus occurs by default ( Lindquist 1986). In this context Tarsonemus is an extensive amalgamate of somewhat morphologically remote forms ( Magowski 2010). Thus it is desirable that any group of species identified as a separate evolutionary branch be removed from Tarsonemus . This is the case for Metatarsonemus which, except for Tarsonemus longitibialis (Attiah) as it is based on males alone, presents itself as a cohesive group with synapomorphies and a well-defined morphological pattern. The presence of fissures may be listed as a set of characters, given the number of fissures in the ventral region, usually close to the apodemes. We have carefully observed these fissures under various microscopic techniques (DIC, Phase contrast, Confocal, LT-SEM). Through these, it is clearly demonstrated that, although adjacent, these fissures are morphologically independent of the associated apodeme, positioned at different levels and in some cases overlapping the apodema. Another apomorphy is the pair of ridges flanking the tegula, which might be homoplastic with a similar structure present in Giselia . Under confocal microscope observation, the fluorescence reaction of these ridges area is identical to other apodemes (R. Ochoa observation). However, histological studies would be necessary to confirm these structures as apodemes.

These mites are very common on plants, especially in the tropical regions of Brazil, where we have found several specimens fitting the morphological pattern of Metatarsonemus described above. Thus it is important to re-establish the genus to avoid future taxonomic misconceptions, given that our analysis has shown that Metatarsonemus, as defined in this study, is a genus evolutionarily independent from Tarsonemus .