Introduction

Ptyctimous mites are important representatives of Oribatida and a very species rich group studied worldwide. More than 1,400 species in 40 genera and subgenera are described, nearly 13% of the known mite species (Subías, 2004). They have the ability to fold their bodies (= ptychoidy) as protection against predators, so they are known as ptychoids. This particular strategy it seems has appeared in the evolution of the group at least 3 times (^{Sanders & Norton, 2004}), as a convergent feature and the group of oribatids that present it are heterogeneous and belong to 2 separate supercohorts: Enarthronotides (Protoplophoridae and Mesoplophoridae) and Mixonomatides (Euphthiracaroidea and Phthiracaroidea known as Ptyctima, a monophyletic group [^{Balogh & Balogh, 1992}; ^{Grandjean, 1954}, ¹⁹⁶⁷]). They are known as "box mites" and constitute one of the most important groups of soil microarthropods; they are macro-phytophagous and feed on dead plant material, being most of them xylophagous. They live in litter and dead leaves, where immature stages build irregular galleries and cavities in decayed wood. They take an important part in the mechanical fragmentation of the organic matter, as well as in the decomposition of organic materials, humification and nutrient cycling processes in soil (^{Norton & Behan-Pelletier, 2009}).

Classification of this heterogeneous group is still under discussion. ^{Niedbała (1986)} proposed a classification of the higher taxa of Phthiracaroidea, and his work has served as a reference in the following years. Recently, ^{Niedbała and Liu (2018)} presented a list of all the worldwide species of ptyctimous mites known.

Superfamily Phthiracaroidea is one of the most species rich oribatid mite superfamilies, with nearly 750 species in 20 genera (^{Norton & Behan-Pelletier, 2009}; ^{Subías et al., 2012}). The number of families and the lower rank classification of the group is under discussion. ^{Norton and Behan-Pelletier (2009)}, ^{Woas (2002)} and other authors recognized a single cosmopolitan family Phthiracaridae. ^{Niedbała (1986)} gave a new hypothesis on the phylogeny of the Phthiracaroidea with cladistic theory, and proposed some new taxa, recognizing 2 families: Phthiracaridae Perty, 1841 and Steganacaridae ^{Niedbała, 1986}, the last with 2 subfamilies, Steganacarinae ^{Niedbała, 1986} and Atropacarinae ^{Niedbała, 1986}. Atropacarinae composed by 5 genera: Austrophthiracarus Balogh & Mahunka, 1978, Calyptophthiracarus^{Niedbała, 1994}, Protophthiracarus Balogh, 1972, Notophthiracarus Ramsay, 1966 and Atropacarus Ewing, 1917. Atropacarus formed by 2 subgenera: Atropacarus (Atropacarus) Ewing, 1917 with Hoplophora stricula C. L. Koch, 1936 as its type species and Atropacarus (Hoplophorella) Berlese, 1923 with Hoplophorella cucullatus Ewing, 1909 as its type species. The genus is characterized by 15 pairs of notogastral setae and only 2 pairs of lyrifissures, ia and im present; 9 pairs of genital setae arranged in a single row and 5 pairs of anoadanal setae present; setae ad₂ remote from the paraxial margin; setae d of tibiae IV minute and coupled with solenidia (^{Niedbała, 1992}, ²⁰⁰⁴; ^{Niedbała & Liu, 2018}).

Knowledge of Mexican ptyctimous mites is broad, most of the known taxa are endemic and have been recorded from few regions in the country (^{Niedbała, 2004}; ^{Palacios-Vargas & Iglesias, 2004}; ^{Vázquez & Prieto-Trueba, 2001}). Records of 60 species from Mexico have been reported (^{Niedbała, 2004}; ^{Niedbała & Liu, 2018}), which include 17 genera from 6 families. Steganacaridae is represented by the highest number (6 genera; 21 spp.) and Mesoplophoridae with just 1 (Table 1). Until now 7 species belonging to Atropacarus are known from Mexico, 5 included in the subgenus Hoplophorella: A. (H.) brachys,^{Niedbała 2004}, A. (H.) brevipilosus,^{Niedbała, 2004}, A. (H.) cucullatus (Ewing, 1909), A. (H.) hamatus (Ewing, 1909) and A. (H.) vitrinus, (Berlese, 1913) and 2 in Atropacarus: A. (A.) plumatus^{Niedbała, 2004} and A. (A.) striculus (C. L. Koch, 1836). For the Yucatan Peninsula records of 2 species are known: A. (H. ) vitrinus Niedbała, 2001 and A. (H.) hamatus (Ewing, 1909) from Cozumel Island, Quintana Roo (^{Niedbała & Ermilov, 2017}) (Table 2).

In this contribution we present new records for 2 species of A. (Hoplophorella), collected recently from litter and soil samples in continental Quintana Roo state, Mexico. We add new localities for A. (H.) hamatus from material previously collected from different localities and states and identified as Hoplophorella sp. 3 by ^{Ojeda (1983)}. We added complementary information to the description of each species and a key to all known species of Atropacarus (Hoplophorella) in Mexico.

Materials and methods

During a collecting trip to the region by JGP-V in 2017, samples of litter at the Environmental Management Unit (UMA in Spanish) "Tepez", located 7 km from the town of Raudales, were obtained and placed in plastic bags for transportation to the laboratory of Dr. Magdalena Vázquez, Universidad de Quintana Roo, Chetumal, Quintana Roo. They were processed by Berlese-Tullgren funnels, and specimens preserved in 70% ethanol. For identification, the mites were macerated in lactic acid, and then mounted in Hoyer's solution. Observations, measurements and illustrations were made using a Zeiss phase contrast microscope equipped with a camera lucida. Terminology follows ^{Niedbała (1992}, ²⁰⁰⁴⁾. All measurements are given in micrometers (|im). All specimens were collected under the scientific collector's license issued to J. G. Palacios-Vargas, and are deposited in the collection of Laboratorio de Ecología y Sistemática de Microartrópodos, Facultad de Ciencias, UNAM (LESM).

Results

Herein we present new records of A. (Hoplophorella) singularis and A. (H.) hamatus for Mexico, giving a complementary description of each species, and an identification key to all the known species of Atropacarus (Hoplophorella) for the country.

Phthiracaroidea Perty, 1841

Steganacaridae ^{Niedbała, 1986}

Atropacarus Ewing, 1917

Type species: Atropacarus (Atropacarus) striculus C. L. Koch, 1936.

Diagnostic characters. Body surface with concavities. Posterior furrows of prodorsum present, lamellar setae minute (length ratio of lamellar setae/prodorsum < 0.18). Ventral region: genital setae in a row or nearly so, distance between setae g6 and g5 longer than g5 and g4, setae ad1 always close to paraxial margin, in a row with anal setae; setae ad2 remote from paraxial margin or close to it. Legs: setae v' on femora I minute (length ratio v"/v' < 2.25); 15 pairs of notogastral setae, 9 pairs of genital setae and 5 pairs of anoadanal setae. Seta ad2 slightly away from paraxial margin (^{Niedbała, 2004}).

Atropacarus (Hoplophorella) Berlese, 1923

Type species: Atropacarus (Hoplophorella) cucullatus (Ewing, 1909).

Diagnostic characters. Fifteen pairs of notogastral setae, as a rule only 2 pairs of lyrifissures, ia and im present. Ventral region, 9 pairs of genital setae and 5 pairs of ano-adanal setae present on ano-adanal plate; setae ad2 remote from paraxial margin. Legs: setae ft" on tarsi I normal (^{Niedbała, 1994}).

Atropacarus (Hoplophorella) singularis (Sellnick, 1959)

(Figs. 1-10)

Size. Measurements: prodorsum: length 318, width 228.2; notogaster: length 541.6, width 278.4; genito-aggenital plate151 × 111; ano-adanal plate 192 × 141. Color cuticule: light to medium brown. Surface of body covered with round to irregular concavities - foveolae (Fig. 1), finely punctuated along the entire surface and inside concavities.

Prodorsum. Length 279 (N = 5): 246.5 - 384.5; width 229 (182.9 - 254.2) (Fig. 2). Prodorsum oval in shape, rostrum with rounded tip, sigillar fields with medium-deep sinus between rostral setae, 1 pair of strong median cristae present. Lateral carinae and posterior furrows well marked. Seta ro acicular, smooth and inserted almost in 1/3 from rostral apex (35). Seta in (120) bearing small barbs in distal half at both sides. Seta le small, smooth and spiniform (19) inserted close to bo (distance between bo and in, 19); in > ro > le > ex. Seta ex, located just down to botridium; small, fine and setiform. Sensillus elongated with a club-like head (Figs. 1, 2). Surface of prodorsum with prominent irregular concavities (foveolae) towards anterior one-third of tip with dense and fine punctuations towards the whole prodorsum surface and inside foveolae (Fig. 2).

Mouthparts. Infracapitulum with rutellum well developed and anterior (or₁) and posterior (or₂) adoral setae are ciliated. Posteriorantiaxial setae (or₃) setiform. Anterior (a) and median (m) smooth and setiform. A pair of minute setiform setae (h) (Fig. 3). Chelicera chelate-dentate type, cha and chb setae on fixed digit; with few small spines on antiaxial surface (Fig. 4). Pedipalp is 3 segmented with setal formula 2-2-7. Solenidion and a euphatidial seta (sul) close to solenidion base on tarsus (Fig. 5).

Notogaster (Fig. 1). Length (N = 5) 526 - (range 453 - 601); width 295 - (range: 246 -74). Notogaster with a globular shape and a rounded posterior end. Surface ornamentation with very prominent, round and deep foveolae. Fifteen pairs of notogastral setae which are rigid, almost erect, c1< cl - dl, covered with small barbs on both sides at distal third. Setae c1, c2 and c3 close to anterior margin. Setae dl, d2, e2, h3 < cl, c2, c3, el. Setae psl, ps2 and h1 longest. Vestigial seta f1 anteriorly of h1. Two pairs of lyrifissures ia and im present.

Figures 1-3 Atropacarus (Hoplophorella) singularis. 1) Lateral view of the body (legs removed); 2) prodorsum, dorsal view; 3) infracapitulum. 

Anogenital region (Fig. 6). Each genital plate rectangular in shape, carrying broad and short aggenital plate with a minute seta ag at its outer corner. Genital setae g_l - g₉ visible, arranged in a row towards the inner margin, g_l - g₃ smaller than others (measuring 5) and very closely arranged. Genital plates ornamented with deep concavities "foveolae", more visible on external border. Ano-adanal plates are broader towards the middle region and rounded in posterior border. Three pairs of adanal and 2 pairs of anal setae present. Anal setae setiform, thin and pointed tip. Ad₂ is much longer than ad_l and ad₃ (146/ 59 respectively). Seta ad₃ with 1/3 distal end medium-size barbs on both sides. Integument with foveolae as well as fine punctuations along all plates (Fig. 6).

Figures 4-6 Atropacarus (Hoplophorella) singularis. 4) Chelicera; 5) pedipalp; 6) right side of ano-genital region. 

Legs (Figs. 7-10). Leg I longest of all. Tiny pores on either trochanter or femur or both. Tarsus of each leg monodactyl. Each claw strongly developed and bidentate midventrally. Anterior tooth is always more developed than the posterior. Formulae of setae "incomplete type". I (1-4-1-4-16), II (1-3-2-2-12), III (2-2-1-1-10), IV (21-1-1-10). Ratio length of v " to v ' on femur I is 2.25. Solenidiotaxy on the genu, tibia and tarsus I (2-1-3), II (11-2), III (1-1-0), IV (0-1-0) typical for all Phthiracaroidea.

Figures 7, 8 . Atropacarus (Hoplophorella) singularis. 7) Leg I; 8) leg II. 

Figures 9, 10 . Atropacarus (Hoplophorella) singularis. 9) Leg III; 10) leg IV. 

Seta d on femora I stout, bent and not bifurcated, situated at distal end of article; seta l' on genua IV present and slightly barbulate.

New record. Material examined: Mexico: Quintana Roo: Municipality Othón O. Blanco, Chetumal: Puente Xulha, 18°33'12" N, 88°27'30" W, 6 m asl, ex litter. 27-VII-2017, J. G. Palacios-Vargas, col.

Atropacarus (Hoplophorella) hamatus (Ewing, 1909)

(Figs. 11-19)

Size. Measurements: prodorsum: length 279, width 229; notogaster: length 526, width 225; genito-aggenital plate 777 × 156; anoadanal plate 92 × 141 (Fig. 11). Color cuticule: light to medium brown.

Prodorsum. Length (n = 5): 279 (range: 246 - 384); width: 229 (range: 182.9 - 254.2). Prodorsum oval in shape, rostrum with slightly pointed tip in lateral view (Fig. 11). Seta ro stiff, slightly bent forward, smooth and acicular, inserted in the last 1/4 of rostral apex, measuring 35 in length. Seta in clavate, almost 1¹/2 times le. Seta le small, acicular and smooth (16), distance between bo and in, 19. Seta ex vestigial. in > ro > le. Sensillus elongated, narrow, sickle-shape with small spine-like ondulate serrations on one side at distal end (Fig. 12). Prodorsum with fine punctuations towards anterior one-third of tip and whole prodorsum surface.

Mouthparts. Infracapitulum stenarthrous type. Rutellum well developed; anterior (orl) adoral setae ciliated; setae (or₃) setiform. Anterior (a) and median (m) smooth setae. Posteriorly, setae h half size of a and m, setiform (Fig. 13). Chelicera, chelate-dentate type, bearing dorsal (cha) and lateral (chb) setae on fixed digit. Pedipalp 3-segmented with setal formula 2-2-8. There is a solenidion (ω) and 1 euphatidial seta (sul) close to solenidion base on tarsus (Fig. 14).

Notogaster. Length 526 - (Range 453.5 - 601.4); width 295.8 (Range 246.5 - 374.7). Notogaster globular shaped, posterior end rounded (Fig. 11). Ornamentation represented by fine punctuations along the whole body. Anterior collar absent. 15 pairs of leaf- shape notogastral setae swollen distally, very conspicuous, bent backwards and spatulate in shape (Fig. 11). Number and arrangement of setae typical of subgenus Hoplophorella. Seta c_l originates further (from the collar line) than c₂ and c₃. Seta c_l < c_l - d_l. Lyrifissures ia located between d₂ and e₂ setae; setae h_l and ps_l arising from a protuberance.

Ano-genital region. Plates clearly visible in lateral view (Fig. 11). Ano-adanal plates somewhat rectangular in shape (Fig. 15). Proximal margin projects anteriorly to form an interlocking device with posterior corner of aggenito-genital plate and all along anterior margin, there are overlapping chitinized lobes on right and left adano-anal plates. Each genital plate is rectangular in shape, carrying broad and short aggenital plate, with a minute seta ag at its outer corner. Genital setae g_l - g₉ visible, arranged in a row towards the inner margin, g_l - g₃ smaller than others (measuring 5 and very closely arranged (formula 6:3). Genital plates ornamented with fine punctuations. Ano-adanal plates are a little broader towards the middle region and rounded in posterior border. Three pairs of adanal and 2 pairs of anal setae present. Anal setae setiform, thick and pointed tip; ad₂ as longer as ad_l and ad₃; ad₂ resembles notogastral setae leaf - shape; ad_l resembles anal setae; ad₃ short, smooth and aciculate. Integument with fine punctuations more visible in the middle part of plates.

Figures 11, 12 . Atropacarus (Hoplophorella) hamatus. 11) Lateral view of body (legs removed), with notogastral seta detail; 12) prodorsum, dorsal view with detail of sensillus. 

Figures 13-15 Atropacarus (Hoplophorella) hamatus. 13) Infracapitulum; 14) pedipalp; 15) ano-genital region. 

Legs (Figs. 16-19). Leg I longest of all. Each leg has 5 segments. Tiny pores similar to ornamentation on epimers are seen on trochanter, femur or both. Tarsus monodactyl. Each claw is strongly developed and with well-developed teeth midventral. Leg chaetotaxy "incomplete type", as follows: I (1-4-1-4-16), II (1-3-2-2-12), III (2-2-1-1-10), IV (2-1-0-1-10). Seta d on femora IV remote from distal end of article; seta l' on genua IV absent. Solenidiotaxy on genu, tibia and tarsus I (2-1-3), II (1-1-2), III (1-1-0), IV (0-1-0) typical for all Phthiracaroidea.

Figures 16, 17. Atropacarus (Hoplophorella) hamatus. 16) Leg I; 17) leg II. 

Figures 18, 19 . Atropacarus (Hoplophorella) hamatus. 18) Leg III; 19) leg IV. 

Material examined. Mexico: Quintana Roo: Raudales UMA El Tepez 18°32'17" N, 88°17'08" W, 7 m asl: 25-vii-2017. J.G. Palacios-Vargas leg. Atropacarus (Hoplophorella) hamatus correspond to the same species cited and preliminary described as Hoplophorella sp. 1 in ^{Ojeda (1983)}. With this work we start the formal naming and publication of ptychoid mites reported by the first author.

Previous records for Mexico (^{Niedbała, 2004}). Jalisco: Puerto Vallarta, above Mismaloya, litter from mixed forest, coastal zone, 2 X 1995, leg. W. Niedbała - (1 specimen); Puerto Vallarta, palm litter near the beach of Hacienda Buenaventura, 26 IX 1995, leg. W. Niedbała - (1 specimen.); Puerto Vallarta, Quimixto, waterfalls, litter from mixed forest near cascade, 27 IX 1995, leg. W. Niedbała - (1specimen); near Puerto Vallarta, El Edén - Mismaloya, near road jungle, litter, 30 IX 1995, leg. W. Niedbała - (1 sp.); 107 km N of Chamela, litter of Pinus oocarpa? leg. F. Mata - (1 sp.). Veracruz: Cabanas, ex trunk, 11 IX 1983, leg. J. Palacios - (1 specimen); Palma Sola (700 m asl), litter of tropical rainforest, 1 X 1976, leg. P. Lavelle - (1 specimen). Guerrero: Taxco, cave "Aguacachil", 25 I 1981, leg. J. Palacios col. - (1 specimen); Sotano Tilaco, soil, 21 XII 1980, leg. A. Guzmán - (4 specimens); Puebla, Yohualichán, litter of coffee plantation, 24 VII 1981, leg. J. Palacios - (1 specimen); Oaxaca, El Suspiro, Carr. Tuxtepec-Oaxaca, Santiago Comaltepec (2,000 m asl), 29 III 1988, leg. A. Luis - (1 specimen.); Yucatán, ca 100 km S of Mérida, between Lol-Tun and Labna, tropical forest, 11 IX 2002, leg. W. Niedbała - (12 specimens). New records: Hidalgo: Cañada Otongo, 700 m.a.s.l., Tropical forest, Pinus-Quercus forest, ex litter, 10- X-80, 10-V-80, R. Johansen leg. Zacualtipán, 24-I-81, R. Johansen leg. Guerrero: Gruta de Acuitlapán, ex soil and litter, 24-I-81, J. Palacios leg. Grutas de Juxtlahuaca, Deciduous forest, ex soil, 11-IV-81, J. Palacios leg., 24-VII-82, M. Ojeda leg. Road to Colotlipa riverside ex soil, decaying trunk and litter, 5-VIII-82, M. Ojeda leg. Piedras Negras, ex detritus, 25-VIII-81, J. Palacios leg. Morelos: Derrame del Chichinautzin, ex litter, decaying trunk, 27-XI-76, J. Palacios leg., 7-XI-76, P. Rico leg. Puebla: Villa Juárez, Rancho Grande, ex soil, 28-X-79, J. Palacios leg. Yohualichán, ex litter in coffee plantation, 24-VII-77, J. Palacios leg. San Luis Potosí: Sótano Tlamayo, ex soil, 14-III-82, H. Guzmán leg. Huichihuayán, rainforest, ex soil, 22-XI-75, J. Palacios leg.

Remarks

Oribatid ptyctimous mites are a diverse and abundant group with few specialists around the world. Due to this, many of the descriptions that are available have not addressed in detail many of the morphological characteristics for the recognition of the species. Among them are the leg chaetotaxy, type of infracapitulum, details that we add for the 2 Atropacarus (Hoplophorella) redescribed here. For Atropacarus (Hoplophorella) hamatus we observe some differences from what was originally described. The Mexican specimens have the spoon-shape notogastral setae but they do not present the small spines on the distal portion. However, all other characters described for the species can be seen in the specimens from Quintana Roo, so the differences could be taken as morphological variations.

The ptyctimous mite fauna of the Neotropical Region is characterized by a great richness and diversity, a large number of endemic species and is much richer than the faunas of other regions of the world (^{Niedbała, 2004}). The total number of species known from this region is 305, with a dominant number of Phthiracaroidea (193 species). Records of 60 species, 17 genera from 6 families have been reported for Mexico (^{Niedbała 2004}; ^{Niedbała & Liu 2018}). The family Steganacaridae is represented by the highest number (6 genera; 21 spp.). According to ^{Niedbała (2004)}Atropacarus (Hoplophorella) species are poorly represented in Mexican territory and are rather common in Central America and the Antilles. The 5 species of Atropacarus (Hoplophorella) previously known from Mexico are recorded in 7 states (Guerrero, Jalisco, Oaxaca, Puebla, Quintana Roo, San Luis Potosí and Yucatán) most of them associated with leaf litter of tropical forest, some living in caves and others under decaying tree trunks (Table 2). A. (H.) cucullatus, A. (H.) hamatus and A. (H.) vitrinus have a semi cosmopolitan distribution, (A. (H.) brachys and A. (H.) brevipilosus) are clearly Neotropical and are distributed in Mesoamerica and the Antilles, and 1 is endemic (Atropacarus (Hoplophorella) plumatus). The record of a new locality for Atropacarus (Hoplophorella) hamatus expands its presence in 7 states of the country. As for the new record of Atropacarus (Hoplophorella) singularis is consistent with the general similarity between the fauna of the Antilles and the lowlands of Mexico, as pointed out by ^{Niedbała (2004)} for the general pattern of neotropical ptichoid mites.