Pygmephoroidea

Venter (left) and dorsum of female Pediculaster americanus, from Walter et al. (2009).

Belongs within: Eleutherengonides.
Contains: Siteroptes, Acinogaster, Bakerdania, Pygmephorus, Imparipes, Scutacarus.

The Pygmephoroidea are a group of primarily free-living mites that mostly feed on fungi. Many species have dimorphic females with distinct phoretic and non-phoretic forms. This has lead to taxonomic confusion with different forms of a single species treated as distinct genera or even families. Members of the Siteroptidae have all setae on the first femur slender and are associated with plants or fungi. The Pygmephoridae have seta d on the first femur thickened and modified in shape, and are free-living or associated with fungi, insects or animal nests. Females of Siteroptidae and Pygmephoridae exhibit extreme hysterosomatic physogastry with offspring developing to or near maturity, and sometimes even mating, within the parent and often emerging when the mother’s body wall ruptures (Walter et al. 2009). Certain species of the pygmephorid genus Pediculaster may infest commercial mushroom houses.

In members of the families Scutacaridae and Microdispidae, the prodorsum of females is usually covered to a greater or lesser extent by tergite C; these families may be separated as the Scutacaroidea. In Scutacaridae, tergite C of females forms a roof completely covering the prodorsum and gnathosoma; tergite C is less expansive in the Microdispidae. Males of Scutacaridae will locate immobile pharate female nymphs and carry them until they emerge as mature females, at which point they mate with them. Some Microdispidae appear to be parasitoids on insects, attaching themselves to the host by blade-like chelicerae or pedipalps (Walter et al. 2009).

The Pygmephoroidea: lives of phoresy and fungi
Published 16 July 2012
Slide-mounted specimen of pygmephorid, Siteroptes sp., photographed by Qing Hai Fan.

Today’s subjects, the Pygmephoroidea, are an assemblage of mites that are mostly free-living feeders on fungi, though exceptions occur. The taxonomic coverage of Pygmephoroidea does vary a bit between systems: Walter et al. (2009) divide the taxa covered in this post between two superfamilies Pygmephoroidea and Scutacaroidea, but Khaustov & Ermilov (2011) combine them all in the one superfamily. Taxonomy of the pygmephoroids is further complicated by presence in many species of distinct adult female forms, some of which were different enough to have been placed by past authors into different families. The normal adult female is a fairly sedentary individual, but other females are what are referred to as ‘phoretomorphs’, specialised to be able to disperse by hitching a ride on other animals (most commonly larger arthropods), a process known as ‘phoresy’. Phoretomorphs tend to be more heavily sclerotised than the normal females, and may also differ in features such as fusion of the tibia and tarsus of the first pair of legs, and enlargement of the claws on the second and third pairs. For the most part, phoretic mites are only passengers of their hosts, and do not cause them any noticeable harm. However, some pygmephoroids in the family Microdispidae have become parasites.

Like ants and wasps, pygmephoroids have what is called a haplodiploid system of sex determination where unfertilised eggs hatch into haploid males and fertilised eggs into diploid females (Camerik et al. 2006). Only larvae and adult females feed; adult males live only to mate. In between the active larval and adult stages of the pygmephoroid life cycle is a quiescent nymphal stage (comparable to the pupal stage in insect development). Adult males of the family Scutacaridae will pick up a quiescent female nymph and carry her until she moults to adulthood and is ready for mating. After mating, normal females of Pygmephoridae and Microdispidae become physogastric: the hind part of their body swells up to relatively enormous size as their eggs develop within them; females of the pygmephorid Siteroptes ceralium may produce up to 500 offspring each (Scutacaridae swell slightly, but not to the extent of the other two families; phoretic females do not generally become physogastric until after they’ve left their dispersal host). In the microdispid genera Glyphidomastax and Perperipes, that feed on the larvae or eggs of the army ants they live with, the elongate physogastric body may mimic a young ant larva (Walter et al. 2009). In at least some species of Pygmephoridae, the progeny of a female will develop through the larval stage while still within the mother, and emerge at or close to maturity (a process that often involves the mother literally bursting open; even if it doesn’t, pygmephoroid females do not survive long past egg-laying). As previously discussed in the related mite Acarophenax, males and females may even copulate while still inside their mother. In the pygmephorid Xenaster longiabdominalis, copulation occurs outside the mother, but the fully developed males are already grabbing their unmated sisters and carrying them away at the time of birth (Walter et al. 2009).

SEM of the scutacarid Imparipes, by Ernst Ebermann. In species of this family, one of the dorsal shields has become extended forward to cover the front of the body.

Some pygmephoroids such as the microdispid Microdispus lambi and the pygmephorid Pediculaster flechtmanni can cause damage to commercial mushroom crops. Other species are vectors of plant diseases, such as Siteroptes cerealium spreading carnation bud rot. The phoretomorphs of many pygmephorids possess structures on their underside to carry spores of their preferred food fungus with them as they travel; in a number of cases, the fungus in question happens to be a plant pathogen.

Systematics of Pygmephoroidea

Characters (from Walter et al. 2009, as Pygmephoroidea + Scutacaroidea): Female gnathosoma capsulate, circular, oval or elongate, visible or hidden from above, with one or usually two pairs of dorsal setae; palpi closely appressed, usually reduced, rarely elongated and blade-lke; movable cheliceral digits stylet-like, partially retractile, rarely vestigial; pharynx divided into two or three muscular sections. Male gnathosoma reduced, tubiform, nonfunctional trophically. Prodorsal shield of females with pair of anterolateral stigmata (extended into troughlike peritremes in some taxa) and associated tracheae, and with two or usually three pairs of setiform setae and with or without a pair of anterolateral bothridia with capitate sensilla, the latter represented by a pair of normal scapular setae in larvae and males. Opisthosoma of females with first dorsal plate C entire, sometimes expanded anteriorly to overlap or cover prodorsum. Adults with coxisternal plates I-II united medially, forming prominent prosternal apodeme; female coxisternal plates III-IV united medially, usually forming poststernal apodeme. Female genital opening small, covered by consolidated aggenital plate, genital and aggenital setae usually absent (two pairs of aggenital setae present in Meristoplaxa). Males with plates C and D consolidated, and with aedeagus tubiform, enclosed in caudal genital capsule formed by consolidated plates H, PS, Ag. Females with legs I similar to or thicker than legs II-IV and with single claw sometimes enlarged for grasping, or thinner than other legs and without ambulacrum and claws; legs II-IV dissimilar in form, each with paired claws and stalked empodium or these absent on legs IV; trochanter IV quadrangular, longer than wide, in contrast to subtriangular trochanters I-III. Adult coxisternal plates I-II together with 4-6 pairs of setae; trochanters I-IV each with a setae; femur I with 3-5 setae; genu I with 2-4 setae; genu II with 2-3 setae. Male leg IV stout, its tarsus variably formed, with or without a single claw, with or without a broad, short empodium, sometimes with 1-2 apical setae elongated and 1-2 other apical setae short, spinelike. With two active postembryonic instars (larva and adult); single nymphal instar an inactive calyptostatic apoderm.

<==Pygmephoroidea [Scutacaroidea]
    |--Neopygmephorus Cross 1965H98 [NeopygmephoridaeZF11]
    |    |--N. arvorum (Jacot 1936)SL71
    |    |--N. luxtoni Mahunka 1970SL71
    |    |--N. novaezelandicus Mahunka 1970SL71
    |    |--N. pannonicus (Willman 1951)SL71
    |    |--N. sellnicki (Krczal 1958)SL71
    |    |--N. tarsalis (Hirst 1921)SL71
    |    |--N. togatus (Willman 1942)SL71
    |    `--N. tripartitus Cross 1964SL71
    |--SiteroptidaeWL09
    |    |--AsensillaWL09
    |    |--DiroptesWL09
    |    |--SevastianovellaWL09
    |    |--PseudobakerdaniaWL09
    |    |--DudichianaWL09
    |    |--MetasiteroptesWL09
    |    |--Meristoplaxa [Meristoplaxinae]WL09
    |    |--ZambedaniaWL09
    |    `--SiteroptesH98
    |--Microdispidae [Microdispini]CT01
    |    |--Perperipes ornithocephalaWL09
    |    |--PhyllodispusWL09
    |    |--CochlodispusWL09
    |    |--TubulodispusWL09
    |    |--VietodispusWL09
    |    |--ScutodispusWL09
    |    |--PunicodoxaWL09
    |    |--GlyphidomastaxWL09
    |    |--Myrmecodispus doryliniWL09
    |    |--UnguidispusWL09
    |    |--CaesarodispusWL09
    |    |--ReductodispusWL09
    |    |--PeponacaraWL09
    |    |--Microdispus (Premicrodispus)WL09
    |    |    |--M. (P.) chandleriWL09
    |    |    `--M. (P.) lambi (Krczal 1964)WL09, H98 [=Pygmephorus lambiH98, Brennandania lambiH98]
    |    `--Brennandania Sasa 1961H98
    |         |--B. angustaM83
    |         |--B. parasilvestrisM83
    |         |--B. scolopendraeWL09
    |         `--B. silvestrisM83
    |--Pygmephoridae [Pygmephorinae]C07
    |    |--Luciaphorus perniciosus [incl. L. auriculariae, L. hauseri]WL09
    |    |--Parapygmephorus costaricanusWL09, WP99
    |    |--ElattomaWL09
    |    |--Pediculitopsis tubiphorusWL09
    |    |--Petalomium fimbrisetumWL09, L86
    |    |--Xenaster longiabdominalisWL09
    |    |--GeotrupophorusWL09
    |    |--SpatulaphorusWL09
    |    |--AcarothorectesWL09
    |    |--SasadaniaWL09
    |    |--AcinogasterRC79
    |    |--Sicilipes costaricanusWL09, WKN94
    |    |--Propygmephorus treatiWL09
    |    |--MahunkaniaWL09
    |    |--PseudopygmephorusWL09
    |    |--AllopygmephorusWL09
    |    |--Siteroptoides trombidiphilusWL09
    |    |--CerattomaWL09
    |    |--RackiaWL09
    |    |--BakerdaniaWL09
    |    |--Microdispodides Vitzthum 1914L86
    |    |--Termitacarus Trägårdh 1906WL09, FH93
    |    |    `--T. ishiharaiWL09
    |    |--PygmephorusH98
    |    `--Pediculaster Vitzthum 1931C07, H98
    |         |--P. americanus (Banks 1904)WL09, H98 [=Pigmephorus (l. c.) americanusH98, Pygmephorus americanusH98]
    |         |--P. australisC07
    |         |--P. flechtmanniWP99
    |         |--P. gautengensisC07
    |         |--P. gracilisC07
    |         |--P. mesembrinae (Canestrini 1881) [=Pygmephorus mesembrinae, Siteroptes mesembrinae]H98
    |         |--P. morelliae Rack 1975C07, H98 [=Siteroptes morelliaeH98]
    |         `--P. norrbomialisC07
    `--Scutacaridae [Disparipedidae]ZF11
         |--ArchidispusWL09
         |--Lophodispus irregularisWL09
         |--RettenmeyerellaWL09
         |--ThaumatopelvisWL09
         |--NasutiscutacarusWL09
         |--Parascutacarus indicusWL09, KK96
         |--RhynchodispusWL09
         |--Lamnacarus Balogh & Mahunka 1963BM63
         |    `--*L. ornatus Balogh & Mahunka 1963BM63
         |--ImparipesWL09
         |--ScutacarusE91
         |--Disparipes Michael 1884L86
         |--Heterodispus Paoli 1911H98
         |    |--H. elongatus (Trägardh 1904) (see below for synonymy)M63
         |    |--H. horridus Mahunka 1967H98
         |    `--H. longisetosus (Womersley 1955) [=Variatipes longisetosus]H98
         |--Pygmodispus Paoli 1911H98
         |    `--P. (Allodispus)E91
         |         |--P. (A.) latisternusE91
         |         `--P. (A.) mancus Mahunka 1967E91, H98
         |--Variatipes Paoli 1911SB63
         |    |--*V. nudus (Berlese 1886) [=Disparipes nudus]SB63
         |    |--V. eucomus (Berlese 1908) [=Diversipes eucomus]SB63
         |    |--V. major Paoli 1911SB63
         |    |--V. montanus Paoli 1911SB63
         |    `--V. tridentinus Paoli 1911SB63
         |--Diversipes Berlese 1903H98
         |    |--*D. exhamulatus (Michael 1886) [=Disparipes exhamulatus]SB63
         |    |    |--D. e. exhamulatusSB63
         |    |    `--D. e. minor Schweizer 1951SB63
         |    |--D. aequalis Mahunka 1967H98
         |    |--D. dilatatus Bal. & Mah. 1962M75
         |    |--D. horridolatus Mahunka 1975H98
         |    |--D. pilosellus Mahunka 1967H98
         |    `--D. setosus Mahunka 1967H98
         `--Reductacarus Mahunka 1963M63
              `--*R. singularis Mahunka 1963M63

Heterodispus elongatus (Trägardh 1904) [=Imparipes elongatus; incl. I. (Heterodispus) elongatus var. capensis Paoli 1911]M63

*Type species of generic name indicated

References

[BM63] Balogh, J., & S. Mahunka. 1963. New scutacarids from Hungary (Acari: Tarsonemini). Acta Zoologica Academiae Scientiarum Hungaricae 9 (1–2): 61–66.

[C07] Camerik, A. M. 2007. Ecological studies on phoretic females of dung-inhabiting Pediculaster (Acari: Heterostigmata, Pygmephoridae) species in South Africa: emigration patterns from cow and horse dung. In: Morales-Malacara, J. B., V. M. Behan-Pelletier, E. Ueckermann, T. M. Pérez, E. G. Estrada-Venegas & M. Badii (eds) Acarology XI: Proceedings of the International Congress pp. 649–655. Instituto de Biología and Faculdad de Ciencias, Universidad Nacional Autónoma de México, Sociedad Latinoamericana de Acarología: México.

Camerik, A. M., E. de Lillo & C. Lalkhan. 2006. The neotype of Pediculaster mesembrinae (Canestrini, 1881) (Acari: Siteroptidae) and the description of all life stages. International Journal of Acarology 32 (1): 45–67.

[CT01] Clift, A., & M. A. Terras. 2001. A quantitative study of phoresy in Microdispus lambi (Acari: Microdispidae) in eastern Australia. In: Halliday, R. B., D. E. Walter, H. C. Proctor, R. A. Norton & M. J. Colloff (eds) Acarology: Proceedings of the 10th International Congress pp. 394–398. CSIRO Publishing: Melbourne.

[E91] Ebermann, E. 1991. Thanatosis or feigning death in mites of the family Scutacaridae. In: Schuster, R., & P. W. Murphy (eds) The Acari: Reproduction, development and life-history strategies pp. 399–401. Chapman & Hall: London.

[FH93] Farrier, M. H., & M. K. Hennessey. 1993. Soil-inhabiting and free-living Mesostigmata (Acari-Parasitiformes) from North America: an annotated checklist with bibliography and index. North Carolina Agricultural Research Service, North Carolina State University, Technical Bulletin 302: i–xvi, 1–408.

[H98] Halliday, R. B. 1998. Mites of Australia: A checklist and bibliography. CSIRO Publishing: Collingwood.

Khaustov, A. A., & S. G. Ermilov. 2011. A new species of the genus Siteroptes (Acari, Heterostigmata, Pygmephoridae) from European Russia. Zoologicheskii Zhurnal 90 (6): 756–760 (translated: Entomological Review 91 (4): 528–532).

[KK96] Kumar, N. R., & R. Kumar. 1996. Mites in plant pollinator ecosystem: influence on conserving biodiversity. In: Mitchell, R., D. J. Horn, G. R. Needham & W. C. Welbourn (eds) Acarology IX vol. 1. Proceedings pp. 133–136. Ohio Biological Survey: Columbus (Ohio).

[L86] Lindquist, E. E. 1986. The world genera of Tarsonemidae (Acari: Heterostigmata): a morphological, phylogenetic, and systematic revision, with a reclassification of family-group taxa in the Heterostigmata. Memoirs of the Entomological Society of Canada 118 (S136): 1–517.

[M63] Mahunka, S. 1963. Beiträge zur Kenntnis der Milbenfauna (Acari) von Säugetiernestern. Acta Zoologica Academiae Scientiarum Hungaricae 9 (3–4): 355–372.

[M83] Martin, N. A. 1983. Miscellaneous observations on a pasture fauna: an annotated species list. DSIR Entomology Division Report 3: 1–98.

[RC79] Ross, L. J., & E. A. Cross. 1979. A revision of the genus Acinogaster (Acari: Pygmephoridae). International Journal of Acarology 5 (3): 231–250.

[SB63] Schweizer, J., & C. Bader. 1963. Die Landmilben der Schweiz (Mittelland, Jura und Alpen): Trombidiformes Reuter, mit 217 Arten und Unterarten und 193 Originalzeichnungen. Denkschriften der Schweizerischen Naturforschenden Gesellschaft [Mémoires de la Société Helvétique des Sciences Naturelles] 84 (2): i–vi, 209–378.

[SL71] Spain, A. V., & M. Luxton. 1971. Catalog and bibliography of the Acari of the New Zealand subregion. Pacific Insects Monograph 25: 179–226.

[WL09] Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 233–420. Texas Tech University Press.

[WP99] Walter, D. E., & H. C. Proctor. 1999. Mites: Ecology, Evolution and Behaviour. CABI Publishing: Wallingford (UK).

[WKN94] Wrensch, D. L., J. B. Kethley & R. A. Norton. 1994. Cytogenetics of holokinetic chromosomes and inverted meiosis: keys to the evolutionary success of mites, with generalizations on eukaryotes. In: M. A. Houck (ed.) Mites: Ecological and Evolutionary Analyses of Life-history Patterns pp. 282–343. Chapman & Hall: New York.

[ZF11] Zhang, Z.-Q., Q.-H. Fan, V. Pesic, H. Smit, A. V. Bochkov, A. A. Khaustov, A. Baker, A. Wohltmann, T. Wen, J. W. Amrine, P. Beron, J. Lin, G. Gabryś & R. Husband. 2011. Order Trombidiformes Reuter, 1909. In: Zhang, Z.-Q. (ed.) Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness. Zootaxa 3148: 129–138.

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