Mesostigmata

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Taxonomy

updated 2025

Diagnostic characters

• Completely divided dorsal shield; setae j2 posterior to margin.
• Pododnotal shield with transverse line extending across surface at level of setae z6
• 3 scleronoduli present, median nodule linear or with procurved lateral arms
• Opisthosnotal shield with similar line at level of setae J1, complete or interrupted between setae
• Chelicerae with arched, serrate-membranous fixed digit
• Paranal setae inserted close to anterior margin of anus
• Genu IV with 8 setae (pl1 absent)
• Tibia IV with 9 setae (pl2 absent); anal enlarged; chelicerae serrate-membranous; corniculi toothed

Similar taxa

Species in the Protogamasellus hibernicus species group have chelate-dentate chelicerae without the serrate-membranous formation, a normal anal opening, and lack scleronoduli. Members of the hibernicus group and Ectoantennoseius Walter, Gamasellodes Athias-Henriot, Asca von Heyden, Antennoseius Berlese all lack scleronoduli.

Key to adult females of Rhodacarus-like mites
(Rhodacaridae, Ologamasidae, Digamasellidae, Ascidae)

1. Arthrodial process simple.............................. 4
   - Arthrodial process produced as a brush.............................. 2

2. Basitarsus IV with 4 setae (pl4 present).............................. Pararhodacarus
   - Basitarsus IV with 3 setae (pl4 absent).............................. 3

3. Tectum with single median process or dentate.............................. Afrogamasellus
   - Tectum with median stalk y-shaped, usually with lateral tines.............................. Afrodacarellus

4. Scleronoduli present.............................. 5
   - Scleronoduli absent.............................. 10

5. Tibia I with 4 ventral setae; palp apotele 3-tined.............................. 6
   - Tibia I with 3 ventral setae; palp apotele 2-tined.............................. 9

6. 3 scleronoduli (median unpaired);  j1–2, z1, and s1 on anterior margin of podonotal shield.............................. 7
   - 4 scleronoduli (median pair +/- fused);  j1 (but not j2), z1 and s1 on anterior margin of shield.............................. 8

7. Pretarsus I absent; without presternal platelets.............................. Rhodacarus (+Mediorhodacarus)
   - Pretarsus I present; with 2 pairs of presternal platelets.............................. Rhodacaropsis

8. Podonotal shield entire.............................. Rhodacarellus
   - Podonotal shield divided between setae j3–j4; tectum denticulate-trifurcate.............................. Minirhodacarellus

9. Tibia I with 5 (rarely 4) dorsal setae; 2 pairs of scleronoduli; sternal shield with 4 pairs of setae; anal opening normal.............................. Dendrolaelaps (Digamasellidae)
   - Tibia I with 6 dorsal setae; 3 scleronoduli; sternal shield with 3 pairs of setae (st4 in soft cuticle); anal opening enlarged.............................. Protogamasellus mica Group (Ascidae)

10. Sternal seta st4 on sternal shield.............................. 11
    - Sternal  seta st4 in soft cuticle.............................. 15

11. Anal opening large; tibia I with 5 dorsal setae.............................. Digamasellus (Digamasellidae)
    - Anal opening normal; tibia I with 6 dorsal setae.............................. 12

12. Sternal shield strongly sclerotized throughout; tibia I with 4 ventral setae.............................. Geogamasus (Ologamasidae)
    - Anterior region of sternal shield poorly sclerotized, setae st1 in soft or granulate cuticle; tibia I with 3 ventral setae.............................. 13

13. Palp apotele 2-tined; coxa I without dorsal spine .............................. 15
    - Palpapotele 3-tined: coxa I with dorsal spine.............................. 14

14. Presternal area granulate.............................. Protogamasellopsis
    - Presternal area with paired stacks of 4 presternal platelets.............................. Rhodacarella

15. Posterior margin of podonotal (at j6) and anterior margin of opisthonotal (at J1) shields with transverse lines; genu IV with 8 setae, tibia IV with 9 setae.............................. Protogamasellus hibernicus Group (Ascidae)
    - Podonotal and opisthonotal shields without transverse lines; genu IV with 9 setae, tibia IV with 10 setae.............................. Gamasellodes (Ascidae)

Key to genera in the subfamily Ascinae

1. With holodorsal shield that captures the R-series.............................. 2
   - Dorsal shield divided into two subequal shields; R-series setae in softcuticle.............................. 3

2. Seta z1 present laterad seta j1 on anterior median region of dorsal shield; femur III with 7 setae; genu I with 13 setae; tibia II with 10 setae.............................. Orthadenella (Blattisociidae)
   - Seta z1 absent; femur III with 6 setae; genu I with 12 setae; tibia II with 9 setae.............................. Neojordensia

3. Opisthonotal shield with 12 pairs of setae (J3, Z2, S1 absent); podonotal shield lacking setae j2, z1, z3; adult leg setation strongly neotenous, genua I–II–III–IV with 11–8–7–7 setae, tibiae with 11–7–7–7 setae; female with anal shield.............................. Diseius
   - Opisthonotal shield with 15 or more pairs of setae (J3, Z2, S1 present); podonotal shield with setae j2, z3, sometimes z1; adult leg setation not strongly neotenous, minimum number of setae on genua 12–11–8–8, on tibiae 13–10–8–9; female with anal or ventrianal shield.............................. 4

4. Hypostome with corniculi bifid apically; anterior rostral setae thickened, spinelike; dorsal shield setae z1 absent.............................. Anephiasca
   - Hypostome with corniculi entire apically; anterior rostral setae of similar thickness to other hypostomal setae; dorsal shield setae z1 present or absent.............................. 5

5. Female with epigynial shield strongly rounded posteriorly and with small subtriangular ventrianal or anal shield bearing 3–5 setae; male with ventrianal shield united to peritrematal shields; soft body cuticle often with microtuberculate striae.............................. 6
   - Female with epigynial shield truncate posteriorly and usually with well developed ventrianal shield (rarely anal shield) bearing 3–9 setae (including circumanal setae); male with ventrianal shield separate from peritrematal shields; soft body cuticle striae without microtubercles.............................. 8

6. Palps elongated, at least half as long as leg I; palp trochanter longer than femur; palp tibia almost 3 times as long as tarsus.............................. Anystipalpus
   - Palps of normal length, about one-third as long as leg I; palp trochanter slightly shorter than femur; palp tibia twice as long as tarsus.............................. 7

7. Legs I with ambulacra and claws.............................. Antennoseius (Vitzthumia)
   - Legs I without ambulacra and claws.............................. Antennoseius (Antennoseius)

8. Opisthonotal shield with setae Z4 & S5 usually approximate and inserted together on pair of prominent posterolateral tubercles; podonotal shield lacking setae z1,  j2 well behind level of j1; peritrematal shields of adults broad, truncate posteriorly; genu I with 12 setae (av2 absent).............................. Asca
   - Opisthonotal shield with setae Z4, S5 well separated, not borne on pair of strong tubercles; podonotal shield with setae z1 present; j2 tending to be displaced anteriorly, often in approximately transverse row with z1 and j1; peritrematal shields of adults narrow or of moderate width, tapering posteriorly; genu I with 13 setae (av2 present).............................. 9

9. Podonotal shield with transverse line extending across surface at level of setae z6, opisthonotal shield with similar line at level of setae J1; paranal setae inserted close to anterior margin of anus; genu IV with 8 setae (pl1 absent); tibia IV with 9 setae (pl2 absent).............................. Protogamasellus
   - Dorsal shields without transverse lines extending completely across surface; paranal setae inserted closer to posterior margin of anus than to anterior margin; genu IV with 9 setae (pl1 present); tibia IV with 10 setae (pl2 present).............................. 10

10. Sternal pore 3 in soft cuticle; setae r3 and Z5 club-shaped, densely plumose; gnathotectum denticulate; movable digit of chelicerae serrate.............................. Ectoantennoseius
    - Sternal pore 3 on sternal shield; setae r3 and Z5 setiform, smooth to barbed; gnathotectum more or less smoothly trifurcate; movable digit of chelicerae usually with 4–6 teeth, not serrate.............................. Gamasellodes

Ecology and distribution

These mites are small soil-inhabiting (near surface to very deep soil) predators and fungivores that usually occur as all female populations (thelytoky).

References

• Evans 1982Evans 1982:
  Evans GO. 1982. Observations of the genus Protogamasellus Karg (Acari: Mesostigmata) with a description of a new species. Acarologia 23: 303–13.
• Evans and Purvis 1987Evans and Purvis 1987:
  Evans GO, Purvis G. 1987. A new Ascid mite from St Helena with observations on the Protogamasellus complex (Acari: Mesostigmata). Journal of Natural History 21: 855–61.
• Evans and Till 1979Evans and Till 1979:
  Evans EO, Till WM. 1979. Mesostigmatic mites of Britain and Ireland (Chelicerata: Acari-Parasitiformes): An introduction to their external morphology and classification. Transactions of the Zoological Society of London 35 (2): 145–270.
• Halliday et al. 1998Halliday et al. 1998:
  Halliday RB, Walter DE, Lindquist EE. 1998. Revision of the Australian Ascidae (Acarina: Mesostigmata). Invertebrate Taxonomy 12: 1–54.
• Karg 1993Karg 1993:
  Karg W. 1993. Acari (Acarina), Milben Parasitiformes (Anactinochaeta) Cohors Gamasina Leach, Raubmilben. Die Tierwelt Deutschlands 59: 1–523.
• Walter and Ikonen 1989Walter and Ikonen 1989:
  Walter DE, Ikonen EK. 1989. Species, guilds and functional groups: Taxonomy and behavior in nematophagous arthropods. Journal of Nematology 21: 315–327.
• Walter and Lindquist 1989Walter and Lindquist 1989:
  Walter DE, Lindquist EE. 1989. Life history and behavior of ascid mites in the genus Lasioseius (Acari: Mesostigmata) from grassland soils in Colorado with taxonomic notes and a description of new species. Canadian Journal of Zoology 67: 2797–2813.
• Walter and Lindquist 1995Walter and Lindquist 1995:
  Walter DE, Lindquist EE. 1995. The distribution of asexual ascid mites (Acari: Parasitiformes) does not support the biotic uncertainty hypothesis. Experimental and Applied Acarology 19: 423–442.