Mesostigmata

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Taxonomy

updated 2025

Diagnostic characters

• Adult female with 4 pairs of setaeseta:
  (pl. setae, from L. = bristle) cuticular process composed of a hollow shaft (sometimes filled with a refractive material) produced from a membranous socket (the alveolus); the hair-like, spine-like, branched or variously expanded structures on the surfaces of the legs and body.  Most setae function as mechanoreceptors, but others (e.g., solenidia) are chemoreceptors or have unknown or ambiguous functions.
  on the sternal shieldsternal shield:
  a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
  
• Dorsaldorsal:
  relating to the upper or back side; opposed to ventral.
  shield divided, 4 scleronoduliscleronoduli:
  3-4 light refractile structures, sometimes comma-shaped, in the podonotal region of many Rhodacaroidea and the genus Protogamasellus (Ascidae) in the Mesostigmata.  
   on pododnotal shield
• Tibiatibia:
  (pl. tibiae) the leg segment between the genu and the tarsus.
  I with 6/4 and genugenu:
  (pl. genua) (= patella) the 4th leg segment, between the femur and tibia.
  I with 6/3 dorsal/ventral setaeseta:
  (pl. setae, from L. = bristle) cuticular process composed of a hollow shaft (sometimes filled with a refractive material) produced from a membranous socket (the alveolus); the hair-like, spine-like, branched or variously expanded structures on the surfaces of the legs and body.  Most setae function as mechanoreceptors, but others (e.g., solenidia) are chemoreceptors or have unknown or ambiguous functions.
  ; genugenu:
  (pl. genua) (= patella) the 4th leg segment, between the femur and tibia.
  IV with 10 setaeseta:
  (pl. setae, from L. = bristle) cuticular process composed of a hollow shaft (sometimes filled with a refractive material) produced from a membranous socket (the alveolus); the hair-like, spine-like, branched or variously expanded structures on the surfaces of the legs and body.  Most setae function as mechanoreceptors, but others (e.g., solenidia) are chemoreceptors or have unknown or ambiguous functions.
  , 5/2 dorsal/ventral
• Chelicerae usually with arthrodial brusharthrodial brush:
  a brush like extension of the arthrodial membrane at the articulation of the movable and fixed digits in some Mesostigmata. Not to be confused with cheliceral excrescences that are produced from openings in the movable digit.
  

Similar taxa

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

1. Arthrodial process simplesimple:
   unadorned; simple setae are needle-like and without hairs or pectins.
   .............................. 4
   - Arthrodial process produced as a brush.............................. 2

2. Basitarsusbasitarsus:
   a basal subdivision of the tarsus.
   IV with 4 setae (pl4 present).............................. Pararhodacarus
   - Basitarsusbasitarsus:
   a basal subdivision of the tarsus.
   IV with 3 setae (pl4 absent).............................. 3

3. Tectumgnathotectum:
   (= epistome, tectum) an often membranous dorsal projection of the anterior margin of the basis capitulum in Mesostigmata; often diagnostic at family-, genus- or species-levels; also often difficult to see without high magnification and good optics.
   with single median process or dentate.............................. Afrogamasellus
   - Tectumgnathotectum:
   (= epistome, tectum) an often membranous dorsal projection of the anterior margin of the basis capitulum in Mesostigmata; often diagnostic at family-, genus- or species-levels; also often difficult to see without high magnification and good optics.
   with median stalk y-shaped, usually with lateral tines.............................. Afrodacarellus

4. Scleronoduli present.............................. 5
   - Scleronoduliscleronoduli:
   3-4 light refractile structures, sometimes comma-shaped, in the podonotal region of many Rhodacaroidea and the genus Protogamasellus (Ascidae) in the Mesostigmata.  
   absent.............................. 10

5. Tibiatibia:
   (pl. tibiae) the leg segment between the genu and the tarsus.
   I with 4 ventral setaeventral setae:
   in the Lindquist-Evans system used in the Mesostigmata, the ventral region has up to three longitudinal rows (Jv, Zv, Lv) with 1-5 transverse row designations.
   ; palp apotelepalptarsal apotele:
   (= palp apotele, palptarsal claw) the most distal segment of the palp; absent in Acariformes, claw-like in Opilioacarida, and a subdistal, tined structure in the Mesostigmata.
   3-tined.............................. 6
   - Tibiatibia:
   (pl. tibiae) the leg segment between the genu and the tarsus.
   I with 3 ventral setaeventral setae:
   in the Lindquist-Evans system used in the Mesostigmata, the ventral region has up to three longitudinal rows (Jv, Zv, Lv) with 1-5 transverse row designations.
   ; palp apotelepalptarsal apotele:
   (= palp apotele, palptarsal claw) the most distal segment of the palp; absent in Acariformes, claw-like in Opilioacarida, and a subdistal, tined structure in the Mesostigmata.
   2-tined.............................. 9

6. 3 scleronoduli (median unpaired);  j1–2, z1, and s1 on anterior margin of podonotal shieldpodonotal shield:
   the anterior shield in mesostigmatans with divided dorsal shields.  
   .............................. 7
   - 4 scleronoduli (median pair +/- fused);  j1 (but not j2), z1 and s1 on anterior margin of shield.............................. 8

7. Pretarsustarsal apotele:
   (also pretarsus) the most distal segments of legs and palps bearing the claws and empodium
   I absent; without presternal plateletspresternal platelets:
   small sclerotized plates anterior to the sternal shield and laterad or posteriad the base of the tritosternum in some Mesostigmata.
   .............................. Rhodacarus (+Mediorhodacarus)
   - Pretarsustarsal apotele:
   (also pretarsus) the most distal segments of legs and palps bearing the claws and empodium
   I present; with 2 pairs of presternal plateletspresternal platelets:
   small sclerotized plates anterior to the sternal shield and laterad or posteriad the base of the tritosternum in some Mesostigmata.
   .............................. Rhodacaropsis

8. Podonotal shieldpodonotal shield:
   the anterior shield in mesostigmatans with divided dorsal shields.  
   entire.............................. Rhodacarellus
   - Podonotal shieldpodonotal shield:
   the anterior shield in mesostigmatans with divided dorsal shields.  
   divided between setae j3–j4; tectumgnathotectum:
   (= epistome, tectum) an often membranous dorsal projection of the anterior margin of the basis capitulum in Mesostigmata; often diagnostic at family-, genus- or species-levels; also often difficult to see without high magnification and good optics.
   denticulate-trifurcate.............................. Minirhodacarellus

9. Tibiatibia:
   (pl. tibiae) the leg segment between the genu and the tarsus.
   I with 5 (rarely 4) dorsaldorsal:
   relating to the upper or back side; opposed to ventral.
   setae; 2 pairs of scleronoduliscleronoduli:
   3-4 light refractile structures, sometimes comma-shaped, in the podonotal region of many Rhodacaroidea and the genus Protogamasellus (Ascidae) in the Mesostigmata.  
   ; sternal shieldsternal shield:
   a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
   with 4 pairs of setae; anal opening normal.............................. Dendrolaelaps (Digamasellidae)
   - Tibiatibia:
   (pl. tibiae) the leg segment between the genu and the tarsus.
   I with 6 dorsaldorsal:
   relating to the upper or back side; opposed to ventral.
   setae; 3 scleronoduliscleronoduli:
   3-4 light refractile structures, sometimes comma-shaped, in the podonotal region of many Rhodacaroidea and the genus Protogamasellus (Ascidae) in the Mesostigmata.  
   ; sternal shieldsternal shield:
   a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
   with 3 pairs of setae (st4 in soft cuticle); anal opening enlarged.............................. Protogamasellus mica Group (Ascidae)

10. Sternal seta st4 on sternal shieldsternal shield:
    a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
    .............................. 11
    - Sternal  seta st4 in soft cuticle.............................. 15

11. Anal opening large; tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    I with 5 dorsaldorsal:
    relating to the upper or back side; opposed to ventral.
    setae.............................. Digamasellus (Digamasellidae)
    - Anal opening normal; tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    I with 6 dorsaldorsal:
    relating to the upper or back side; opposed to ventral.
    setae.............................. 12

12. Sternal shieldsternal shield:
    a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
    strongly sclerotized throughout; tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    I with 4 ventral setaeventral setae:
    in the Lindquist-Evans system used in the Mesostigmata, the ventral region has up to three longitudinal rows (Jv, Zv, Lv) with 1-5 transverse row designations.
    .............................. Geogamasus (Ologamasidae)
    - Anterior region of sternal shieldsternal shield:
    a shield in the anterior intercoxal region of parasitiform mites that bears one or more pairs of sternal setae.
    poorly sclerotized, setae st1 in soft or granulate cuticle; tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    I with 3 ventral setaeventral setae:
    in the Lindquist-Evans system used in the Mesostigmata, the ventral region has up to three longitudinal rows (Jv, Zv, Lv) with 1-5 transverse row designations.
    .............................. 13

13. Palp apotelepalptarsal apotele:
    (= palp apotele, palptarsal claw) the most distal segment of the palp; absent in Acariformes, claw-like in Opilioacarida, and a subdistal, tined structure in the Mesostigmata.
    2-tined; coxacoxa:
    the basal segment of the leg, articulating with (Parasitiformes) or fused to (Acariformes) the body wall.
    I without dorsaldorsal:
    relating to the upper or back side; opposed to ventral.
    spine .............................. 15
    - Palpapotele 3-tined: coxacoxa:
    the basal segment of the leg, articulating with (Parasitiformes) or fused to (Acariformes) the body wall.
    I with dorsaldorsal:
    relating to the upper or back side; opposed to ventral.
    spine.............................. 14

14. Presternal area granulate.............................. Protogamasellopsis
    - Presternal area with paired stacks of 4 presternal plateletspresternal platelets:
    small sclerotized plates anterior to the sternal shield and laterad or posteriad the base of the tritosternum in some Mesostigmata.
    .............................. Rhodacarella

15. Posteriorposterior:
    the back part of the body or towards that region in comparison, e.g., 'posterior to'.
    margin of podonotalpodonotal:
    relating to the dorsal podosoma, the region of the idiosoma over the legs.
    (at j6) and anterior margin of opisthonotalopisthonotal:
    dorsal opisthosoma.
    (at J1) shields with transverse lines; genugenu:
    (pl. genua) (= patella) the 4th leg segment, between the femur and tibia.
    IV with 8 setae, tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    IV with 9 setae.............................. Protogamasellus hibernicus Group (Ascidae)
    - Podonotalpodonotal:
    relating to the dorsal podosoma, the region of the idiosoma over the legs.
    and opisthonotalopisthonotal:
    dorsal opisthosoma.
    shields without transverse lines; genugenu:
    (pl. genua) (= patella) the 4th leg segment, between the femur and tibia.
    IV with 9 setae, tibiatibia:
    (pl. tibiae) the leg segment between the genu and the tarsus.
    IV with 10 setae.............................. Gamasellodes (Ascidae)

References

• Athias-Henriot 1961aAthias-Henriot 1961a:
  Athias-Henriot C. 1961a. Mesostigmatides (Urop. excl.) edaphiques Mediterraneans (Acaromorpha, Anactinotrichida). Acarologia 3: 381–509.
• Hurlbutt 1973Hurlbutt 1973:
  Hurlbutt HW. 1973. The Afrogamasellus Loots and Ryke and Afrodacarellus n. gen. (Acarina: Rhodacaridae) of Tanzania. Acarologia 15: 565-619.
• Lee 1970Lee 1970:
  Lee DC. 1970. The Rhodacaridae (Acari: Mesostigmata); classification, external morphology and distribution of genera. Records of the South Australian Museum 16: 1-219.
• Loots 1969Loots 1969:
  Loots GC. 1969. Notes on Rhodacarus Oudemans and its related genera with descriptions of new species from the Ethiopian region. Publicações Culturais da Companhia de Diamantes de Angola 81: 45-82.
• Moraza 2004Moraza 2004:
  Moraza ML. 2004. Rhodacarella, a new genus of Rhodacaridae mites from North America (Acari: Mesostigmata: Rhodacaridae). Zootaxa 470: 1-10.