Mesostigmata

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Taxonomy

updated 2025

Diagnostic characters

• Sternal shield split medially and bearing 3 pairs of setae
• Hologastric ventral shield, genital region acuminate, metasternal plates large
• Associated with bark beetles (Scolytidae) and their galleries

Similar taxa

Some Laelapidae parasitic on millipedes have split sternal shields but lack parasitine genital regions and enlarged metasternal shields.

Key to some Parasitine genera (adult females) (modified from Hyatt 1980)

1. Lateral setae of palp genu (al1–2) entire, either setiform or spatulate.............................. 2
   - Lateral setae of palp genu (al1–2) bifid.............................. Eugamasus

2. Posterior ventral region (opisthogaster) with 30 or fewer pairs of setae; associated with a variety of insects, but usually not bumblebees (Bombus).............................. 3
   - Opisthogaster with extensive hypertrichy (>40 pairs of setae); associated with bumblebees (Bombus) or their nests.............................. Parasitellus

3. Corniculi of normal length, not reaching the anterior margin of the palp femur; anterior margin of anterior dorsal shield convex.............................. 4
   - Corniculi elongate, extending beyond palp femur-genu articulation; margin of anterior dorsal shield concave.............................. Cornigamasus

4. Genital shield (or genital region of hologastric shield) subtriangular, acuminate.............................. 5
   - Genital region of hologastric shield distally tricuspid.............................. Porrhostaspis

5. Lateral seta of palp femur (al) entire, usually spatulate..............................8
   - Lateral seta of palp femur bifid, serrate, or serrate-spatulate.............................. 6

6. Setae of dorsal hexagon dissimilar, seta z5 longer and usually thicker than j5 and j6............................... 7
   - Setae of dorsal hexagon similar in size and ornamentation.............................. Vulgarogamasus

7. Sternal shield entire; seta z5 usually thicker than j5 and j6 and distally pilose associated with dung (and dung beetles), compost, etc.............................. Parasitus
   - Sternal shield split medially; seta z5 usually longer than j5 and j6 but similar in form; associated with bark beetles.............................. Schizosthetus

8. Dorsal shield completely divided.............................. 9
   - Schizodorsal shield.............................. Gamasodes bispinosus group

9. Seta z5 of dorsal hexagon thickened and distally pilose; associated with various insects.............................. 10
   - Seta z5 simple; associated with carrion beetles (Silphidae).............................. Poecilochirus

10. Articulation between sternal and metasternal shields oblique; ambulacra of legs II–IV simple, rounded.............................. Gamasodes
    - Articulation (or suture) between sternal and metasternal shields transverse; ambulacra of legs II–IV long and acuminate.............................. Trachygamasus

Ecology and distribution

Predators found in bark beetle galleries; deutonymphs phoretic on bark beetles.

References

• Kaluz et al. 2003Kaluz et al. 2003:
  Kaluz S, Masan P, Moser JC. 2003. Morphology and ecology of Schizosthetus simulatrix (Acari, Mesostigmata) associated with galleries of bark beetles (Scolytidae). Biologia (Bratislava) 58: 165–172.
• Karg 1971Karg 1971:
  Karg W. 1971. Acari (Acarina), Milben Unterordnung Anactinochaeta (Parasitiformes) Die freilebenden Gamasina (Gamasides), Raubmilben. Die Tierwelt Deutschlands 59: 1–475.
• Micherdzinski 1969Micherdzinski 1969:
  Micherdzinski W. 1969. Die Familie Parasitidae Oudemans 1901 (Acarina, Mesostigmata). Zakład Zoologii Systematycznej Polskiej Akademii Nauk, Krakow. 690 pp.