Forelia mutata (Piersig 1901) – Identification almost certain

Fig 10 - Forelia mutata female - dorsal

This water Mite was caught in Roëzé (France – Sarthe) the same day I caught a bunch of Mideopsis orbicularis, the 7th of April 2012. The length of this water mite is 815µm and the width 635 µm.

Before finding this identification to Forelia mutata, I spent some time looking in the Piona genus. I’ve put my identification log in a blog article.

Description of Forelia mutata female in Soar & Williamson 1929.

Body of female.

  • Length about 0.85mm and breadth about 0.65mm. Almost a perfect match even if this one is a little bit smaller.
  • Outline oval, portion between the antenniform bristles deeply emarginate. OK, see fig 60.
  • Postero-lateral portion slightly emarginate. OK, see fig 60
  • Dorsum gradually sloping towards the anterior end of the body.
  • Skin reticulate.
  • Antenniform bristles long and curved obliquely outwards and upwards. See my fig 50 and also visible on fig 10.
  • Eyes standing at the lateral edge of body fairly well back from the anterior emargination, pigment black. Ok for the position, the eyes of mine are more red than black.
  • Colour transparent brownish yellow.
  • Bright yellow T-shaped marking on the dorsum surrounded by a broad brown, irregularly marked border. Perfect match.


  • Palpi in the basal segments somewhat stouter than the adjacent segment of the first pair of legs. Maybe visible on fig 10.
  • Length about one-fourth that of the body, and rather stoutly formed.
  • Fourth segment shorter than usual, not exceeding the second in length. See fig 70.
  • Outer papilla on flexor surface of fourth segment resting on a thickened portion of segment.
  • Inner papilla rather nearer to anterior and lying on an inner lower ridge running up to the small chitinose peg at the distal end of segment.
  • Extensor surface with two bristles midway and one distal, about equalling in length those springing from the papillae on flexor surface.
  • Extensor surface of second and third segments with a few spinose bristles.


  • Epimera resembling in the main those of Forelia liliacea, but the first three pairs rather narrower and the posterior of the fourth pair drawn out much sharply.


  • Legs short, fourth pair not so long as the body.
  • Proximal end of segments much thinner than the thick distal end. See my fig 80 for example.
  • Claws on the first three pairs of legs large, basal portion laminate. See my fig 90.
  • All segments well furnished with bristles, some of these pectinate. See my fig 80.
  • Distal end of fifth segment of second, third and fourth pairs of legs with a few swimming hairs. Fig 80.


  • Anus lying midway between the genital area and the posterior end of body. Fig 30.

Genital area.

  • Genital area fairly large.
  • Anterior end of aperture lying on a level with the inner end of the third and fourth pairs of epimera.
  • Posterior end lying beyond the extreme tips of fourth pair.
  • Genital plates lying obliquely a little beyond posterior end of aperture.
  • Plates very short and very wide, extending outwards slightly beyond extremity of fourth pair of epimera. Perfect match.
  • Outer ends of plate rounded.
  • Inner ends almost stright.
  • Each plate with from thirteen to fifteen acetbula, one of those lying apart from the others in the anterior inner corner of the plate. Perfect match on my fig 30 for the disposition of this last (or first) acetabula. I count « only » 11 acetabula on my fig 30.


In Soar & Williamson 1929, among the synonyms given for Forelia mutata is given the name Acercus brevipes (Piersig 1893 & 1897) and Tiphys ornatus (Piersig 1901 & Halbert 1903). Forelia brevipes (Neuman, 1880) is not described in Soar & Williamson and Neuman doesn’t appear in the references used in any of the species of Forelia described. One can think that Forelia brevipes and Forelia mutata are synonym in Soar & Williamson.

The problem is that actually, in the collection of accepted names these 2 species are considered distinct (Hallan :

I’ve found the description of Nesaea brevipes in Neuman 1880 and the general shape of the genital plates match Forelia mutata.

Neuman 1880 P131 Nesaea brevipes female

I’ve also checked Piersig 1900 and his illustration of Acercus brevipes is also almost the same as Soar & Williamson Forelia mutata.

Piersig 1900 P515 Acercus brevipes ventral female

At this point the synonymy between Forelia mutata and Forelia brevipes may be established but some other author are presenting a rather different shape (more triangular) for the genital plates of Forelia brevipes.

Koenike in 1909 identify Forelia triangularis (which is an accepted synonym of Forelia brevipes) a little bigger than mutata (1mm instead of 0.85 for mutata) with larger and triangular genital plate. This author recognize also as different specie a Forelia mutata of 0.85mm.

Piersig 1900 present Acercus brevipes (illustration above) (0.85mm – genital plate long and narrow) and Acercus triangularis which is an other synonym for the actual Forelia brevipes. This is very confusing and was the spring of all this discussion. In fact Acercus brevipes Piersig is not the actual Forelia brevipes but Forelia mutata.

According to Smit & Gerecke 2010 Forelia mutata is a new specie for France.


  • Neuman, C.J. (1880). Om sveriges hydrachnider. Kongl. Vet. Akad. Handl.
  • Piersig, R (1900). Deutschlands Hydrachniden. Zoologica
  • Smit, H. & Gerecke, R. (2010) : A checklist of the water mites of France (acari: hydrachnidia). Acarologia 50(1): 21-91.
  • Soar, C.D. & Williamson, W. (1929) – The British Hydracarina. Volume III. Ray Society, London.

Other pictures of this water mite.

Fig 20 - Forelia mutata - ventral

Fig 30 - Forelia mutata - Genital area

Fig 40 - Forelia mutata - Palpi

Fig 50 - Forelia mutata - Lateral

Fig 60 - Forelia mutata - Dorsal

Fig 70 - Forelia mutata - Palpi and first leg - lateral

Fig 80. Forelia mutata. Leg II an III

Fig 90. Forelia mutata. Claws on leg II.



About yann