Why the water mite #327 is not an Unionicola ypsilophora

fig 100 : unionicola ypsilophora - lithograph of 1956

I have a good set of pictures for this Unionicola and unlike the Unionicola #199, I have a picture on which one can discern a large number of acetabulae.

With the Soar & Williamson in hand, my first idea was to recognize Unionicola ypsilophora on my pictures.

I write this article to keep a track of my identification process.

Unionicola ypsilophora – male – Soar & Williamson

Reference : Soar & Williamson – 1927 – British hydracarina – Vol 2 – Page 123  -Plate 36

In favor of Unionicola ypsilophora.

  • Male size up to 1.3mm in length.
  • Genital area of male with two plates united at both end.
  • Body outline oval, envely rounded at both ends, dorsum well arched.
  • Skin soft.
  • Colour a dirty transparent yellow with dark patches.
  • Malpigian vessel Y shaped bright yellow.
  • Eyes small, not quite at the edge of the body.
  • Eyes reddish-brown to brownish black (I would say brownish black on my pictures)
  • Capitulum widest at it anterior end, evenly rounded posteriorly
  • Suture on the surface of the capitulum well marked for the whole length.
  • Palpi in length about one-fourth that of the body.
  • Epimera groups well separated from one another especially on the median line.
  • First pair of coxae rather longer and narrower than the second (see my fig 25)
  • Third pair of coxae wider at the outer end than at the inner.
  • Suture between third and fourth pair of coxae continuous.
  • Fourth pair of coxae quadrate. Inner margin nearly straight, posterior margin convex, angles rounded.
  • First pair of legs scarcely any thicker than the other pairs.

Not in favour of Unionicola ypsilophora

  • Palpi much stouter than the first pair of legs. 
  • First pair of legs shorter than the body.
  • Third pair of legs shorter than the second but longer than the first (on all my pictures, the third pair of leg looks shorter than the first).

Elements impossible to check on my pictures or questionnable.

  • Skin finely ridged, but having here and there a scaly or granular appearance arising from small patches being more or less chitinized.
  • Antenniform bristles short and thin.
  • Anchorla process of capitulum broad and short.
  • Posterior end of capitulum produced into a recurved hook at each corner.
  • Rostrum moderate in size.
  • Mandible short and broad in the proximal portion.
  • Palpi : a lot of description in Soar and Williamson but I haven’t any good picture of them.
  • Common margin of two first pair of coxae drawn out into a claw like process
  • Surface of epimera granular, porose.
  • Fourth segment of the first 3 pair of legs and fifth of fourth pair of legs longest.
  • Extensor surface surface of segments of legs well furnished with hairs.
  • Swimming hair short and scanty.
  • Bristles on flexor surface chiefly on middle segments, those of thirst pair without pappilae  found in other species of this genus.
  • Claw-sheaths with wing-like extensions. Claw with large accessory claw.

Observation I made which are not in the S&W description.

  • First 2 pairs of legs with strong setae
  • Legs III and IV with small setae.
  • 2 dark sclerites on each side of the genital area

The checklist of the water mite of France.

In their “checklist of the water mites of France”, Smit and Gerecke found references of Unionicola ypsilophora in Nord but not in Pas de Calais (french departements of the same area). Moniez & Barrois had found it and it was called Atax ypsilophorus in their catalogue.

Other name given to the Unionicola ypsilophora.

I found  this list done by Joel Hallan and learnt also that there are 2 recents description of this specie : Vidrine in 1986 and Viets in 1949.

TYPE-SPECIES      Unionicola ypsilophora (Bonz, 1783) [ Redescription, Vidrine 1986:141; Viets 1949?]
                                        [=Acarus ypsilophorus Bonz, 1783]
                                        [=Atax adensameri Thon, 1901]
                                        [=Unionicola humerosa Haldeman, 1842]
                                        [=Unionicola lactea Haldeman, 1842]
                                        [=Unionicola lugubris Haldeman, 1842]
                                        [=Unionicola oviformis Haldeman, 1842]
                                        [=Unionicola personata Haldeman, 1842]
                                        [=Unionicola proxima Haldeman, 1842]
                                        [=Unionicola reticulata Haldeman, 1842]
                                        [=Unionicola symmetrica Haldeman, 1842]
                                        [=Unionicola unicolor Haldeman, 1842]

Other synonimous indicated in the Soar & Williamson :

Trombidium notatumHydrachna triangularis (?) – Limnochares anodontaeHydrachna concharumHydrachna formosa (?) – Unionicola oviformis (?) – Atax ypsilophora Atax ypsilophorusUnionicola ypsilophorus.

My opinion about this identification.

Among all the Unionicola described in Soar & Williamson, ypsilophora is definitely the best candidate for the identification of this water mite.

main points in favor of Unionicola ypsilophora

  • The shape of the genital area and the ressemblance of the drawing of the numerous acetabula.
  • The latin name ypsilophora (from grec upsilon = Y and the grec suffic -phoros = to bear) even if the Y is not as clear as on these other pictures.
  • The size of the animal wich match roughly S&W

Main points againt this identification.

  • The two first pair of legs are very different on my pictures and the length of the legs given by S&W don’t match.
  • The sclerites on each side of the genital area are not in the description of S&W.
  • The ovoid shape of the body of Unionicola Ypsilophora is very different from my pictures.

For the time being, I don’t consider this water mite as Unionicola ypsilophora.

Next step to try to identify this Unionicola not ypsilophora.

  1. Use the checklist of Smit & Gerecke to list all the Unionicola already found in France
  2. Check in Soar & Williamson to eliminate obvious impossibility.
  3. Use the work of Hallan to get the more synonimous possible.
  4. Search in Google for these synonimous to try to get an old and precise description (with this method I’ve found for exemple the nice plates of Thon 1901 – Atax adensameri)


  • Thon (1901) Ueber eine neue parasitische Atax-Art aus Texas – In german –  Download a good pdf at landesmuseum.at.
  • Hallan Joel – Biology Catalog – Texas A&M University. Link to Unionicoladae species.
  • Beneden P.J. van (1848): Recherches sur l’histoire naturelle et le développement de l’Atax Ypsilophorus (Hydrachna concharum) – Mémoires de l’Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique, tom. xxiv. 1850 (24 pp. ed. 1 pl.) (good scan of this paper in french available on the internet).
  • Soar & Williamson (1927): British hydracarina – Vol 2 – Page 123  -Plate 36
  • Smit & Gerecke  (2010): A CHECKLIST OF THE WATER MITES OF FRANCE (ACARI: HYDRACHNIDIA) – Acarologia 50(1): 21–91.

fig 10 unionicola general

Fig 300 : Atax adensameri Thon

Note on this Atax adensameri fig 300.

This illustration is taken in Thon 1901 (see my references for downloading the entire article in german). It is now accepted that Atax adensameri is an old name for Unionicola ypsilophora.

Main differences between this plate and my pictures :

  • First par of legs of much smaller than on my pictures.
  • All the four pair of legs with the same width but on mine the 2 first pair are very different.
  • Size of the body much narrower than mine.
  • Coxae plates not more than half the ventral surface (on mine they take more place but il may be a difference between the female on the Thon figure and a male on my pictures).
  • Fourth pair of coxae plates jointed on my pictures (maybe a sexual dimorphism).
  • Shape of the first two pair of coxae plates not at all the same as on my pictures.


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