Mideopsis orbicularis (Müller, 1776) – Identification almost certain


Fig 10 - Mideopsis orbicularis - Dorsal

This water mite was Found in the Sarthe river in the village of Roëzé in France the 7th of April 2012. The length of this water mite is 890µm and the width 790µm.

Element taken from the description made by Soar & Williamson (1929).

My notes are in italic.


  • Length up to 1mm for the female and smaller for the male.
  • Breadth very slightly less than the length.
  • Outline nearly circular.
  • Anterior margin of body sometimes flattened between the eyes. Look at the fig 50.
  • Body dorso -ventrally compressed, epimeral area more or less arched.
  • Skin heavily chitinized, porose.
  • Round the edge of the dorsum a narrow groove, with a softer skin than that of the rest of the body.
  • Gland pores ranges in four rows on the dorsum but not readily observed.
  • Guard-hairs fairly long.
  • Besides each eyes a group of three hairs.
  • Eyes lying close to the edge of body, dark red to black in colour. Red on mine.
  • Colour of body rather variable, yellowish to brown or even reddish.
  • Legs rather lighter in colour.
  • Dorsum with dark brown patches.
  • Malpighian vessel paler or orange tinted with a trace of blue.
  • Epimera sometimes with a tinge of bluish green round the edges or even very dark coloured. I agree, see fig 20 & 30.


  • Capitulum with no external ancoral process visible
  • Rostrum inclining downwards toward the front. See my picture fig 50.
  • Rostrum exhibiting a fairly sharp conical outline when viewed ventrally. I don’t understand.


My pictures aren’t good enough to see anything interesting.

  • Palpi short and about as stout as the adjacent legs.
  • Second segment only a little shorter than the fourth.
  • Distal end of segment about twice as thick as the proximal end.
  • Extensor surface arched, with some finely pectinate bristles.
  • Flexor surface slightly concave.
  • Third segment short, with pectinate bristles on its extensor surface similar to those of the end of the segment.
  • Fourth segment thickened ; near the middle of the flexor surface a pair of conical papillae, each tipped with a hair.
  • Inner papilla much larger than the outer.
  • Distal outer flexor end of segment wih a few hairs.
  • Fifth segment about as long as the third and terminated by two claws.


  • Epimeral area consisting of a basal plate much wider than than the epimera and with the third , second and first pairs in that order gradually rising above the plate.
  • Outer end of first pair drawn out to a point.
  • Inner end passing without definition into the basal plate. Fig 20.
  • Third and fourth pair with scale-like patches having their rounded edge toward the posterior.
  • Plate porose.


  • Legs short and slender, and articulating with the epimera in the basal furrow between the epimera and the basal plate.
  • First pair little more than half the length of the body.
  • Fourth pair rather more than three-fourths of the length of the body.
  • Third pair slightly shorter than the second.
  • Extensor surface of all segments with short bristles, flexor surface with slightly longer, partly pectinate ones.
  • Fourth and fifth segments of second and third pairs of legs and third, fourth and fifth segments of fourth pair with small groups of swimming hairs.
  • Claw like those of Midea Orbiculata, but itp of accessory claw not blunt as in latter species.


  • Lying in a small chitinous ring midway between the genital area and posterior end of body.

Genital area.

  • Genital area lying partly within the bay formed by the inner edges of the fourth pair  of epimera.
  • Genital plates broad, lunate, meeting together at each end.
  • Plates finely porose, and with seven or eight fairly long hairs distributes over the surface.
  • On the inner edge of each plate three acetabula lying one behind the other. Fig 40.
  • Between the anterior surface end of eacch plate and the fourth pair of epimera a large pore. Well visible on my fig 30.

 Processus of identification.

Not as usual, the Hopkin’s key didn’t help to recognize at least the family of this water mite.

By looking in the plates in the  Soar & Williamson (1929 – plate 43) I found some similarities with the plates illustrating a Mideopsis.

Then I checked in the checklist of Smit & Gerecke (2010) the species of mideopsis already found in France :

  • Mideopsis (Mideopsis) crassipes Soar, 1904 (from Soar & Williamson 1929 : third and fourth pair of each side of epimera nearly meeting together on the median line)
  • Mideopsis (Mideopsis) orbicularis (Müller, 1776)
  • Mideopsis (Mideopsis) roztoczensis Biesiadka & Kowalik, 1979 (Pesic 2010 : can be distinguished from Mideopsis orbicularis at a glance owing to the high convexity of its dorsal shield).

At this step it was already almost certain that this water mite is a Mideopsis orbicularis.

According to Smit & Gerecke (2010) it’s the first water mite reported from the department of Sarthe. It’s even the first water mite reported from the French region « Pays de la Loire ».

During my identification process I also checked the original reference (Müller, 1781) and even if my Latin is not very good, it seems that Müller had made a brief but rather concordant description of this water mite  (page 24) :

Lutea : yellow – Depressa : flattened – Orbicularis : round – Maculis disci nigris : black circular spot – Furca rufa : red fork (red malpighian vessel ?)

See also at the bottom of this article.

Other names.

  • Mideopsis depressa.


  • Müller, O.F. (1781) : Hydrachnae, quas in aquis Daniae palustribus. Lipsiæ
  • Pesic, V. & Turan, D. (2006) – New Records and Description of a New Subspecies for the Water Mite Fauna (Acari, Hydrachnidia) of Turkey from the Eastern Black Sea Coast. Turk J Zool 30 (2006) 405-411
  • 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 page 78 & plate 43. Ray Society, London.

References non used.

  • There may be a redescription of Mideopsis orbicularis from Tuzovsky in 1984. Dejtonimfy vodjanych klescej rodov Midea i Mideopsis (Mideopsidae, Acariformes) fauny SSSR
  • Biesiadka, E. and Kovalik, W. (1979). A new species of Mideopsis Neuman (Hydrachnellae, Acari) from Poland. Bulletin de l’Academie Polonaise, Seried des Sciences Biologiques, Classe II, 26: 695-702.

Other pictures.

Fig 20 - Mideopsis orbitularis ventral from back

Fig 20 - Mideopsis orbicularis - Ventral from back

Fig 30 - Mideopsis orbicularis - Ventral from front

Fig 30 - Mideopsis orbicularis - Ventral from front

Fig 40 - Mideopsis orbicularis - Acetabula

Fig 50 - Mideopsis orbicularis - Dorsal from front

 Hydrachna orbicularis described by Otho Fridericus Müller in 1781.

Page LII (52).

Tab V :


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