Hygrobates longipalpis (Herman, 1804) – male – Identification almost certain

Found the 26 of February in Saint Marin au Laërt in France.

My observations

• 2 eyes reddish with a dark spot inside.
• Size unknown (maybe 1.5mm but I’m unsure)
• Limnesian peg very small (is there a limensian peg on fig 30?)
• Genital area far away the coxae plates.
• Colour orange/yellow with darker area.
• Coxae plates occupying a little bit less than the half of the ventral area.
• Legs thin
• Skin slightly striated
• Third coxae very small
• Gnathosoma longer than first pair of coxae
• White ornamentation around the genital area (excretory organ ?)
• Coxae plates I and II ending like a needle.
• Genital area with 1 pair of 3 acetabulae grouped with 2 acetabulae at the posterior side.

The description of Soar & Williamson (1927)

Body.

• Attaining a length of 2.5mm and a breadth of slightly over 2mm for the female. The male is smaller.
• Colour yellow with dark brown patches.
• Malpigian vessel very much branched, ranging from yellow to brown in colour.
• Epimera, palpi and legs brownish yellow or tinged with blue.
• Outline oval; anterior portion between the antenniform bristles more or less emarginate.
• Dorsum well arched, with the anterior portion at times slightly depressed.
• Antenniform bristles short and stumpy.
• Skin with a fine ridging.

Capitulum.

• Posteriorly slightly narrowed, fused with the first pair of epimera ; point of fusion indeterminate.
• Mouth aperture large, anteriorly forming a furrow with two lateral tapering process.

Palpi

Epimera.

• Epimera occupying about two-fifths of t
• he length of the ventral surface, situated somewhat back from anterior edge.
• First pair united to the posterior end of capitulum and to each other without any suture to indicate point of fusion, extending well back and rounded posteriorly.
• Second pair almost triangular, united to first pair, but not of the same length.
• At the posterior ends of second pair a short wedge shaped process pointing outwards.
• Third pair slightly larger than second and separated from them by a space, varying with the size of the mite.
• Fourth pair somewhat rectangular in outline with the inner edges tending to be rounded, the anterior portion being more oblique to accommodate the posterior end of the first pair in the younger form.
• On the anterior margin adjacent to inner ends of third pair of epimera, a gland pore.

Legs

• First pair about 1.4mm, fourth pair about 1.85mm.

Genital area of female.

• Genital area about twice as broad as long, lying about its own length behind the fourth pair of epimera.
• Aperture with very broad, but very slightly inflated lips.
• Outside these a pair of irregularly sickle-shaped plates, each with three acetabula.
• Anterior acetabula usually the smallest and rather elongate.
• Second pair of acetabula placed somewhat to the side f the posterior pair, but well forward.
• Outer side of each plate as well as the anterior and posterior inner ends of the same with a number of long fine hairs.
• At each end of the genital aperture a chitinous arcuate bridge, with a median process extending into the body.

Male

• Distinguished by its smaller size and more slender limbs as well as by its genital area.

Genital area of male :

• Genital aperture much shorter than in female.
• Lips very much narrower at the centre end tapered toward each end.
• Plates very broadly lunate and united at their extremities.
• Acetabula very much larger, but placed very much as in female.

My opinion on the matching with the description of Soar & Williamson.

Elements that match perfectly.

• The small peg visible on my fig 30, on the first segment of the palpi.
• The shape and the disposition of the acetabulae on the genital plates.

Elements that doesn’t match.

• On my water mite the capitulum is not fused with the first pair of epimera (fig 20 & 30). A suture line between the first pair of coxae and the gnathosoma is well visible and continued.

My opinion on these elements.

I’m very annoyed by the obvious difference between my water mite and Hygrobates longipalpis. For the time being, this identification seems highly questionable. I have to pursue my enquiry.

Last note : with the comment made by Gerard (see below) I rank this identification as almost certain.

Synonymous of Hygrobates longipalpis.

Hygrobates rotundatus – Nesea dentata – Hydrarachna longipalpis – Hygrobates impressus

 

Presence reported in France.

Smit & Gerecke (2010) report it from the two departement of my area : Nord and Pas de Calais.

 

Other idea of identification.

• My first idea was to think of a Limnesia (shape, limnesian peg ?…) but this water mite have only two eyes while the Limnesia have 4 eyes and there is no limnesia with such a configuration of acetabulae (Van Haaren & tempelman 2009).
• Hygrobates nigromaculatus Lebert, 1879 : in Piersig (1900) plate AAAA. More similar at the level of the junction of the gnatosoma and the first coxae plates but the shape is ovoid while mine is dorso ventrally depressed.

References.

• Haaren, T. van & Tempelman, D. (2009): the dutch species of limnesia, with ecological and biological notes (acari: hydrachnidia: limnesiidae). Nederlandse Faunistische Mededelingen 30 ‒ 53:74
• Soar, C. & Williamson, W  (1927): The British hydracarina. Ray Society. Volume II. Page 101. Plates 24 – 35.
• Smit, H. & Gerecke, R. (2010): A cheklist of the water mites of France (acari: hydrachnidia). Acarologia 50(1): 21-91.
• Piersig

Web links.

• Agroeconet with some pictures of Hygrobates longipalpis. In russian.

More pictures.