watermijt, Hydrodroma species
A red watermite (Hydrodroma sp.)

watermite, drawing underside
watermite, underside (Piona sp.)

GENITAL Adult watermites bear structures around the genital pore that are specific for the species. Sometimes these are two movable flaps. On top of, or below these flaps there are acetabula, which often look like suckerdisclike structures. Some species have three on either side, other species many more. Below here two examples:
watermite, drawing underside
The acetabula were thought to be real suckerdiscs to connect the mating individuals, but that's not true, probably they have an osmoregulating function (Wesenberg Lund, 1939, Davids, 1979). The structure of the genital is an important item for the identification of the species.
LIKE A PRETTY LITTLE RED DOT, THE WATERMITE SWIMS IN THE GREEN DITCH. Other animals flash to there hideouts when you advance to the ditch, but not the watermite: it carries on swimming in a seemingly leisurely manner. Many people have seen a red watermite, because the red colour is conspicuous and you can easy observe them. It almost seems as if the watermite wants to attract the attention of possible predators and in fact that is just how it is: they secrete an unpalatable substance on their skin, which makes them inedible for many animals. When fishes and newts accidently gobble up a watermite, they immediately spit it out again. The red colour is a warning: ”I'm nauseous tasting, remember?” and so the mite may swim undisturbed. It doesn't always work: hydra's do eat watermites and I found the water stick insects I kept sometimes sucking on a watermite. It seems Coots aren't put off by the taste either, as can be seen on a beautiful picture by Hans Gebuis. And not all watermite species are red.

EPIMERS Chitineous plates on the underside, on which the hips (coxa) of the legs are attached. Many species have the epimers fused with the ventral shield. The shape and position of the epimers are characteristic and differ between species. Below a few examples, from left to right: Hygrobates, Piona, Hydrodromas and Arrenurus.
epimer examples of four watermite species

EYES Watermites have two pairs of eyes, on many species these are lying so close together that there seems to be only one pair. Some species have chitineous structures. around or near the eyes.

four eyes:
eyes watermite, Limnesia sp.
Limnesia sp.
two eyes:
eyes watermite, Arrenurus sp.
Arrenurus sp.
eyes watermite, Eylais sp.
Eylais sp.

Is the red colour really a warning?

Proctor H. & Garga N. (2004) found many red watermites in temporary waters without any fish, but in waters were fish lived the not red watermites were dominant. They suggest ”...that the main role of red and orange carotenoid pigments may be to act as photoprotectants, and hypothesize that redness originated in the terrestrial ancestors of water mites.” They further suggest that because red watermites are more conspiceous, they developed the distastefulness. - so this is the opposite of the original idea!
ANATOMY (v.d. Hammen, 1979, Wesenberg Lund 1939).
Watermites have amazing properties. There are many different species, which are difficult to identify. They are closely related to terrestrial species, and the changes that were needed to live under water are not profound.
The body is not segmented like that of insects and spiders, but it is shaped like a sack or a globular lump. Hence the name Trombidiformes, (the "lump shaped") for the order in which watermites are placed. A few species have a rather formless body, that is so soft that these mites will suffer a lethal collapse when brought outside the water, others are more sturdy and some species are really hardened, by an armoured skin. The body is called the idiosoma, a small part at front is called the gnathosoma and carries two pair of appendages, the first frontal pair being
the chelicerae, but these are almost not visible, because they are retracted in the mouthparts (in red on the drawing), and even further in the body. They are build like little spears, not as fangs like the chelicerae of the spiders, but serve to rip through the skin of their prey. Through the surrounding mouthparts, protein solving saliva is then brought in the wound after which they suck up the liquified tissue.
The palps (as a mites pedipalps are called for short) are the second pair of appendages (marked green on the drawing at left) and on some species might be mistaken for a front pair of legs . They may seem like a sort of antennae (ped- foot, palpus feel,touch gently), but they serve several other functions. The palps differ from species to species: long or short, with or without small scissors or pincers. The mite uses them to grab and hold his prey, or sometimes as an extra pair of legs to climb or hang from a waterplant. For the identification of a watermite species they are an indispensable help.
The eight legs are attached to chitineous plates on the underside, the epimers. There are eight legs, many species they have fringes or tufts of long hairs on all or some of the legs which allow them to swim.
Feeding, digestive track. The prey animals are caught with the legs and palps, and after piercing the skin with the chelicerae, liquified protein is sucked up into the stomach (also called midgut) which has a number of blind ending sacks. This stomach is not connected to a real endgut, but the meal is absorbed into the body. Because their are no insoluble leftovers, a normal anus is not necessary. However, the relative large excretory organ that is lying over and after the stomach, does have an opening to the outside, through which now and then translucent globules are discharged. This opening is situated on the sternal part of the underside and is called the excretory pore, anal pore or simple: anus. The globules are the waste products of the body that are passed to the membrane of the excretory organ. When filled the organ shimmers through the skin as a Y-form, in some species it has fine branches which give the mite an attractive colour pattern.
Breathing. There are no lungs, but tracheae, widely branched, very fine tubes, comparable to those of insects. The two main tubes end in stigma's (little openings) on top of the mouthparts, covered by the skin. Through the skin oxygen from the water is taken in. Watermites never come up to breathe, yet the tracheae are filled with air.
The genital is positioned at the underside of the body (see left).


Proctor, H.C., Garga, N. (2004). Red, distasteful water mites: did fish make them that way? Exp. & appl. acarology 2004 34 (1-2) 127-47
   Excerpt retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15597605



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rev. 23-05-2015

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