watermijt, nimf op duikerwants
Watermite, nymph on a lesser waterboatman nymph

watermijten, levenscyclus
Life cycle
IN THE SUMMERTIME WE OCCASIONALLY MAY SEE INSECTS THAT HAVE ONE OR MORE TINY RED BULBS ATTACHED TO THEM. Those little globes are nymphs (larvae) of mites, in a legless, parasitic phase of development. Watermites have a complex life cycle, which has four extra phases between the egg and the adult. The cycle is not the same for all species, but the main line is described below: (Wesenberg Lund, 1939, Davids, 1979,2004, Smit 2008).

MATING differs widely from species to species. Often the male anchors itself to the female, with clamping structures on his legs or even with a kind of glue. Sperm is always transfered in a small packet, a spermatophore. In many species the male puts this packet in the genital pore of the female, sometimes with special adapted tarsi of the third pair of legs. On other species the male glues the spermatophores to a substrate (e.g. leafs or a stone) and the female picks it up in her genital pore, either by itself or because the male forces her to walk over it.
THE EGG The females of some species lay thousands of eggs together with other females and cover them with a fluid they secrete that hardens out, forming large, smooth plaques on the bottom and waterplants (Wesenberg Lund, 1939). Other species lay smaller, uncovered egg packets, or inject their eggs in waterplants. The number of eggs varies widely from ten to a few thousand. In the egg a second egg skin is formed, it envelopes the prolarval stage, which is like a kind of embryo of the larva.
THE LARVAE that crawl out the egg are very different from the adult: they have six legs instead of eight and relative large, specialised mouthparts with which they can anchor themselves to an insect on which they will parasitise. And that is the only goal of the larvae: if they do not find a host they will perish within two weeks. The success factor is low: most larva will not survive. Larvae of some species, for example Hydrodroma rise in masses to the water surface, where they run incredibly fast, like a swarm of gliding little red dots. They try to find and jump on a right victim, for example a water strider. Thyas larvae are able to jump over centimeters, an enormous distance in relation for these minute animals! Other species climb up in waterplants, or find their hosts under water by swimming. Victims may be waterbugs, waterbeetles and their larvae, further are midges dragonflys important hosts. The larvae attach themselves to the body, the legs or the wings.
THE NYMPHOPHANE stage is shown on the picture above left and at right, on an infected Lesser waterboatman larva and adult. After a larva successfully has attached itself to a host, its legs deteriorate and it changes to a pear or sacked shaped object. Species that live under the elytra of insects grow broad and flattened. By sucking in blood and dissolved proteins from their hosts these sacks may grow much larger, until the nymph, that develops within this bag, is visible through the outer membrane. In the end the nymph gets out the bag and so the insect is relieved from its parasite. If the host is able to fly, transportation to other waters is an added advantage!
THE NYMPH closely resembles the adult watermite, it hunts the same prey. The genital however has not fully developed. After a number of days the nymph drills its chelicerae in a waterplant and enters a new quiescent phase. It seems just a watermite that takes a long pause, but this stage is called
THE TELEIOPHANE stage and within the skin of the nymph grows the adult. Finally the skin ruptures and the adult watermite, the IMAGO shows up to complete the life cycle.

Like a bear in a tree
- -
I once saw, in a very shallow corner of a recreation lake, a lot of very tiny red dots with great speed swarming together on the surface, in random directions. They seemed to glide, because of their tininess legs could not be observed. I understood they were watermite larvae, searching for a host to jump on. And like a bear, fled in a tree for the jackals, a small Haliplus beetle sat in the top of a sprit that stood up from the water between the dots. Did he really flee for his attackers?
Adult watermites and the nymphs eat waterfleas, ostracods and other small animals. On the drawing up left the prey is a zebra ostracod. Many species are very voracious and will stab their chelicerae in other watermites. Especially Limnesia species overpower soft skinned mites that happen to come in reach of their palps. Hydrachna species suck out insect eggs (Davids, 1979). I once saw how a few eggs of a Hyphydrus beetle were sucked out by a watermite.


Davids, C. (1979). De watermijten (Hydrachnellae) van Nederland. Levenswijze en voorkomen. Wetensch Meded K N N V, 132, 1-78.

Davids, C. (1997). Watermijten als parasiten van libellen. Retrieved from http://www.brachytron.nl/Brachytron/brachytronsums/Brachytron%201-2-4.pdf.

Davids, C. (2004). Parasitisme bij watermijten. Retrieved from http://www.nev.nl/eb/EB-2004/EB-64(2)/2004-051-058-Davids.pdf.

Peyrusse, P. Bertrand, M. (2001). Les Acariens aquatiques de France. Insectes 4 № 123 - 2001 (4) retrieved june 20 2012 from http://www.inra.fr/opie-insectes/pdf/i123peyrusse-bertrand.pdf

Smit, H. (2008). Watermijten, ter land, ter zee en in de lucht. in: Passie voor kleine beestjes, 33,3 jaar Stichting EIS-Nederland (2008), p. 60-61. Retrieved from http://www.repository.naturalis.nl/document/124827


Pictures of eggs, larvae ao. by Ferry Siemensma.



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All pictures on this site were made by Gerard Visser (Almelo, Netherlands), unless stated otherwise. All rights remain with him. These pictures may not be used for purposes any other than private viewing or printing. Do NOT hardlink to these pictures or place them on other websites without the author's approval. Should you need them for purposes which include third parties, you must ask the author permission by e-mail. People, who want to use this pictures for exhibitions or publications or educative material are much encouraged to do so, after approval as mentioned and giving the normal credits.
© G.H. Visser 09-02-2010
rev. 17-11-2012

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