Watermite, nymph on a lesser waterboatman nymph
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. Parathyas
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.
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