Hydrachna sp., from here.

Belongs within: Neohydrachnidia.

Hydrachna is a cosmopolitan genus of water mites found in temporary pools, ponds and lakes; larvae are parasitic on Hemiptera and Coleoptera (Walter et al. 2009). Eggs are laid in the stalks of aquatic plants with females using their ovipositor to push eggs into internal air spaces (Walter & Proctor 1999). Larvae remain much of the year in diapause attached to their hosts while the vernal pools in which the adults breed are dry (Walter et al. 2009).

Hunter balls: the Hydrachna water-mites
Published 4 December 2014
Hydrachna sp., copyright J. C. Schou.

Hydrachnae are among the most rapacious of living animals, bold, fierce and cruel, the natural and inveterate enemies of all their congenera; they are no less hostile to each other, against which is waged a permanent war of extermination. Neither do they hesitate in attacking such animals as are suitable to their appetites, though double the size of their assailant.

This lurid description was applied to the water-mites by John Graham Dalyell in his 1851 book, The Powers of the Creator displayed in the Creation, or observations on life amidst the various forms of the humbler tribes of animated nature. The water-mites are a diverse group found mostly in fresh waters around the world; Dalyell probably intended the name Hydrachna to cover all water-mites, but modern authors recognise a large number of genera and families in addition to Hydrachna (which is distinctive enough that it is placed in its own family. Whether Hydrachna proper deserves the full force of Dalyell’s description may be debatable, but there is no denying that they are predators.

Water-mites belong to a group of mites, variously referred as Parasitengonae, Parasitengonina or some variation thereof (depending where you look), that is characterised by a life cycle including parasitic larvae and predatory adults. In the case of Hydrachna, the adults, which are nearly spherical in shape and bright red in colouration, feed on the eggs of aquatic bugs such as water boatmen or backswimmers that they find attached to submerged plants (so for all his charcterisation of Hydrachna as ‘bold, fierce and cruel’, Dalyell probably committed no less horrific an act of cruelty to chickens when he sat down to a fried breakfast). Despite their aquatic habits, Hydrachna are only clumsy swimmers themselves. After all, it does not require any great athleticism to hunt down an egg.

Larvae of Hydrachna, copyright Pfliegler Walter.

As well as finding their food on submerged plants, female Hydrachna lay their own eggs in them. They have needle-like chelicerae that they use to cut into the plant’s stem, and then lay their eggs in air spaces within the plant cells (up to 1500 at a time: Walter et al. 2009). When the eggs hatch, the emerging larvae (which are kind of rugby ball-shaped) swim in search of a suitable host. Usually, this is a water-bug of much the same sort whose eggs were being devoured by the larvae’s parents, though some Hydrachna species have also been recorded parasitising aquatic beetles. While some water-mites are quite picky about where exactly they choose to attach to a host, Hydrachna are not so: they may attach pretty much anywhere. They also do not exclude each other: a single host insect may end up with a large number of Hydrachna larvae attached to it (enough to have a serious impact on the host’s health). The palps on either side of the chelicerae are used to initially hold on to the host before the larva cuts into the host cuticle with its chelicerae; once its hold with the chelicerae is firm, the palps are folded out of the way (Redmond & Lanciani 1982). Once attached, feeding on the host’s haemolymph may not commence immediately: if the larva has found itself on a host that has not yet reached maturity, it will often wait until after the host has moulted. This is because the feeding larva becomes massively engorged and may swell up to hundreds of times its original size. In this swollen state, it obviously becomes immobile (one cannot walk if one’s legs no longer touch the ground); should the host shed its cuticle with the engorged larva attached, the larva would be unable to reattach itself to the host.

After about two weeks of feeding, the larva is ready to mature, but this does not necessarily mean leaving the host. In parasitengonines, the first nymphal instar (the protonymph) after the larval stage is dormant as the mite metamorphoses into something closer to its adult form, the first of the two ‘pupal’ stages that the mite will go through in its life (the second comes between the active deutonymphal instar and maturity, but involves less of a radical change in morphology). Hydrachna passes this ‘pupal’ stage while still attached to the host, only detaching when it becomes an active deutonymph. As well as saving the larva the inconvenience (and danger) of dropping off the host while in an engorged state, this helps ensure that the deutonymph emerges in a suitable habitat. Hydrachna prefers still waters, such as ponds and lakes. Some species of Hydrachna prefer to breed in temporary seasonal pools, and may remain attached to the host for several months while their home pools are dry. Somehow they can tell the difference between the temporary pools and the more permanent waters in which the hosts spend the rest of their time.

Systematics of Hydrachna

Characters (from Walter et al. 2009): Adult gnathosoma typically with long, curved, pointed rostrum. Palpi chelate, with dorsodistal tibial claw extending well beyond base of tarsus; genu longer than tibia. Chelicerae one-segmented and styletiform. Idiosoma with lateral eye capsules present and not borne on dorsal plate; body nearly spherical, variable in degree of sclerotisation, ranging from soft with dorsum bearing only glandularia platelets to heavily armoured with dorsum entirely covered by a dorsal shield. Genital field heart-shaped in males and rounded in females, with gonopore located posteriorly; bearing numerous genital acetabula borne on plates flanking gonopore; with conspicuous tubular ovipositor in females. Larvae aquatic. Gnathosoma lacking elaborate camerostome. Idiosoma with lateral eyes borne on dorsal plate; dorsum with large dorsal plate bearing eight pairs of setae, including verticils, scapulars, c3, and three additional pairs of hysterosomatic setae. Venter with coxal plates all separate; with paired urstigmata laterally between coxal plates I–II; coxal plates III located near midlength of body and laterally directed; excretory pore plate lacking setae. Leg with five movable segments, with basifemur and telofemur fused; tarsus with claws absent but empodium present.

<==Hydrachna Müller 1776RFF05
    |--H. (Anohydrachna) maramauensis Stout 1953SL71
    |--H. (Diplohydrachna Thor 1916)RFF05
    |    `--H. (D.) chilensis Gervais 1849RFF05
    |--H. (Rhabdohydrachna Viets 1931)RFF05
    |    |--H. (R.) rectirostris Viets 1940RFF05
    |    `--H. (R.) silvestrii Ribaga 1902RFF05
    `--H. (Scutohydrachna Viets 1933)RFF05
         |--H. (S.) miliaria Berlese 1888RFF05
         |    |--H. m. miliariaRFF05
         |    `--H. m. constricta Lundblad 1941RFF05
         |--H. (S.) nonlamellata Viets 1940RFF05
         `--H. (S.) portigera Lundblad 1941RFF05
Hydrachna incertae sedis:
  H. agilis Gervais 1849RFF05
  H. approximata Halik 1940H98
  H. australica Lundblad 1941H98
  H. bilobata Halik 1940 [incl. H. laceriscuta Lundblad 1941]H98
  H. conjecta Koenike 1895D91
  H. conjectoides Lundblad 1947H98
  H. cruentaWWO01
  H. cyanipes Lucas 1847E12
  H. erythrina Lucas 1847E12
  H. fuscataWP99
  H. geographicaMK91
  H. globosaD01
  H. hamata Lundblad 1947H98
  H. heterophthalma Viets 1954RFF05
  H. inermis Daday 1901D01
  H. linderi Lundblad 1947H98
  H. magniscutataWL09
  H. odontognatha Canestrini 1884 [=H. odontognathus]H98
  H. palustris Smit 1992H98
  H. pusilla Daday 1905RFF05
  H. rossica Daday 1901D01
  H. rostrata Lucas 1847E12
  H. scutataP-C06
  H. simulans Marshall 1928 [=Hydrarachna (l. c.) simulans]H98
  H. tasmanica Lundblad 1947H98
  H. testudinataWL09
  H. tomentosa Lucas 1847E12
  H. triscutata Lundblad 1947H98
  H. ventrifissa Viets 1954RFF05
  H. virella Lanciani 1979D91

*Type species of generic name indicated


[D01] Daday, E. 1901. Édesvizi mikroszkópi állatok [Mikroskopische Süsswasserthiere]. In: Horváth, G. (ed.) Zichy Jenő Gróf Harmadik Ázsiai Utazása [Dritte Asiatische Forschungsreise des Grafen Eugen Zichy] vol. 2. Zichy Jenő Gróf Harmadik Ázsiai Utazásának Állattani Eredményei [Zoologische Ergebnisse der Dritten Asiatischen Forschungsreise des Grafen Eugen Zichy] pp. 375–470. Victor Hornyánszky: Budapest, and Karl W. Hierseman: Leipzig.

[D91] Davids, C. 1991. Water mites: the impact of larvae and adults on their host and prey populations. In: Dusbábek, F., & V. Bukva (eds) Modern Acarology: Proceedings of the VIII International Congress of Acarology, held in České Budĕjovice, Czechoslovakia, 6–11 August 1990 vol. 1 pp. 497–501. SPB Academic Publishing: The Hague.

[E12] Evenhuis, N. L. 2012. Publication and dating of the Exploration Scientifique de l’Algérie: Histoire Naturelle des Animaux Articulés (1846–1849) by Pierre Hippolyte Lucas. Zootaxa 3448: 1–61.

[H98] Halliday, R. B. 1998. Mites of Australia: A checklist and bibliography. CSIRO Publishing: Collingwood.

[MK91] Meyer, E., & K. Kabbe. 1991. Pigmentation in water mites of the general Limnochares Latr. and Hydrodroma Koch (Hydrachnidia). In: Schuster, R., & P. W. Murphy (eds) The Acari: Reproduction, development and life-history strategies pp. 379–391. Chapman & Hall: London.

[P-C06] Pickard-Cambridge, O. 1906. The wild fauna and flora of the Royal Botanic Gardens, Kew. Arachnida. Bulletin of Miscellaneous Information, Additional Series 5: 53–65.

Redmond, B. L., & C. A. Lanciani. 1982. Attachment and engorgement of a water mite, Hydrachna virella (Acari: Parasitengona), parasitic on Buenoa scimitra (Hemiptera: Notonectidae). Transactions of the American Microscopical Society 101 (4): 388–394.

[RFF05] Rosso de Ferradás, B., & H. R. Fernández. 2005. Elenco y biogeografía de los ácaros acuáticos (Acari, Parasitengona, Hydrachnidia) de Sudamérica. Graellsia 61 (2): 181–224.

Smith, I. M., D. R. Cook & B. P. Smith. 2010. Water mites (Hydrachnidiae) and other arachnids. In: Thorp, J. T., & A. P. Covich (eds) Ecology and Classification of North American Freshwater Invertebrates, pp. 485–586. Academic Press.

[SL71] Spain, A. V., & M. Luxton. 1971. Catalog and bibliography of the Acari of the New Zealand subregion. Pacific Insects Monograph 25: 179–226.

[WL09] Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 233–420. Texas Tech University Press.

[WP99] Walter, D. E., & H. C. Proctor. 1999. Mites: Ecology, Evolution and Behaviour. CABI Publishing: Wallingford (UK).

[WWO01] Wohltmann, A., H. Witte & R. Olomski. 2001. Organismal patterns causing high potential for adaptive radiation in Parasitengonae (Acari: Prostigmata). In: Halliday, R. B., D. E. Walter, H. C. Proctor, R. A. Norton & M. J. Colloff (eds) Acarology: Proceedings of the 10th International Congress pp. 83–99. CSIRO Publishing: Melbourne.

Leave a comment

Your email address will not be published. Required fields are marked *