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.
As explained in an earlier post, 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.
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.
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.
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.
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.