Arachnida


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Digitizing and providing web access to this text was funded in part by the Alberta Conservation Association and the University of Alberta, Department of Biological Sciences

Hydrachnidia (Water Mites)


Introduction

The class Arachnida contain such well-known arthropods as scorpions, spiders, mites, ticks, and many others. The subclass Acari includes the mites and ticks, some being economically important, because they transmit pathogens to livestock or directly to humans, e.g. the organisms that cause tularemia, Rocky Mountain Spotted Fever and Lyme disease. A few Acari are aquatic; these are the water mites, or Hydracarina ( Plate 19.1). Some workers consider Hydrachnidia (also called Hydrachnida, Hydracarina or Hydrachnellae) to be a suborder, or cohort division of the Acari; others consider the terms Hydrachnidia and Hvdracarina to be only terms of convenience.

Life Cycle

The generalized life cycle of a water mite features three active stages: larva, deutonymph, and adult, with the larval and nymphal stages often subdivided into additional stages ( Fig 18.1). The larva, which hatches from the egg, can be distinguished from nymphs and adults by its smaller size and possession of only three pairs of legs. The free-living larva attaches to an immature aquatic insect, and enters into a parasitic stage, in which the larva feeds on its host. How host-specific water mites are, if host-specific at all, is a matter of dispute. Smith (1988) reviews host-parasitic interactions of water mites with their insect hosts. Once engorged, the larva drops off its host. If it lands in water, the larva enters a quiescent stage called the nymphochrysalis, which metamorphoses into an active deutonymph. However, some species spend the nymphochrysalid stage on the host. The deutonymph has four pairs of legs, but can be distinguished from the adult by the nymph's immature genital features (see HYDRACHNIDIA A pictorial key ). Nymphs, as well as adults, are active predators on small crustaceans or aquatic insect eggs and larvae. The nymph eventually attaches to one of various types of substrata, enters into a second quiescent stage called the tritonymph (or teleiochrysalis) and then metamorphoses into the adult.

Collecting, Identifying, Preserving

Adults are needed for identification. Vigorous pond-net sampling, using a fine meshed net (about 150 micrometers openings), in both standing and running water usually yields numerous water mites. But some water mites occur in special habitats, e.g. adults of one group are found only in the mantle cavity of clams. Water mite samples can be fixed in the field with about 80-90% ethanol, or the nonpreserved sample can be brought to the laboratory where the active adults can easily be spotted and picked from the sample. A 75% ethanol solution is a suitable preservative, but for detailed work at the species and often the genus level, the following procedure, suggested by Heather Proctor, Department of Biological Sciences, University of Calgary, is recommended.

  1. Kill and preserve mites in Koenike's fluid (45 parts glycerin: 45 parts water: 10 parts glacial acetic acid).
  2. Puncture the body of the mite (through the dorsum of a soft mite, through a suture line of a hard-shelled mite).
  3. Place in about 8% potassium hydroxide (KOH) to clear specimen.
  4. Remove from KOH after 2 hours and place in a mixture of glycerin and water for dissection.
  5. For large red, soft-bodied mites and hard-shelled mites, identify under the dissecting microscope and then place in a vial of Koenike's fluid with a proper label.
  6. For soft-bodied mites, squeeze out the gelatinous body contents, remove palps, and mount body and palps in PVA (or glycerine jelly).

Alberta's Fauna and Pictorial Key

Heather Proctor has recently completed a M. Sc. thesis (1988) in the Department of Biological Sciences, University of Calgary, on the biology of some Alberta water mites. She constructed the pictorial keys, adapted mainly from Cook (1974), for the water mites of this book.

Genera

Genera that have been reported or are presumed to occur in Alberta (Proctor, unpublished report) are listed below.

Survey of References

The following references have information on Alberta's water mite fauna: Aiken (1985), Conroy (1968, 1985), Peck (1988), Proctor (1988, 1989), Proctor and Pritchard (1989), Smith (1989a, 1989b, 1989c), Zacharda and Pugsley (1988).

For anatomical features important in identifying water mites, see Figure 18.1.

Pictorial Keys

Figures

  • Figure 18.1
    • A generalized life cycle of a water mite, chrysalides stages (see text).