Arachnids are spiders, and their close kin. They all have 8 legs, plus 2 long appendages around the head called pedipalps that are used by the males in mating. The body is divided into 2 parts, the abdomen, and the cephalothorax or combined head and chest. Very few arachnids are aquatic. The most common aquatic arachnids are fishing spiders (Dolomedes) and water mites (Hydracrina or Hydrachnida).

The fishing spider, also called the raft spider or the nursery web spider, is a large spider with very long legs. One prominent identification mark is the light bands around the edges of the abdomen and cephalothorax. They can get up to an inch long, excluding legs.

Fishing spiders are not truly aquatic because they do not actually live in the water. However they are frequently found running over the surface of ponds, and can dive under the water to hide or to catch prey.

They feed on aquatic insects and occasionally small fish or tadpoles.

Fishing Spider (Dolomedes)

Green spotted water mite 0.6 mm long

Water mites are very tiny, one of the smallest creatures that can be found with the naked eye in the pond. Without magnification, they look like tiny black dots. They are usually less than 1 mm in diameter. Their bodies are round, and the cephalothorax is very small, so that even under magnification, it is hard to distinguish it.

Water mites come in a variety of colors, black, blue, red, and green, and sometimes spotted. The legs are small and the pedipalps are large.

The picture at the right was taken under 100 X magnification. The cephalothorax can be seen to be a separate part of the water mite's body. Also visible are the long hairs on the water mite's legs that aid in swimming.

Water mites are usually found on the bottoms of ponds. They are predators, feeding on very small insect larvae and small crustaceans. Early larval stages are parasites, attacking insect larvae. Although water mite larva do not kill their hosts, female insects that have been parasitized by them are less reproductive than those who have not been. Thus water mites are important in controlling the populations of insects, such as mosquitoes and midges.

Water Mite 0.4 mm long

Large red Water Mite 1.5 mm in diameter

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Crustaceans

Dobsonflies and Fishflies

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