Spiders

Spiders belong to the phylum Arthropoda, class Arachnida, order Araneae. There are approximately 32,000 described spider species in the world, with thousands more yet to be discovered. Saskatchewan has recorded about 455 species, and just over 600 species are estimated for the province. Major challenges in spider taxonomy include revising a number of the family groups and the identification of immatures.

There are twenty spider families in the province, including eleven families that are specialized web-builders. The major web-building families include the Araneidae (orb-web weavers), Agelenidae (funnel-web weavers), Theridiidae (cobweb weavers), and the Dictynidae. The other web-building families are the Linyphiidae, Amaurobiidae, Hahniidae, Uloboridae, Tetragnathidae (long-jawed orb weavers), Mimetidae (pirate spiders), and the Pholcidae (daddy-long-legs spiders). The nine remaining families are hunting spiders, including the Philodromidae and Thomisidae (crab spiders), Lycosidae (wolf spiders), Anyphaenidae and Clubionidae (sac spiders), Salticidae (jumping spiders), Gnaphosidae (ground spiders), Pisauridae (nurseryweb spiders), and the Oxyopidae (lynx spiders).

The eyesight of the hunting spiders tends to be more acute than that of the web-builders. The salticids (jumping spiders) use their keen vision to stalk their prey, much like a cat, before pouncing on resting insects. The crab spiders often lie in wait on flower heads for their prey, which includes flies, butterflies and even bumble-bees. One species of crab spider, Misumena vatia (goldenrod spider), has the ability to change its colour from white to yellow and back, which helps to camouflage it against the flower head.

Spiders can readily be distinguished from insects as they have two body segments, eight legs, eight eyes (occasionally six), and no antennae. Almost all spiders have a pair of poison glands which aid them in subduing their prey. The strength and toxicity of the poison varies from species to species. Except for allergic reactions, bites from Saskatchewan spiders are not harmful to humans. However, the bite of Saskatchewan's most venomous spider, the black widow spider (Latrodectus hesperus), can produce a severe reaction.

A major attribute of spiders is their ability to spin silk. Spiders may use their silk to construct webs that aid in capturing prey, to wrap their eggs in, and to set down a dragline that may serve as a safety line or as a way to retrace their path. Adults and immatures also use silk to “balloon” from one area to another. A ballooning spider will climb up on vegetation, a fence post or another higher vantage point, lift its abdomen into the air and release some silk threads. As the wind catches this thread, it will lift the spider up and transport it from a few metres to even thousands of kilometres as it is completely at the mercy of the prevailing air currents.

Spiders can be found in almost every part of the world. In Saskatchewan, they range from the grassy Prairies of the south to the Forests and lakes of the north. On a wet morning, the ground may be covered with hundreds of dew-covered webs. These webs often belong to the funnel-web weaving spiders, the Agelenidae. Their web has a funnel built into the web that allows the spider to retreat and lie in wait for unsuspecting prey. Another common type of web is constructed by the orb-weaving spiders. Their webs are circular and can be found almost anywhere. One of the more common orb-weaving spiders is the Plains orb-weaver, Araneus gemmoides. Its webs can be found attached to buildings and among vegetation. Later in the summer and early fall, the female becomes quite large as she fills up with eggs. Soon after she lays her eggs in a silken case, she dies. The females of the wolf spiders, the Lycosidae, have the unique characteristic of carrying their egg sacs attached to their spinnerets. Once the spiderlings hatch, the female carries them on her back for several days before they disperse. The females of the nurseryweb spiders, the Pisauridae, also carry their egg sacs, but in this case they use their chelicerae or front pair of jaw-like organs. The nurseryweb spiders are also unique in being almost semiaquatic. Dolomedes triton will dive beneath the water's surface in search of prey among aquatic vegetation; these spiders are also called “fishing spiders” as they are capable of catching small fish.

While many people have an aversion to spiders, most spiders are extremely beneficial. They are carnivorous and feed on numerous arthropods including insects. They probably consume more insects than all the other insectivorous animals combined, helping to maintain the balance of nature. Silk strands have also been used for the cross hairs in certain optical instruments.

Keith Roney

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Further Reading

Gertsch, W.J. 1979. American Spiders. New York: Van Nostrand Reinhold; Levi, H.W. 1968. Spiders and Their Kin. New York: Golden Press; Preston-Mafham, R. and K. Preston-Mafham. 1984. Spiders of the World. New York: Facts on File Publications.