Wild species 2010: chapter 10

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Spiders

Araneae - Order of arachnids that include predatory species with eight legs, no antennas, two poison fangs, and usually two silk-spinning organs at the back end of the body; they spin silk to make cocoons for their eggs or webs to catch prey.

Quick facts

• There are approximately 40 000 known species of spiders in the world, of which 1379 have been found in Canada.
• When excluding species ranked as Extinct, Extirpated, Undetermined, Not Assessed, Exotic or Accidental, the majority (86%) of spiders in Canada have Canada General Status Ranks (Canada ranks) of Secure, while 7% have Canada ranks of Sensitive and 7% have Canada ranks of May Be At Risk. However, it should be noted that many of the spider species were ranked as Undetermined.
• A total of 70 species of spiders found in the wild in Canada are Exotic, meaning that they have been introduced by human activities.
• Spiders are found on every continent in the world except for Antarctica.
• Unlike most arthropods, spiders do not have extensor muscles in their limbs and instead extend them using hydraulic pressure.

Background

Spiders (order Araneae) are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms. There are approximately 40 000 confirmed species of spiders world-wide (with an estimated total of 170 000) of which 1379 species have been confirmed in Canada. Spiders are found world-wide on every continent except for Antarctica, and live in almost all terrestrial and some aquatic habitats.

Spiders belong to a large group of organisms called arthropods. Arthropods have a hard outer coat called an exoskeleton that protects their soft insides. The bodies of arthropods are divided into sections. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend their limbs using hydraulic pressure. The abdomens of all spiders include spinnerets that produce silk. The most well known use for the silk is in the production of spider webs, which are primarily used to catch prey and provide a protective retreat for the maker.

Most spiders have fangs through which venom is ejected. Although a few spiders are vegetarian when very young, all spiders are predators. They mostly prey on insects (flies, mosquitoes, grasshoppers, beetles, and butterflies) and on other spiders. The largest species of Canadian spiders are from the family Pisauridea; they can reach 15 cm including their legs and are able to feed on fishes and salamanders! Spiders capture prey by trapping it in their webs or by active hunting. Spiders that use webs detect their prey by movement in their webs. Most spiders have six or eight eyes, though some species have fewer or even none.

Most spiders will live for at most one or two years, however tarantulas and other mygalomorph spiders have been known to live up to 25 years in captivity. Male spiders of most species have a shorter life span than the female. During the reproductive phase, females weave silk egg-cases, each of which may contain hundreds of eggs. Females of some species care for their young by carrying them around and by sharing food. A small number of species are social, and will build communal webs.

Spider bites can be painful but only a very few spider species are known to be potentially dangerous to humans. Scientists are now researching whether some spider venoms could be used as medicines and as non-toxic pesticides.

Status of knowledge

Due to their interesting range of prey capture and reproductive behaviours and the associated complex morphology of various body parts, spiders have attracted attention for study by scientists. In fact, several international scientific journals are devoted primarily to the study of spiders. However, in spite of the interest in spiders, in general, they remain poorly known; probably less than a quarter of the world’s spider species have been described and named and life history is well known for only a few of these. In Canada, only a few habitats have been well sampled for spiders and new additions to the list of known Canadian spiders will undoubtedly occur.

Fortunately, good taxonomic literature exists for most spiders likely to occur in Canada. Because of this and the facts that spiders are important predators wherever they occur and generally show strong associations with particular habitats, spiders are favoured subjects for ecological study in North America, including as indicators of biodiversity. Spiders are becoming increasingly important to scientists, conservationists, and managers within Canada and elsewhere in the world.

Richness and diversity in Canada

The three provinces where the most species of spiders can be found are Ontario (751 species), British Columbia (701 species) and Quebec (679 species). Yukon, Northwest Territories and Nunavut generally have fewer species of spiders than the other provinces of Canada (figure 10). However, Prince Edward Island has the lowest number of spider species (38).

Species spotlight - Western Black Widow Spider

The Western Black Widow (Latrodectus hesperus) is a hairless species with a black shiny body about 6 mm long. On the underside of its very round abdomen there is usually, but not always, a distinctive red mark which can often be in the shape of an hourglass. Immature individuals usually have white or pink markings on their abdomen which occasionally will remain through adulthood. Black widow spiders are cobweb spiders, meaning their webs look like a tangle of lines with no pattern.

The Western Black Widow is found from southern Saskatchewan, Alberta and British Columbia in Canada and south all the way to Mexico. Individual spiders are occasionally brought into other areas of Canada in shipments of grapes from California.

Black widow spiders reproduce in the summer. Occasionally, after mating, the female black widow spider will eat the male! The females typically produce four to nine egg sacs at a time and each sac can contain from one-hundred to four-hundred individual eggs (of which approximately thirty eggs will survive). The baby spiders will take on average six to nine months to fully mature into adults. Female black widow spiders may live for several years, but the males will typically survive for a much shorter period. The black widow spider usually preys on insects, but will also feed on other spiders. Most prey become caught in the webs of the black widow spiders; spiders subdue the prey by wrapping them in silk threads and then biting them to inject their venom. The black widow begins feeding after the venom takes effect.

Black widows tend to prefer dark shaded habitats such as abandoned squirrel tunnels, wood piles and the underside of rocks in sunny outdoor areas. The spiders remain active until the first good frost, and then hibernate for the winter, reappearing in the spring when temperatures warm up. A hibernating spider drops its metabolic rate, tucks its legs into its body and remains huddled in a shelter during the coldest months of the year.

Contrary to popular belief, people are rarely bitten by black widow spiders, and in the unusual circumstance of an actual bite, the person seldom suffers serious damage. In very rare cases, bites may result in medical problems or even death. The Western Black Widow has a Canada Rank of Secure.

Species spotlight - the dung mosses

The gnaphosid ground spider family is a relatively well known group with about 2000 recognized species world-wide. The Georgia Basin Bog Spider (Gnaphosa snohomish) is a very rare gnaphosid spider, known to exist only from a few sites in the Puget Sound / Georgia Basin area of northwestern Washington State and southwestern British Columbia. Current knowledge of the Georgia Basin Bog Spider indicates that it almost always selects peat and bog land as its habitat. The spiders follow a one year life cycle, overwintering before they are adults and then maturing late in the spring.

In British Columbia, researchers found what is considered to be the most substantial population in Burnaby, outside of Vancouver. The site of the find is an old commercial cranberry bog situated beside the Fraser River. The location is covered in peat and other mosses and overlain with cranberry bushes and various types of grasses. As of July 2007, the Burnaby site was redeveloped as a commercial cranberry bog. With little suitable habitat remaining (the site is surrounded by agricultural, commercial and industrial development), it is unknown whether any members of the Burnaby population of the Georgia Basin Bog Spider still exist.

Because of these confirmed threats to this very rare spider, the Georgia Basin Bog Spider was accepted for assessment by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). COSEWIC is preparing a report describing the status of the Georgia Basin Bog Spider which is anticipated to be completed in April 2012. The Georgia Basin Bog Spider has a Canada general status rank of May Be At Risk.

Results of general status assessment

This is the first assessment of spiders by the National General Status Working Group. The majority of Canada’s 1379 species of spiders have Canada ranks of Secure (714 species, 52%, figure 10 and table 14). However, many species of spiders are ranked as Undetermined (477 species, 35%). A total of 62 species (4%) are ranked as May Be At Risk and 56 species (4%) are ranked as Sensitive. Finally, 70 species of spiders (5%) are Exotic in Canada.

Threats to Canadian mosses

Many spider species show preferences for particular habitats. These spiders are vulnerable to habitat loss and disturbance due to natural succession, changes in drainage patterns, erosion control and other forms of habitat conversion, and climate change. Spiders are also very vulnerable to many pesticides.

Conclusion

Although spiders have attracted the attention of researchers and of the general public, most species are not very well studied. The ecology and distribution of these species need more studies. Much remains to be learned about their range and status in Canada.

Further information

Bennett, R. G. 1999. Canadian spider diversity and systematics. Newsletter of the Biological Survey of Canada (Terrestrial Arthropods) 18: 16-27.

Bristowe, W. S. 1971. The world of spiders. HarperCollins, London.

Foelix, R. F. 1996. Biology of spiders, second edition. Oxford University Press, New York.

Gertsch, W. J. 1979. American spiders. Van Nostrand Reinhold, New York.

Levi, H. W. and Levi, L. R. 1996. Spiders and their kin. St. Martin’s Press, New York.

References

Bennett, R., Blades, D., Buckle, D., Dondale, C. and West, R. C. 2006. The spiders of British Columbia. In E-Fauna BC: Electronic atlas of the fauna of British Columbia (B. Klinkenberg, editor). Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. (Accessed April 10, 2010).

Bennett, R., Fitzpatrick, S. M. and Troubridge, J. T. 2006. Redescription of the rare ground spider Gnaphosa snohomish (Araneae: Gnaphosidae), an apparent bog specialist endemic to the Puget Sound / Georgia Basin area. Journal of the Entomological Society of Ontario 137: 13-23.

Buckle, D. J., Carroll, D., Crawford, R. L. and Roth, V. D. 2001. Linyphiidae and Pimoidae of America north of Mexico: checklist, synonymy, and literature. Fabreries Supplement 10: 89-191.

Canadian Food Inspection Agency. 2010. Food safety tips when finding spiders in grapes. (Accessed January 13, 2010).

Coddington, J. A. and Levi, H. W. 1991. Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics 22: 565-592.

Dondale, C. D., Redner, J. H., Paquin, P. and Levi, H. W. 2003. The insects and arachnids of Canada. Part 23. The orb-weaving spiders of Canada and Alaska (Araneae: Uloboridae, Tetragnathidae, Araneidae, Theridiosomatidae). NRC Research Press, Ottawa.

Dondale, C. D. 1979. Araneae. In Canada and Its Insect Fauna (H. V. Danks, editor). Memoirs of the Entomological Society of Canada 108: 247-250.

Dondale, C. D., and Redner, J. H. 1978. The insects and arachnids of Canada. Part 5. The crab spiders of Canada and Alaska (Araneae: Philodromidae and Thomisidae). Agriculture Canada Publication 1663: 255 pp.

Dondale, C. D. and Redner, J. H. 1982. The insects and arachnids of Canada. Part 9. The sac spiders of Canada and Alaska (Araneae: Clubionidae and Anyphaenidae). Agriculture Canada Publication 1724: 194 pp.

Dondale, C. D. and Redner, J. H. 1990. The insects and arachnids of Canada. Part 17. The wolf spiders, nurseryweb spiders, and lynx spiders of Canada and Alaska (Araneae: Lycosidae, Pisauridae, and Oxyopidae). Agriculture Canada Publication 1856: 383 pp.

Dondale, C. D., Redner, J. H. and Marusik, Y. M. 1997. Spiders (Araneae) of the Yukon. In Insects of the Yukon (H. V. Danks and J. A. Downes, editors). Biological Survey of Canada (Terrestrial Arthropods), Ottawa: 1034 pp.

Paquin, P. and Dupérré, N. Guide d’identification des araignées du Québec. Fabreries Supplement 11: 1-251.

Platnick, N. I. 2010. The world spider catalog, version 10.5. American Museum of Natural History. (Accessed April 10, 2010).

Platnick, N. I. and Dondale, C. D. 1992. The insects and arachnids of Canada. Part 19. The ground spiders of Canada and Alaska (Araneae: Gnaphosidae). Agriculture Canada Publication 1875: 297 pp.

Royal Alberta Museum. 2010. Bug Facts: Western Black Widow. (Accessed January 13, 2010).

Shear, W. A. (editor). 1986. Spiders: webs, behavior and evolution. Stanford University Press, Stanford: 492 pp.

Ubick, D., Paquin, P., Cushing, P. E. and Roth V. D (editors). 2005. Spiders of North America: an identification manual. American Arachnological Society.