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Arachnida is a large class of chelicerate arthropods (segmented, jointed-limbed animals) including the orders Araneae (SPIDERS), Scorpiones (SCORPIONS), Opiliones (HARVESTMEN), Pseudoscorpiones (PSEUDOSCORPIONS), Solifugae (WIND-SCORPIONS) and the subclass Acari (MITES and TICKS). Second only to INSECTS in species numbers, an estimated one million arachnid species may occur worldwide, about 13 000 in Canada. The great majority are mites; spiders are next most common.
Characteristically, arachnids have 2 body parts, the prosoma (cephalothorax) and opisthosoma (abdomen). The prosoma has 2 pairs of mouthparts, 4 pairs of legs, and, if present, 2-8 simple eyes above the first 2 pairs of legs. The opisthosoma has the genital and anal openings as well as various organs (eg, pectines of the scorpions and spinnerets of spiders). The Acari have diverged from this basic plan in having a movable headlike anterior body region, the gnathosoma, which bears the mouthparts and is distinct from the rest of the body. Arachnids lack wings. Arachnids are an ancient lineage: the earliest known fossils are of 420 million-year-old scorpions. All major living groups had originated by the end of the Devonian age, 360 million years ago. Mites, spiders and harvestmen occur throughout most of Canada. Some mites and spiders and one species of harvestmen even occur in the High Arctic. Ticks and pseudoscorpions are transcontinental north to the TREELINE, excluding arctic regions. Scorpions and wind-scorpions live only in southernmost western Canadian desert areas. Most arachnids are predators of other arthropods, although some harvestmen feed on dead organic matter; ticks are obligate blood feeders (parasites) of vertebrates, and mites have diverse feeding habits.
 PseudoscorpionPredatory small arthropods, pseudoscorpions are poisonous, but in their front claws, not their tails (photo by Klaus Bolte).
 MiteOribatid mite (photo by Barbara Eamer).
 TickLarval tick (photo by Klaus Bolte).
Author
EVERT E. LINDQUIST
Links to Other Sites
Canadian Biodiversity Website
A great information source for all budding biologists. Learn about biodiversity theory, natural history, and conservation issues. From McGill’s Redpath Museum.
The Northern Scorpion, Paruroctonus boreus
Carefully catch this illustrated natural history of Canada’s only native species of scorpion. From the Biological Survey of Canada. A University of Lethbridge website. A pdf file.
The Canadian Arachnologist
This site spins an extensive web of information about the science of arachnology, including checklists, databases, and reports about various spidery species found in Canada. Their online guide to Nearctic spiders features images, species lists and descriptions, real-time distribution maps, and more. "The Canadian Arachnologist" also provides news of interest to Canadian arachnologists and other spider enthusiasts.
Pseudoscorpions
Find out about the scaaary looking but harmless arachnids called pseudoscorpions. Features a great close-up photograph of a specimen. From the Royal Alberta Museum.
A primer on pseudoscorpions and taxonomic status in Canada
This article focuses on the life history, ecology, and taxonomy of species belonging to the arachnid order Pseudoscorpiones. Also includes a bibliography and table of families and species. From the “Newsletter of the Biological Survey of Canada (Terrestrial Arthropods.”)
Entomology
Learn all about angry arachnids and irascible insects in this online introduction to entomology. Features a glossary, great photos, graphs, charts and more. From Natural Resources Canada.
The Canadian National Collection of Insects, Arachnids and Nematodes
This website provides information about the scope and contents of the Canadian National Collection of Insects, Arachnids and Nematodes. Check the “Index” link for illustrated descriptions of various taxonomic groups.
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