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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

Arthropoda: Introduction and Key to Major Taxa


The great phylum Arthropoda contains most of the described species of animals. There are probably over 900,000 described species of extant arthropods, about 80% being insects (there are over 200,000 species of beetles alone). Features of arthropods include an exoskeleton and usually jointed appendages, hence the name arthropod (jointed leg). The phylum is usually separated into nine extant classes, although today there is a tendency to treat many of these classes as phyla or subphyla.

Three classes of arthropods have freshwater representatives: Arachnida, Crustacea, and Insecta. Freshwater arachnids and insects are considered by most workers to be secondarily aquatic. That is, they are descendants of formerly terrestrial arachnids and insects that have invaded freshwater. In contrast, the crustaceans are primarily aquatic.

Key to Major Arthropod Taxa

See KEYS TO MAJOR AQUATIC ARTHROPOD TAXA AND ORDERS OF AQUATIC INSECTS OF ALBERTA for pictorial keys to the major taxa of aquatic arthropods of Alberta and to orders of aquatic insects. These keys should be used with caution because they must encompass a great diversity of invertebrates. In some cases it is possible to have a specimen that might not properly key in this section. The keys can be used in conjunction with the various whole specimen figures and colored photos to determine the major arthropod taxon of the specimen in question.

Pictorial Keys