Introduction to the Arthropods
The phylum Arthropoda is one of the largest phyla. There are more than a million species of arthropods, including the extinct trilobites, spiders, mites, ticks, crabs, crayfish, lobsters, water fleas, copepods, millipedes, centipedes, and other insects. The insects include the cockroaches, beetles, bedbugs, fleas, bees, ants, wasps, mosquitoes, butterflies, moths, grasshoppers, lice, silverfish, and dragonflies. With more than 900,000 species, insects are by far the largest class within the phylum Arthropoda.
Key Morphologic Features
The name arthropod means “jointed foot” (arthro means “joint,” and pod means “foot”). All adult arthropods have jointed appendages. All arthropods are covered with a chitinous exoskeleton that is composed of segments. Chitin is a hard (yet elastic) body covering that envelops the entire body of all arthropods. Whenever you step on a cockroach, the chitin makes the “crunch” sound. Arthropods have a hemocoel, a body cavity filled with hemolymph. Hemolymph is a bloodlike fluid that bathes the internal organs of an arthropod. When a flying insect hits the windshield of a speeding car and “splats,” hemolymph is the fluid that creates the “splat.” Arthropods have a very simple circulatory system composed of a heartlike dorsal tube. This dorsal tube is actually a primitive heart that pumps the hemolymph to the arthropod’s head. The digestive system of arthropods begins with a ventral mouth and ends with a terminal anus. Arthropods possess a variety of respiratory systems; they may use gills, book lungs, or tracheal tubes as respiratory organs. Arthropods have complex nervous and excretory systems. However, the most important organ system of the arthropods is the reproductive system. Arthropods have separate sexes; they are dioecious. Reproduction is by means of eggs. Arthropods have a tremendous reproductive capacity.
Divisions of the Phylum Arthropoda
The phylum Arthropoda is made up of the following subphyla: Trilobitomorpha (the extinct trilobites), Onychophora (onychophorans, or “living fossils”), Tardigrada (water bears), Pycnogonida (sea spiders), Chelicerata (mites, ticks, spiders, scorpions, and others), and Mandibulata (crustaceans, centipedes and millipedes, and insects). Most of the arthropods important to veterinary parasitology are members of Chelicerata and Mandibulata.
Members of the phylum Arthropoda are important because (1) they may serve as causal agents themselves, (2) they may produce venoms or toxic substances, (3) they may serve as intermediate hosts for protozoan and helminth parasites, and (4) they may serve as vectors for bacteria, viruses, spirochetes, rickettsiae, chlamydial agents, and other pathogens.