Phylum: Arthropoda [jointed foot] insects, crustaceans, spiders and ticks, millipedes, centipedes.

Arthropods are the most successful group of animals on earth. There are over 900,000 described species and approximately another million species yet undescribed. Arthropods are found in all habitats and are often the most abundant organisms. Arthropods are classified as eucoelomate protostomes.

The arthropod body plan is segmented (metamerism) and often is composed of three sections (head, thorax, and abdomen) of fused segments (tagmosis). Tagmosis is the process by which segments are fused to form larger body sections. Support for the animal comes from a hardened exoskeleton of chitin, a polysaccharide. Because the exoskeleton can not expand as the animal grows, it is periodically shed by a process called molting (also called ecdysis). After the animal molts, the individual produces a new, larger exoskeleton. Because the cuticle is hardened over most of the body, movement of body parts only occurs at joints where the cuticle is thin and flexible.

Respiration in arthropods varies between groups but is usually by gills in aquatic habitats and a tracheal system in terrestrial forms. The tracheal system is composed of tubes that distribute air throughout the body. Some spiders have a specialized lung, called a book lung. Although most arthropods have specialize respiratory structures, diffusion of gases across thin areas of the cuticle also contributes to respiration. The circulatory system is open.

Nitrogenous waste of terrestrial forms is usually converted to uric acid to reduce water loss. Malpigian tubules excrete uric acid into the digestive system where it is removed from the body with the feces. In aquatic forms, nitrogenous waste is removed as ammonia. Specialized structures, called green glands, in crayfish funtion in excretion and help maintain proper water balance in the body.

The sexes are usually separtate (dioecious) in Arthropods and many have specialized life stages within the life cycle that minimize competition between young and old individuals of the same species. The body of larvae is often very different from that of the adult (compare a catepillar to a butterfly) and go through a pupal stage where they metamorphose into the adult form.

Two major categories of arthropods based upon jaw apparatii:
A. Chelicerata - the more primitive group, feeds by sucking fluids and juices from victims by modified mouthparts, Chelicera. Chelicera inject toxin or poison into victim before sucking liquids. Horseshoe crab, spiders, ticks, mites, daddy longlegs.
B. Mandibulata - the more modern group, feeds by normal chewing with traditional mouthparts, Mandibles. Insects, millipedes, and centipedes (Uniramia), and crustaceans (Biramia).

Taxonomy of Arthropoda:

Subphylum Chelicerata - chelicera mouthparts

Class Merostomata - aquatic chelicerates = horseshoe crab.

Class Pycnogonida - sea spiders, usually 4 pairs of legs.

Class Arachnida - scorpions, spiders, ticks, mites.

Subphylum Crustacea - mandibulate mouthparts, 2 pair antennae

Class Malacostraca - aquatic gill breathers, biramous limbs. Head and thoraxfused into a Cephalothorax

Subphylum Uniramia - mandibulate mouthparts, 1 pair antennae, uniramous limbs.

Class Diplopoda - millipedes, 2 pair legs/segment

Class Chilopoda - centipedes, 1 pair legs/segment.

Class Insecta - insects, 3 pairs legs.

Grasshopper dissection

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