Classes of Arthropods

Key Physical Components

• All arthropods share distinct external characteristics. These include an exoskeleton; a body with bilateral symmetry (body divided into two equal sides); jointed appendages; segmented, regional bodies (bodies divided into parts according to function); and additional, specialized appendages. Specialized appendages improve locomotion, feeding, defense and sensing. The arthropod's major classes can be categorized by comparing the number of body segments, legs and antennae.

Common Classes

• There are five common classes of arthropods: Crustacea, which includes crayfish lobsters and shrimp; Arachnida, which includes spiders, ticks and scorpions; Insecta, which comprises insects; and Chilopoda for centipedes and Diplopoda for millipedes.

Crustaceans

• There are more than 30,000 species of crustacean. Crustaceans are primarily aquatic and live in saltwater oceans and seas. This class is defined as having two tagmata (body segments): the cephalothorax (head and thorax) and abdomen. They have two pairs of antennae, though one may be hard to see, and five pairs of legs. The pair of leg appendages closest to the head will possess large claws.

Arachnida

• These arthropods actually belong to an Arthropoda subclass known as the Chelicerata. They are characterized by two body regions, an abdomen and cephalothorax. Chelicerates have six pairs of appendages; the foremost two pair are parts of the mouth and the remaining four pairs are its legs. They do not possess antennae.

Insecta

• This is the largest and most abundant form of life known. There are more than 1 million species of insects that have been scientifically quantified and named. The Insecta class insects have three tagmata (head, thorax and abdomen) and one pair of antennae. They have three pairs of legs, and many have either one or two pairs of wings. The named species of insects number greater than all other known and named animals.

Chilopoda and Dilopoda

• Centipedes belong to the class Chilopoda and are elongated and somewhat flattened. They usually try to find protected places to live and hide, such as under loose soil or leaves, or in rotten logs. Chilopoda will have two tagmata (head and trunk), one pair of antennae and many appendages. The jaws of this class animal hold a poison with which they paralyze their prey.

  Diplopoda, the millipedes, have two tagmata also (head and trunk), one pair of antennae and a segmented trunk with many legs. Millipedes are usually cylindrical, although scientists recently have found and named some Dilopoda species that are flattened in appearance.