What is an arthropod?
The arthropods constitute over 90% of the animal kingdom and are classified in the phylum Arthropoda. They are distinguished from other animals by: an exoskeleton (a skeleton on the outside of the body) body divided into distinct partsjointed legs and appendagesbilateral symmetry (both sides of the body are the same) Class Distinctions Insects Grasshoppers, butterflies, beetles, ants, etc. 1,000,000 described world species: three body regions: head, thorax, abdomensix legs attached to the thorax (which has 3 segments)adults with one or two pairs of wings attached to the thorax (some have none)two antennaelateral compound eyes Arachnids Spiders, scorpions, ticks, mites, etc. 65,000 described world species: two body regions (cephalothorax, abdomen)eight legsno antennaeMouth parts are chelicerae (modified appendages) which in spiders are fangs Crustaceans (technically a subphylum) Classes include crabs, shrimps, lobsters, barnacles, isopods etc. 44,000 described world species: two body regionstwo pairs of antennae5 or more pairs of legsprimarily aquatic, few terrestrial Chilopods Centipedes. 2,800 described world species: well-defined headfirst pair of legs modified for envenomationflattened top to bottomone pair of legs per segmentone pair of antennae Diplopods Millipedes. 10,000 described world species: two pairs of legs per segments, first four segments have 1 pair legsone pair of antennaewell-defined headusually cylindrical.