Arthropod, an animal of the phylum Arthropoda. This phylum of invertebrates (animals without backbones) includes insects, spiders, ticks, lobsters, and centipedes. The name is derived from Latin words meaning "joint-footed," and refers to the jointed legs and other appendages of these animals. Arthropods are the most numerous and varied of all animals---more than one million species are known. They are found on land, in seas, and in fresh water.

Arthropods are of great importance to humans. Lobsters, shrimps, crabs, and crayfish are prized as food. Honeybees produce honey and help pollinate plants. Some insects and spiders destroy agricultural pests.

However, arthropods as a group are the greatest transmitters of human disease other than humans themselves. Mosquitoes and other flies spread malaria, sleeping sickness, yellow fever, dengue fever, and sandfly fever. By contaminating food, cockroaches spread amoebic dysentery, poliomyelitis, diphtheria, and many other serious diseases. Certain ticks, mites, and lice infest human bodies and carry such diseases as typhus fever, bubonic plague, and Rocky Mountain spotted fever. The bites or stings of many arthropods are poisonous to humans. In addition, arthropods spread disease among domestic animals, and many of them---such as the boll weevil and corn borer---destroy crops.

Arthropods vary in size from microscopic mites to giant spider crabs with a leg span of 12 feet (3.7 m). They have segmented bodies, well-developed nervous systems, and exoskeletons (outer skeletons). The exoskeleton, formed of a substance called chitin, is shed periodically as the animal grows. In many arthropods, including the centipede, the body segments are alike except for the head and anal segments. In others there are distinct body regions: head, thorax, and abdomen. In lobsters and other crustaceans, and in arachnids (spiders, ticks, and mites), head and thorax are fused into a cephalothorax .

Most arthropods lay eggs. In some species, the young are born live. Certain arthropods reproduce by parthenogenesis, the process by which the young develop from unfertilized eggs.

Classification

The large variety of arthropods has led to some disagreement among taxonomists (scientists who classify plants and animals). The following is a widely accepted classification of living arthropods:

Class Pauropoda,

a group of soft-bodied animals with nine pairs of legs.

Class Symphyla,

a group of soft-bodied animals with 12 pairs of legs.

Class Merostomata,

horseshoe crabs. Horseshoe-shaped shell; spikelike tail; six pairs of legs.

Class Pycnogonida,

a group of about 500 species of marine animals, including the sea spiders.

Class Crustacea,

crustaceans (shrimps, lobsters, crabs, sow bugs). Mostly water-dwelling; hard shells; two pairs of antennae; cephalothorax.

Class Chilopoda,

centipedes. Long, flat, many-legged bodies; one pair of antennae; poison glands.

Class Diplopoda,

millipedes. Long, round bodies; many legs; one pair of antennae; no poison glands.

Class Insecta,

insects (flies, bugs, beetles, ants, bees). Three main body divisions; one pair of antennae; three pairs of legs. Most kinds have wings.

Class Arachnida,

arachnids (spiders, scorpions, mites, ticks). Lack antennae; usually have four pairs of legs; cephalothorax.