Members of the phylum Arthropoda are distinguished by having segmented bodies and segmented appendages. Metamerism, being segmented, is evident in embryos and in many adult features. In most cases it disappears through loss or fusion. The bodies of most arthropods are made of two or three distinct parts—a cephalum (head), a thorax (chest), and an abdomen. In origin, each orthropodan metamere bears a pair of appendages, but most may be lost. They have a hardened external skeleton, often called the exoskeleton, made of the nitrogen-rich polysaccharide chitin. As the animal outgrows the exoskeleton, it is shed and a new one is regenerated. The molting is controlled by hormones. The nervous system is made up by the dorso-anterior situated brain, followed by a ventral nerve cord with ganglia in each segment. The heart, a dorsal vessel, circulates an open system and has primitively one pair of ostia in each segment.
Many arthropods metamorphose: the egg develops into a larval form that differs considerably from the sexually mature adult. Although arthropods range in size from 100 µm to 60 cm long, most adults are about 1 mm long.
In number of species, the phylum Arthropoda is by far the largest in the animal kingdom. Nearly half a million species of insects alone have been described; some zoologists feel that if the tropical groups were better known there might be as many as 10 million living species of insects.
Arthropods are of great economic importance. Most fruit trees and many crop vegetables rely on insects for pollination. Insects and other arthropods are crucial predators of plant pests and other noxious species. Many cause diseases of plants by transmitting pathogenic fungi and bacteria. Others transmit human pathogens, such as those that cause trypanosomiasis and malaria. Arthropods are crucial sources of nutrient for many other animals—and indeed, for some carnivorous plants. Seafood that is not mollusk or fish is generally arthropod.
There are three great subphyla of living arthropods: Crustacea, Uniramia, and Chelicerata. Traditionally the living arthopods have been split into two subphyla; the Mandibulata and the Chelicerata. The Chelicerata lacking antennae and being named from the fact that the first posteral appendages, the feeding appendages, are called chelicerae. Belonging to this group are the scorpions, spiders, mites, horseshoe crabs and ticks. The Mandibulata consist of all arthropods with antennae and derive their name since the first posteral appendages are mandibles. This assemblage includes insects, crabs, millipeds, shrimps and centipedes.
Today most taxonomists agree that the Mandibulata is an artificial assemblage of unrelated groups. It is more likely that the origin of arthropod evolution consists of four lines rather that the above mentioned two. The general idea of this classification is the result of the work of S. M. Manton (1964), leading to the following four groups: The extinct Trilobitomorpha, Crustacea, Uniramia, and Chelicerata, see arthropoda.
In the Chelicerata, the first two body regions are combined into a single cephalothorax; other arthropods have three distinct body parts. The Crustacea includes the aquatic gill-breathing crustaceans, whose major members are the water fleas, fairy shrimp and tadpole shrimp (Branchiopoda), ostracods (Ostracoda), copepods (Copepoda), and the barnacles (Cirripedia). All of these classes have a bivalved carapace, called a straca. Crustaceans, belonging to class Malacostraca, are primarily marine organisms, although a few species live in fresh water or on land. Amphipods and isopods lack a carapace and have flattened bodies. Most are marine (for example, beach fleas), although the common pill bug or sow bug is an isopod that lives on land, in moist soil litter under fallen logs and stones. In the order Decapoda, the entire cephalothorax is covered by the carapace. Lobster, crayfish, crabs, and shrimp are all decapods.
There are five classes in the subphylum Uniramia: the Diplopoda (millipedes), the Chilopoda (centipedes), the Pauropoda (centipedelike animals having branched antennae and nine or ten pairs of legs), the Symphyla (also similar to centipedes but having from 10 to 12 pairs of legs), and the Insecta, by far the largest class of the Uniramia.
Insects have three pairs of legs, three body sections, generally one or two pairs of wings, and one pair of antennae. There are some 25 orders of insects, and more than 600 families. Suffice it here to mention some major orders: Isoptera (termites), Ephemeroptera (mayflies), Odonata (dragonflies), Orthoptera (grasshoppers, roaches, and crickets), Hemiptera (true bugs), Homoptera (cicadas and aphids), Lepidoptera (butterflies and moths), Diptera (flies), Siphonaptera (fleas), Hymenoptera (ants, wasps, bees, and chalcids), and Coleoptera (beetles), which contains more than 290,000 described species. The Hymenoptera is the largest order if taken into account the undescribed species known upto now.
Most members of the subphylum Chelicerata have six pairs of appendages, of which the first two differ from each other. The first pair, called chelicerae, are grasping and jawlike; the second pair are usually feelerlike or clawlike; the third and other more posterior pairs are usually leglike. Unlike other arthropods, chelicerates lack sensory antennae. They include the classes Pycnogonida (sea spiders), Merostomata (horseshoe crabs), and Arachnida (scorpions, daddy-long-legs or harvestmen, spiders, and the mites and ticks of the order Acarina). Nearly all the arachnids have four pairs of segmented legs. Most are carnivorous; many prey on insects. They play an important role in the balance of nature.
Apparently, the insects arose in the late Paleozoic Era—by 200 million years ago, modern cockroaches lived and were fossilized. The phylum Arthropoda itself appeared far earlier; indeed, some fossils of simple joint-footed animals have been seen in Ediacaran (upper Proterozoic) rocks.
Some contend that arthropods are polyphyletic, based, for example, on their rich variety of life cycles.