ARTHROPOD

any member of the phylum Arthropoda, the largest in the animal kingdom, which includes such familiar forms as lobsters, crabs, spiders, insects, centipedes, and millipedes. About 84 percent of the known species of animals are members of this phylum, and they are very diverse in structure, in lifestyles, and in types of habitat. The distinguishing feature of arthropods is the presence of a skeletal covering composed of chitin (a complex sugar) bound to protein. This nonliving exoskeleton is secreted by the underlying epidermis (which corresponds to the skin of other animals). The body is usually segmented, and the segments bear paired, jointed appendages, from which the name arthropod (jointed feet) is derived. More than 879,000 arthropod species have been described, of which most are insects. This number, however, may be only a fraction of the total. Based on the number of undescribed species collected from the treetops of tropical forests, zoologists have estimated the total number of insect species alone to be as high as 10,000,000. The 30,000 described species of mites, another group of arthropods, may also represent only a fraction of the existing number. The phylum Arthropoda may be divided into four subphyla: Trilobita, Chelicerata, Crustacea, and Uniramia. The subphylum Trilobita contains only the trilobites, which were the dominant arthropods in the early Paleozoic seas (540 to 245 million years ago) but became extinct during the Permian Period (286 to 245 million years ago), at the end of the Paleozoic Era. Most members of the subphylum Chelicerata belong to the class Arachnida, containing the spiders, scorpions, ticks, and mites. They are largely terrestrial arthropods, living beneath stones and logs, in leaf mold, and in vegetation, but there are some aquatic mites that live in fresh water and in the sea. There are also many parasitic mites. Two small classes of chelicerates, the Merostomata, containing the horseshoe crabs, and the Pycnogonida, containing the sea spiders, are entirely marine. The merostomes are an ancient group and probably gave rise to the arachnids. Indeed, the earliest known fossil scorpions were aquatic. The subphylum Crustacea contains mostly marine arthropods though many of its members, such as the crayfish, have invaded fresh water, and one group, the pill bugs (sow bugs) has become terrestrial, living beneath stones and logs and in leaf mold. In the sea, large crustaceans such as crabs and shrimps are common bottom-dwelling arthropods. Many minute species of crustaceans are an important component of the zooplankton (floating or weakly swimming animals) and serve as food for other invertebrates, fishes, and even whales. Uniramia is the largest of the arthropod subphyla. It contains not only the class Insecta but also four closely related classes of long-bodied arthropods collectively known as myriapods: class Chilopoda (centipedes), class Symphyla (symphylans), class Diplopoda (millipedes), and class Pauropoda (pauropods). They are mostly terrestrial and, in contrast to the other arthropod subphyla, the uniramians are believed to have had a terrestrial origin. Centipedes, symphylans, millipedes, and pauropods live beneath stones and logs and in leaf mold; insects are found in all types of terrestrial habitats and some have invaded fresh water. The sea has remained the domain of the crustaceans, however, and only at its very edges are insects found. This article discusses the arthropods as a group. For specific information on the most significant subphyla and classes of arthropods, see the articles crustacean; arachnid; and insect; see also the article myriapod. any member of the phylum Arthropoda, the largest such division in the animal kingdom, consisting of at least 1,000,000 invertebrate species. Arthropoda is the largest animal phylum and is one of tremendous diversity. By far the largest group within the phylum comprises the insects, one class of the subphylum Uniramia. The remaining three subphyla are Chelicerata (e.g., spiders, scorpions, ticks, and mites), Crustacea (shrimp, crabs, lobsters, crayfish, sand fleas), and Trilobita, an early group of arthropods that became extinct during the Permian period (286 million to 245 million years ago). All arthropods are bilaterally symmetrical and possess a segmented body in which each segment may bear a pair of limbs. The most characteristic feature of the arthropods is a covering (exoskeleton) that, when thickened, forms an articulated armour over body and limbs and provides a surface area for muscle attachment. The head carries a dorsal brain, sense organs, and the feeding apparatus. Among the several classes a variable number of trunk segments have been incorporated into the head, the head structure alone serving to differentiate the groups. Terrestrial arthropods possess tracheal tubes or book lungs, or both, as respiratory organs; aquatic arthropods possess gills. The exoskeleton is periodically molted to allow growth. Some arthropods, especially crustaceans, are able to regenerate missing limbs. Arthropods exhibit every type of feeding mode. They are carnivores, herbivores, detritus feeders, filter feeders, and parasites. Each arthropod group has specialized the paired appendages around the mouth in accordance with the particular diet. With few exceptions, the sexes of arthropods are separate. Sperm are commonly transferred to the female within sealed packets called spermatophores. Some arthropods (especially arachnids) exhibit courtship behaviour prior to fertilization. Crustaceans usually go through a larval stage upon hatching, while most young chelicerates and insects resemble the adult. Arthropods have successfully adapted to nearly all environments, both aquatic and terrestrial. A species of jumping spider, for example, is able to live permanently at latitudes above 6,000 m (19,680 feet) on the upper slopes of Mount Everest. In contrast, a certain kind of blind crab thrives at depths of 2,500 m (8,200 feet) in the Galapagos Rift several hundred miles off the coast of Ecuador. Additional reading Introductory texts include Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987); Vicki Pearse et al., Living Invertebrates (1987); Richard S. Boardman, Alan H. Cheetham, and Albert J. Rowell (eds.), Fossil Invertebrates (1987); Alfred Kaestner, Invertebrate Zoology, vol. 2 and 3, trans. from German and adapted by Herbert W. and Lorna R. Levi (196870); Arthropoda, in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, vol. 2 (1982), pp. 71728, taxonomic classifications; and Friedrich Schaller, Soil Animals (1968; originally published in German, 1962). Advanced treatments of arthropod characteristics include Kenneth U. Clarke, The Biology of the Arthropoda (1973), emphasizing the unity of functions among arthropods; Clyde F. Herreid II and Charles R. Fourtner (eds.), Locomotion and Energetics in Arthropods (1981); Anthony C. Neville, Biology of the Arthropod Cuticle (1975), on the phylogeny, physical properties, and physiology of cuticle; and J. Bereiter-Hahn, A.G. Matoltsky, and K. Sylvia Richards (eds.), Biology of the Integument (1984), vol. 1, Invertebrates, with several chapters on the arthropod integument. Opposing viewpoints on the evolution of arthropods are presented by H. Bruce Boudreaux, Arthropod Phylogeny with Special Reference to Insects (1979), favouring a monophyletic or single-ancestor origin; and S.M. Manton, The Arthropoda: Habits, Functional Morphology, and Evolution (1977), taking a polyphyletic origin. Robert D. Barnes