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The crustaceans are characterized by two pairs of antennae and two pairs of modified appendages (maxillae) used for food handling. There are over 40,000 species of crustaceans, including lobsters, shrimps, crayfish, crabs, copepods, barnacles, and a large number of minute planktonic forms. Crustaceans are the only arthropods that are mainly aquatic, and most of them are marine. Some have spread to humid areas near water. They use gills for respiration. The thoracic region typically bears walking legs (pereiopods), also used for capturing prey. The abdominal region often is equipped with swimmerets (pleopods) and a tail fan made up of a pair of appendages (uropods) and the telson. Their excretory organs are modified nephridia, as a rule producing a dilute urine that contains a great deal of ammonia.
Crustaceans are herbivores, carnivores, or scavengers and are often vital elements of the food chain. Some, such as lobsters, shrimp, and crayfish, are important economically as edible shellfish. Barnacles are notorious as fouling organisms of ship bottoms and harbor installations. Some crustaceans, such as the copepods known as sea lice, are significant parasites of other aquatic organisms. As a rule they pass through a complex set of molts during development, involving a series of larval stages. The characteristic larva is called a nauplius, with three pairs of appendages. More appendages are added as the organism passes through its developmental molts. The cuticle of crustaceans, unlike that of other arthropods, contains calcium deposits. The most familiar classes are the Branchiopoda, which includes the orders Notostraca (tadpole shrimps) and Diplostraca (clam shrimps and water fleas); the Malacostraca, which includes the orders Stomatopoda (mantis shrimps), Mysida (opossum shrimps), Isopoda (isopods), Amphipoda (amphipods), and Decapoda (crayfish, lobsters, shrimps, and crabs); and the Maxillopoda, which includes the order Copepoda (copepods) and the infraclass Cirripedia (barnacles).
Class Insecta is the largest of the arthropod classes, containing hundreds of thousands of species. Except for a few primitive or highly modified forms, insects are characterized by having one or two pairs of wings attached to the thorax. The head region bears a pair of antennae, a pair of mandibles, and two pairs of modified maxillae forming the mouthparts. The abdomen is well set off from the thorax and has no appendages except reduced ones that are modified as reproductive organs. The typical insect head bears compound eyes and one or more simple eyes and is covered by a continuous exoskeletal armor. The thorax is made up of three segments, each bearing a pair of legs. The last two segments usually bear a pair of wings. Insects are predominantly terrestrial and have tracheae for gas exchange. Insects are also characterized by unique excretory organs, known as Malpighian tubules, which are useful in conserving water.
Members of the class are extremely varied. They have adapted to many different kinds of feeding and play a variety of important roles in their ecological communities. Mouthparts may be adapted to chewing either plant or animal food, for sucking plant sap or blood, or for lapping or swabbing moisture such as fruit juices or animal body fluids. Some burrow and feed in soil or plant tissue, some are runners or jumpers that feed at or near the ground level, and others feed on the wing.
Most primitive insects are wingless and have a relatively weak exoskeleton. These are forced to seek humid, protected habitats. Juveniles of primitive insects closely resemble the parents and undergo little change other than growth after hatching. This is called ametaboly. Many of the winged insects undergo paurometabolous development, hatching as nymphs that resemble the parent in many ways but that have small buds instead of wings. With each molt these juveniles change somewhat, and the wings increase in size as the young gradually assume the form of the adult. Some insects have adapted to an aquatic life to a certain extent, and in their juvenile stages they are found in ponds and streams. Some of these are hemimetabolous; the juveniles are naiads, i.e., they resemble the nymphs of paurometabolous insects, but their wings do not grow during the juvenile molts, even though other body changes occur. Instead, the last molt before the adult stage is reached involves full development of the wings, after which the insect takes up a terrestrial existence. The least primitive of the insects are termed holometabolous. In holometaboly, the eggs hatch to release the usually wormlike larvae, which are often equipped with false legs in the abdominal region to aid in locomotion. Wing buds are entirely lacking. Although the larvae grow at each molt, they do not begin to resemble the adult until later. During the larval stage the young insect enters into a quiescent pupal stage. At the end of this stage a major metamorphosis occurs, and the insect emerges with all the adult organs.
Insects often cause great losses in agriculture, attack stored products, parasitize humans and domesticated animals and plants, and serve as important carriers of disease organisms. They are also beneficial, producing honey and silk and pollinating the flowers of the majority of flowering plants.
See H. B. Boudreaux, Arthropod Phylogeny (1979, repr. 1987); G. Eisenbeis and W. Wichard, Atlas on the Biology of Soil Arthropods (1987); J. L. Thompson-Cloudsley, Evolution and Adaptation of Terrestrial Arthropods (1988).
A phylum that includes the well-known insects, spiders, ticks, and crustaceans, as well as many smaller groups, some of which are known only as fossils. Arthropodous animals make up about 75% of all animals that have been described. The estimated number of known species exceeds 780,000. Of this number the class Insecta alone contains about 700,000 described species. Arthropods vary in size from the microscopic mites to the giant decapod crustaceans, such as the Japanese crab with an appendage span of 5 ft (1.5 m) or more.
The adult arthropod typically has a body composed of a series of ringlike segments, muscularly movable on each other. The integument is sclerotized by the formation of hardening substances in the cuticle, and the segmental limbs are many-jointed. These characteristics, taken together, distinguish the arthropods from all other animals. Young stages may be quite different from the adults, and some parasitic species differ very radically from their relatives.
Arthropod evolution is no longer the clear-cut subdivision of a single phylum, Arthropoda, into three structurally divergent subphyla. Advances in functional morphology, comparative embryology, spermatology, serology, and paleontology have brought an array of new hypotheses about relationships of arthropodous animals. At the center of debate is the question of monophyly versus polyphyly: Did all arthropodous animals evolve from a common ancestor or did several distinct lineages evolve along similar pathways? Two opposing classification schemes are presented; numerous variations on these schemes can be found in the literature. The first pair of classifications is as follows:
- Phylum Uniramia
- Subphylum: Onychophora
- Hexapoda (Insecta)
- Phylum Trilobita (Trilobitomorpha)
- Phylum Crustacea
- Phylum Chelicerata
- Phylum Arthropoda
- Subphylum Arachnata
- Superclass: Trilobita
- Subphylum Mandibulata
- Superclass: Crustacea
- Phylum Onychophora
Alternatively, a slightly different and expanded pair of classifications is as follows:
- Phylum Uniramia
- Subphylum Onychophora
- Subphylum Myriapoda
- Class: Chilopoda
- Superclass: Arthropleurida
- Subphylum Hexapoda
- Class: Protura
- Pterygota (Insecta)
- Phylum Crustacea
- Class: Cephalocarida
- Phylum Cheliceriformes
- Subphylum Pycnogonida
- Subphylum Chelicerata
- Class: Merostomata
- Phylum Trilobitomorpha
- Class: Trilobitoidea
- Phylum Onychophora
- Phylum Arthropoda
- Subphylum Cheliceromorpha
- Infraphylum: Pycnogonida
- Superclass: Xiphosurida
- Class: Eurypterida
- Subphylum Ganthomorpha
- Infraphylym: Trilobitomorpha
- Class: Trilobita
- Infraphylum: Mandibulata
- Class: Cheloniellida
Body segmentation, or metamerism, is the most fundamental character of the arthropods, but it is shared by the annelid worms, so there can be little doubt that these two groups of animals are related. The limbs of all modern arthropods develop in the embryo from small lateroventral outgrowths of the body segments that lengthen and become jointed. Hence it may be inferred that the arthropods originated from some segmented worm that acquired similar lobelike limb rudiments and thus, as a crawling or walking animal, became distinguished from its swimming relatives. Then, with sclerotization of the integument, the limbs could lengthen and finally become jointed, providing greater locomotor efficiency. In their later evolution, some of these limbs became modified for many other purposes, such as feeding, grasping, swimming, respiration, silk spinning, egg laying, and sperm transfer. The body segments, corresponding to specialized sets of appendages, tend to become consolidated or united in groups, or tagmata, forming differentiated body regions, such as head, thorax, and abdomen. Annelida; Metameres.
Sclerotization of the cuticle may be continuous around the segments. More usually, it forms discrete segmental plates, or sclerites. A back plate of a segment is a tergum, or notum; a ventral plate is a sternum; and lateral plates are pleura. The consecutive tergal and sternal plates, unless secondarily united, are connected by infolded membranes, and are thus movable on each other by longitudinal muscles attached on anterior marginal ridges of the plates. Since nearly all the body and limb muscles are attached on integumental sclerites, there is little limit to the development of skeletomuscular mechanisms.
All arthropods have all the internal organs essential to any complex animal. An alimentary canal extends either straight or coiled from the subapical ventral mouth to the terminal anus. Its primary part is the endodermal stomach, or mesenteron, but there are added ectodermal ingrowths that form a stomodeum anteriorly and a proctodeum posteriorly. The nervous system includes a brain and a subesophageal ganglion in the head, united by connectives around the stomodeum, and a ventral nerve cord of interconnected ganglia. Some of the successive ganglia, however, may be condensed into composite ganglionic masses. Nerves proceed from the ganglia. Internal proprioceptors and surface sense organs of numerous kinds are present, chiefly tactile, olfactory, and optic. A usually tubular pulsatory heart lies along the dorsal side of the body and keeps the blood in circulation. In some arthropods arteries distribute the blood from the heart; in others it is discharged from the anterior end of the tube directly into the body cavity. The blood reenters the heart through openings along its sides.
Aquatic arthropods breathe by means of gills. Most terrestrial species have either flat air pouches or tubular tracheae opening from the outside surface; some have both. A few small, soft-bodied forms respire through the skin. Excretory organs open either at the bases of some of the appendages or into the alimentary canal. Most arthropods have separate sexes, but some are hermaphroditic, and parthenogenesis is of common occurrence. The genital openings differ in position in different groups and are not always on the same body segment in the two sexes. See Chelicerata, Crustacea, Insecta, Onychophora
the highest and largest phylum of invertebrates, including approximately 1.5 million terrestrial, aquatic, and parasitic species. Arthropods evolved from forms with a single metamerism but eventually acquired heteronomous segmentation manifested both as specialization of individual body segments and their appendages and as the formation of body regions consisting of relatively similar segments. The primitive marine annelid worms are believed to be the ancestors of arthropods, but whether arthropods are monophyletic, that is, derived from a single group of ancestors, remains a contentious issue.
Arthropods have a bilaterally symmetrical body, usually consisting of a head, thorax, and abdomen. Thoracic segments are often attached to the head, thus forming a cephalothorax. The limbs are articulated, in the form of multijointed levers; they were probably biramous initially. (The ancient trait of biramosity is, for example, characteristic of Paleozoic trilobites and is preserved to this day in many crustaceans.) The body is covered with a chitinous cuticle, forming a protective skeletal shell to which the internal muscles are attached. Growth is intermittent in arthropods, occurring after the old cuticle is shed and before the new cuticle has hardened.
The nervous system consists of three pairs of united supra-esophageal ganglia (brain) and an abdominal nerve net in which the segmental ganglia often come together and fuse. The advanced differentiation of the brain is related to the complexity of movement and behavior and to the high level of development of the sense organs, among which compound eyes are especially typical. The alimentary canal consists of an ectodermal pharynx and hind-gut that are lined with chitin and an endodermal mid-gut into which open ducts from a digestive (hepaticopancreatic) gland, which is also referred to as the liver. The respiratory organs are gills, book lungs, or tracheae.
During the embryonic period, paired coelom sacs form. The sacs subsequently break down, and the individual cavities merge together and with the remains of the original body cavity. The result is the formation of a mixed body cavity, which contains the internal organs and is filled with hemolymph. Arthropods have an open circulatory system; only the large arteries and aorta are present. The metameric heart, which is located above the alimentary canal and which is homologous with the dorsal circulatory vessel of annelid worms, is always arterial.
The excretory organs are in the form of coelomoducts (coxal, green, and maxillary glands), or, in terrestrial forms, Malpighian tubules. The sexes are separate. Reproduction is sexual or, sometimes, parthenogenetic. Embryonic development often involves metamorphosis.
The various systems of classification of arthropods differ greatly from one another. According to the most widely accepted classification, the phylum is divided into four subphyla: (1) Trilobitomorpha (containing the extinct class Trilobita), (2) Chelicerata (containing the classes Merostomata and Arachnida), (3) Branchiata (containing the single class Crustacea), and (4) Tracheata, or Atelocerata (containing the classes Myriopoda and Inserta).
The behavior of arthropods is extremely diverse. Relatively complex instincts are characteristic of the higher arthropods. For example, many insects form associations in which there is distribution of labor among polymorphic individuals. Many arthropods are beneficial to man. Some are edible (crayfish, lobsters) or manufacture important products, for example, honey, wax, or silk (the honeybee and the silkworm). Some beneficial arthropods are raised by man. A significant number of arthropod species are crop pests (Colorado potato beetle, locusts), human and animal parasites (itch mite, fleas, lice), and disease vectors (malarial mosquito, ixodid ticks).
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A. V. IVANOV