Arthropod
From Encyclopædia
{ahr'-throh-pahd} An arthropod is any member of the phylum Arthropoda, which includes the CENTIPEDES,
crustaceanS, INSECTS, MILLIPEDES, MITES, SCORPIONS, SPIDERS, and other related forms. Arthropods constitute about 75 percent of known animal species. More than 930,000 have been described, and the total has been estimated at more than 6,000,000. Arthropods are characterized by a resistant integument, or cuticle, forming an EXOSKELETON, a segmented body, and jointed appendages; the name Arthropoda means "jointed legs." Other characteristics include bilateral symmetry,
growth by MOLTING, the presence of specialized cuticular sense
organs, blood-filled spaces in the body (the hemocoel), and a greatly reduced body cavity (coelom). Respiratory tracheae, or book lung, occur in most terrestrial forms. Eyes are simple or compound; both types may occur in the same animal.Millions of years before animal life succeeded in colonizing the land, the primeval oceans were teeming with arthropods. These had probably evolved from segmented, wormlike forms similar to the
polychaete ANNELIDS of today, but fossil
evidence is lacking. It is now generally accepted, however, that the phylum is polyphyletic; that is, some of its distinctive features--an exoskeleton, tracheal respiration, Malpighian excretory tubes, compound eyes, and others--must have originated from at least three separate ancestral lines.Paths of EvolutionFrom the wormlike ancestor, one evolutionary path led to the extinct
trilobiteS and the chelicerates.
trilobites were marine organisms that were particularly numerous in the Cambrian Period, 570 million to 500 million years ago. They had flattened bodies molded longitudinally into three lobes whose segments bore paired limbs with fringed hairs. The chelicerates include ARACHNIDA--false scorpions, harvestmen, mites, scorpions, solifugids (see
SUN SPIDER), and spiders--as well as KING
crabS (Xiphosura),
sea spiderS (Pycnogonida), and the EURYPTERIDA--gigantic, scorpionlike marine animals, now extinct. The body of a chelicerate is divided into two regions, or tagmata: an anterior prosoma, or cephalothorax, and a posterior abdomen, or opisthosoma. It has no distinct head, and the prosoma is without ANTENNAE. It bears pincerlike chelicerae (jaws) and sometimes pedipalps. These may take the form of claws in scorpions and false scorpions; tactile and feeding
organs in solifugids, harvestmen, and mites; or reproductive
organs in male spiders.The second evolutionary path led to the crustacea--
barnacleS,
brine shrimpS,
crabS,
copepodS, LOBSTERS,
ostracodS, SHRIMPS,
water fleaS, and their allies. With the exception of wood lice, slaters, and land
crabs,
crustaceans are mostly aquatic, respiring by means of GILLS. They are equipped with two pairs of antennae; the eyes, when present, may be stalked or unstalked, and the limbs are often specialized. Many
crustaceans have adopted a parasitic mode of life and show appropriate adaptations of their
anatomy and life history.The last assemblage of arthropods has evolved through animals like
Peripatus, which retains many wormlike characteristics, to millipedes, symphylans, centipedes, and SPRINGTAILS and other insects. These groups possess one pair of antennae and simple and compound eyes; like
crustaceans, they feed with the aid of mandibles. A fundamental difference, however, is that the crustacea use the basal part, or gnathobase, of a modified limb for masticating their food; whereas in the groups under discussion an entire limb has become a jaw, and its tip is used for biting. The
crustaceans also resemble the arachnids in that the excretory coxal glands are divided from coelomoducts, whereas the Malpighian tubules of insects and their allies are outgrowths from the alimentary canal.Body SystemsMany small and primitive arthropods, such as those that enjoy a hidden or cryptozoic m?de of life, respire through the body wall. The sites of gaseous exchange in
crustaceans are varied. Many arachnids make use of book lung. These are derived directly from the gill books of their marine ancestors and contain numerous leaflike respiratory filaments. Tracheae and tracheoles, tiny breathing tubes that
lead directly to the tissues, are the routes by which
oxygen enters the body of most terrestrial arthropods. These tubes are present in their most primitive condition in the
Peripatus and are the principal respiratory
organs of insects, myriapods, and some arachnids. Studies of the evolution of arthropods indicate that the tracheal system must have evolved a
number of separate times.The modes of nutrition of arthropods are even more diverse than are their
respiratory systems. Some are vegetarians, some are carnivores, and many of the
crustaceans are filter feeders; others feed on fluids obtained from the sap of
plants or the blood of other animals. In these, different parts of the alimentary canal may be modified as sucking
pumps--the foregut in arachnids, the pharynx in insects.The basic pattern of the
nervous system is as follows. From an anterior dorsal brain a pair of nerve cords passes ventrally, one on each side of the gut; lying in the hemocoel close to the ventral surface, each cord continues posteriorly to the extremity of the body. Paired ganglia in successive segments
fuse together to produce more complex nerve masses from which segmental nerves
lead off to the limbs and other appendages. Apart from the eyes, the sense
organs of arthropods occur mainly in the form of hairs, the function of which may be tactile, auditory, chemoreceptive, or proprioceptive (responding to internal stimuli).
color change, retinal pigment migration,
growth, molting, METAMORPHOSIS, and
sexual development are controlled by HORMONES.In arthropods whose limbs are moved by their own muscles, the coelom is greatly reduced, and the functional body cavity is a blood-filled space known as the hemocoel. At the time of molting, arthropods swallow air or water. The
pressure induced is distributed by the blood so that when the new cuticle hardens, the entire body will have expanded.Sex and Life CycleWith few exceptions, both sexes occur separately in arthropods. The reproductive
organs are directly continuous with their ducts, the latter being connected to ectodermal invaginations that
lead to the exterior. Sex ducts open near the rear of the body in
Peripatus, centipedes, and insects. Among
crustaceans, they nearly always open at the hind end of the thorax; in millipedes, pauropods, and symphylans their opening is not far behind the head. In arachnids, they open near the middle of the body. The abdomen of the
sea spiders is so reduced that it contains little more than the terminal part of the alimentary canal, and the gonads are displaced into the proximal segments of the legs.Spermatozoa are usually transferred to the female arthropod in sealed packets known as spermatophores. In this way the sperms are not diluted by the surrounding medium in the
case of aquatic forms, nor do they suffer from desiccation on land. Transfer of spermatozoa is often accompanied by ritual courtship and mating ceremonies. In some groups there has been a specialization toward indirect sperm transfer, resulting in dissociation of the male and female. In some springtails, for instance, the male deposits spermatophores at random, and these are afterward picked up casually by passing females.Most arthropods lay eggs in masses, but in some species the eggs hatch inside the body of the mother and the young are born alive. Newly hatched arthropods are often very different from the adults in appearance and habits. In this way competition is avoided, and alternative food sources can be exploited. Many
crustaceans have larval development
stages that are planktonic; these play an important role in dispersal, especially in the
case of parasitic species. Among endopterygote insects, the larvae are entirely different from the adults and pass t?rough a pupal, or CHRYSALIS,
stage, during which metamorphosis occurs. In the
case of exopterygote insects, on the other
hand, the nymphal
stages, although wingless, are essentially similar to the adults, as they are in most arachnids.Limits to SizeChief among the characteristics to which the arthropods owe their success is the presence of a rigid and almost impervious exoskeleton; this assists in the osmotic regulation of aquatic forms and enables many terrestrial species to become almost independent of water.Animals encased in armored skeletons can grow only by molting, or ecdysis. The need for this also limits absolute size, because in larger animals, a considerable increase in weight results in a relatively small gain in linear dimensions. Molting is a hazardous process at all times. If everything does not go exactly right, the animal may die, for it is extremely vulnerable to enemies until its new cuticle has hardened. Thus the benefits conferred by an exoskeleton are increasingly offset by its disadvantages as the size of the animal increases.Among arthropods, only aquatic forms whose bodies are supported by
seawater are able to attain any great size or weight.
crustacean integuments are heavy because they are strongly calcified, and weight is advantageous to bottom-dwelling aquatic animals. The cuticles of terrestrial arthropods, such as centipedes, insects, and arachnids, are strengthened by sclerotization, a chemical process analogous to the tanning of leather. In this way they become tough and rigid but at the same time remain
light in weight.
