Arthropoda

Arthropoda

The phylum Arthropoda is the largest and most varied in the animal kingdom. It includes well over one million described species. This represents approximately three-quarters of all known biological organisms, living or extinct. Countless arthropods remain undescribed (not yet named and studied), and the actual number of living species could be as high as ten million or more. Some of the more well-known arthropods include insects, crustaceans, and spiders, as well as the fossil trilobites . Arthropods are found in virtually every known marine (ocean-based), freshwater, and terrestrial (land-based) ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences.

Characteristics of Arthropods

Despite the remarkable variety of arthropod species, all share aspects of a single basic body plan. All arthropods possess a stiff exoskeleton (external skeleton) composed primarily of chitin . In some species, lipids, proteins, and calcium carbonate may also contribute to the exoskeleton. The external skeleton offers organisms protection as well as support for the body. Its walls provide anchors for the attachment of muscles. The exoskeleton is incapable of growth, and is molted (shed) repeatedly during the growth of the animal. This process is called ecdysis. Molting allows for rapid growth until the newly secreted exoskeleton hardens.

Arthropod bodies are divided into segments. However, a number of segments are sometimes fused to form integrated body parts known as tagmata. This process of fusion is called tagmosis. The head, thorax, and abdomen are examples of tagmata. Arthropods also have appendages with joints (the word "arthropod" means "jointed feet"). In early, primitive anthropods, each body segment was associated with a single pair of appendages (attachments). However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. Arthropod appendages may be either biramous (branched) or uniramous (unbranched).

Some arthropods have highly developed sense organs. Most species have paired compound eyes , and many also have a number of simpler eyes called ocelli. Arthropods have an open circulatory system (without blood vessels) that consists of a tube that is the heart and an open hemocoel , the coelom of the animal, in which blood pools. Arthropods also have a complete gut with two openings, the mouth and the anus.

Gas exchange in the phylum occurs in various ways. Some species have gills, while others employ tracheae, or book lungs. The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues. Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in the oral cavity. A double nerve cord extends backwards along the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells. In most arthropod species, the sexes are separate. Fertilization usually occurs internally, and most species are egg laying. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form.

Major Groups of Arthropods

Arthropods are divided into four subphyla. These are the Chelicerata, the Crustacea, the Uniramia, and the Trilobita. The last consists exclusively of extinct forms.

Subphylum Chelicerata.

The chelicerates include the horseshoe crabs , scorpions, spiders, ticks, mites, sea spiders, and other related species. They are characterized by the presence of two tagmata (fused segments), a cephalothorax (fused head and thorax), and an abdomen. They possess six pairs of unbranched appendages. These include a pair of chelicerae , a pair of pedipals, and four pairs of legs.

The class Arachnida includes scorpions, spiders, ticks, and mites. There are over 100,000 described species in this class. The majority are land-based and most are found in fairly warm, dry habitats. Like other chelicerates, arachnids have six pairs of appendages. The first pair, the chelicerae, is typically adapted for killing and consuming prey. The second pair, pedipals, have a sensory function, and may include both receptors sensitive to touch and receptors sensitive to chemical changes. The final four pairs of appendages are walking legs. Arachnids have fairly simple eyes that register only changes in light levels. Of the arachnids, spiders (which make up the Order Araneae) are the most diverse. All spiders are able to spin webs using modified appendages called spinnerets. These are located in the rear abdomen. Webs are used for a variety of purposes in different species. In many, they are used to catch prey and to build nests. Spiderwebs can even be used for movement, as in those species that create parachutes to catch the air, enabling them to descend safely. Many spiders have toxic poisons to immobilize prey or to use in self-defense; perhaps the most famous of these is the black widow. Spiders prey primarily on insects, and are often ecologically important for this reason. Scorpions (order Scorpiones) are arachnids characterized by a pair of claws and a long, jointed tail with a poisonous sting at the end. Ticks and mites (order Acari) are ectoparasites. They embed themselves in the skin of vertebrate animals and feed on blood. Certain tick species carry diseases such as Lyme disease and Rocky Mountain spotted fever.

The class Merostomata includes the horseshoe crabs. Horseshoe crabs are an extremely ancient marine lineage. Only five species have survived to the present. They are characterized by a long appendage called a telson that projects from the rear end of the body, which is used in flipping the animal over when it is lying on its carapace. They use book gills to breathe and generally feed on small invertebrates.

The class Pycnogonida consists of the sea spiders. There are 2,000 described species, all of which are marine. Most species are fairly small. Like spiders, they have small bodies with long legs. They use an extensible proboscis to suck nutrients from the bodies of soft invertebrates.

Subphylum Crustacea.

The subphylum Crustacea includes lobsters, crabs, shrimp, barnacles, and other related organisms. There are approximately 40,000 described species. The majority are marine, but there are freshwater and land-based representatives as well. Unlike other arthropods, the crustacean exoskeleton often includes calcium carbonate, which offers added rigidity. Crustaceans generally have three tagmata: a head, a thorax, and an abdomen. There are two pairs of antennae, complicated mouthparts consisting of two pairs of maxillae (upper jaws) and one pair of mandibles (lower jaws) used in food processing, and a series of branched appendages. These appendages are associated with the thorax. Some function as walking legs while others may be specialized for capturing prey. The abdomen is sometimes equipped with swimmerets (small swimming legs that are also used for other purposes, including as copulatory organs in males and for egg carrying in females) and a tail that is composed of modified appendages in addition to a telson. Some crustacean species have well-developed sensory systems, including highly sensitive compound eyes on stalks, ears, chemoreceptors for taste and/or smell, telson and hairs or bristles that function as touch receptors. Crustaceans have a wide variety of ways to capture food. Some are filter feeders , while others are scavengers or predators. In most species, the sexes are separate. Some species pass through what is called a nauplius larval stage prior to metamorphosing into adults, while others have direct development and bypass the larval stage. Crustaceans use gills to inhale and exhale air.

The class Branchiopoda include the brine shrimp, water fleas, and other related groups. Species in this class are generally small and tend to live in freshwater habitats or in salty lakes. Most species have a large number of segments with minimal fusing of segments, or tagmiosis. The majority are filter feeders.

The class Maxillopoda includes the barnacles and related groups. Maxillopods have a head, thorax, and abdomen along with a telson projecting from the back end of their bodies. Most species are small and feed using their maxillae. Barnacles, however, are sessile (immobile) filter feeders. They are often seen in large numbers, anchored to structures such as ship bottoms or piers.

The class Malacostraca has over 20,000 species and is the largest group within the Crustacea. Most species are marine, but others are freshwater or terrestrial. The largest order, Decapoda, includes shrimp, crabs, crayfish, and lobsters. Other well-known malacostracans include krill as well as a terrestrial group, the sowbugs. The malacostracans exhibit a variety of feeding strategies. The more primitive species tend to be filter feeders. Others are scavengers. Crabs and lobsters are active predators. They have a pair of chelipeds, also known as claws or pincers, which are used to capture and carry prey. Pincers have evolved to serve other functions as well, however, and in various species are used for digging, defense from predators, or in courtship rituals. Some malacostracan species are parasites. Many malacostracans, including many of the larval forms, are critical components of oceanic plankton, a critical component of oceanic food webs.

Subphylum Uniramia.

Uniramia is the largest subphylum within the arthropods. It includes the centipedes, the millipedes, and the insects, as well as a few smaller related groups. The name Uniramia comes from the unbranched appendages that characterize members of the group. Species generally have two or three tagmata. There are one pair of antennae and two pairs of maxillae. Respiration occurs via tracheae. Uniramians generally have separate sexes.

The class Chilopoda includes the centipedes, a diverse group of over 5,000 species. These terrestrial organisms are characterized by a very large number of segments, often well over 100. The largest centipedes reach lengths of up to 25 centimeters (10 inches). Each centipede body segment, aside from a few at the head and tail of the organism, is associated with a single pair of legs. All centipedes are carnivorous, and the appendages that are frontmost have been modified to form large poisonous fangs that are used to immobilize prey. Centipedes feed primarily on earthworms and insects. Species of centipedes are generally egg laying, and in some, the female remains to guard the eggs. Development is direct—there is no larval stage. In some species, juveniles hatch with the same number of segments as an adult, while in others, individuals add segments with each molt.

The class Diplopoda consists of the millipedes, a group that includes over 8,000 described species. Like centipedes, millipedes have a large number of segments. However, they differ from centipedes in that each segment has two pairs of legs rather than just one. Millipedes do not have fangs, and in fact, most species are either herbivorous or scavengers. Many millipedes do, however, exude (ooze) poisonous or noxious substances as a defense against potential predators. Millipedes are often found in decaying organic matter or in moist soils. They are effective burrowers. Like some species of centipedes, they lay eggs in nests that are attended by the female. Millipedes add body segments as they grow and molt.

The class Insecta is the largest class in the animal kingdom. There are nearly one million described species, and no doubt countless others that have yet to be named. Insects are found in a wide variety of terrestrial and freshwater habitats, and there are even a few marine forms.

Insects have three tagmata, or fused segments: a head, a thorax, and an abdomen. They have a pair of antennae; a series of complex, highly variable mouthparts, which vary greatly from species to species; and three pairs of legs. Both the antennae and mouthparts are evolved from modified appendages (walking legs, most likely). Most insect species also have two pairs of wings, although these are absent in a few very primitive species and have been reduced in others, becoming nonfunctional or adapted for a different purpose. Insect legs and wings are associated with the thorax, not the abdomen, which does not usually carry appendages except for appendages that are evolved into reproductive organs. A theory of the origin of insect flight maintains that wings evolved from external gills that were present in certain primitive groups. Aside from their breathing function, these gills served as flaps that assisted insects in leaping and jumping, and were advantageous because they made escape from predators more likely. Gradual increases in wing size allowed for gliding movement, and ultimately for flapping flight.

Insects have highly elaborated sense organs. For example, they may possess a pair of compound eyes as well as several cranial ocelli, or simple eyes. The compound eye is made up of hundreds of individual facets, or parts. Each facet points in a different direction. An individual facet provides information regarding the color and intensity of light but does not provide a complete image. Together, however, the numerous facets create a combined, mosaic image of the world. Compound eyes are particularly effective for seeing nearby objects; distance vision is not as good. The greatest advantage of compound eyes is that they are able to register changes in the visual field much more quickly than eyes with lenses. This is particularly important for detecting motion, as well as for the rapid maneuvering required during flight. Many insects also have well-developed ears. Some species also have an extraordinary ability to detect chemicals. This is especially true in species that use chemical signals called pheromones for detection of a sexual partner. The pheromones are emitted by receptive females and picked up by males, which use them to locate potential mates.

Insects breathe through the tracheal system, described earlier. Because of limits on the spread of gas in the trachea, insects are restricted to a comparatively small size. The excretory system of insects consists of structures known as Malpighian tubules. The sexes are separate in insects, and fertilization occurs internally in most species.

The variety in patterns of insect development is exceptionally high. Most insects pass through several stages before reaching the final adult form. Insects may be described as either hemimetabolous or holometabolous. In hemimetabolous forms, the hatched young resemble adults reasonably closely, although they may be sexually immature and may lack wings. In holometabolous insects, on the other hand, there is a distinct larval stage that is dramatically different from the adult stage in almost all ways: morphology (form and structure), diet, and habitat. In holometabolous insects, there are usually several different larval stages separated by molts. After a period in which the larva grows, it then enters a sessile pupal phase during which a dramatic metamorphosis occurs, and the insect emerges from the pupa with its adult form.

Certain insect groups are highly social. Termites and many species of Hymenoptera (ants, wasps, and bees) are eusocial , meaning that their colonies include a caste (a segment of the population) that reproduces as well as a large number of individuals that do not. The evolution of nonreproductive species seems to pose a problem because it appears to defy natural selection, which emphasizes the production of offspring. However, direct reproduction is not the only way for an individual to pass on its genes. For example, because an individual's siblings share some of its genes, contribution to the production of a large number of siblings will also result in an individual's genes being represented in the population. This is what occurs in the eusocial insects. In addition, unusual behaviors in termites (repeated cycles of inbreeding) and unusual genetic systems in hymenopterans (haplodiploidy, in which males of the species are haploid while females are diploid) increase the proportion of genes shared by siblings.

Insects play many vital roles in maintaining ecological systems. Many insects act as pollinators to higher plants. Others are important in decomposition. Many species are agricultural pests or parasites, and have a dramatic impact on humans. The fruit fly Drosophila melanogaster is one of the most well-studied biological organisms and serves as a model species for studies of genetics , development, and evolution.

Some well-known insect groups include the Thysanura (silverfish), Ephemeroptera (mayflies), Odonata (dragonflies), Orthoptera (grasshoppers, crickets, katydids), Blattaria (cockroaches), Isoptera (termites), Heteroptera (true bugs), Homoptera (cicadas and aphids), Coleoptera (beetles), Siphonaptera (fleas), Diptera (flies), Lepidoptera (butterflies and moths), and Hymenoptera (ants, bees, and wasps).

Subphylum Trilobita.

The subphylum Trilobita includes only extinct species found in fossil form. The trilobites were a primitive group of marine species that was particularly abundant during the Cambrian (570 million years ago) and Ordovician (505 million years ago) periods. The group became extinct at the end of the Permian (286 million years ago). Trilobites had flattened, oval-shaped bodies. Most were a few inches long, although one species is known to have attained a length of 0.6 meters (2 feet).

see also Phylogenetic Relationships of Major Groups.

Jennifer Yeh

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