Crabs, lobsters, spiders, insects, centipedes, and millipedes — these creatures all belong to the phylum Arthropoda, the most successful group of animals on Earth. Indeed, arthropods far outnumber all other types of animals combined, with an estimated 1 million species. Insects alone number more than 700,000 species, accounting for some 80 percent of all animals on Earth. Scientists predict that nearly as many await discovery. The arthropods, or "joint-footed" animals, also fill every imaginable environment, from the deepest ocean trenches to the highest mountain peaks, from the frozen wasteland of Antarctica to the broiling Mojave Desert in the western United States. Arthropods such as mites and lice flourish on the bodies of almost every kind of animal on Earth.
A Basic Body Plan
Despite their vast number and varied existence, all arthropods share certain characteristics. The most important may be their stiff covering, or exoskeleton. This development set the first arthropods apart from their soft-bodied ancestors, and may be what allowed some early arthropods to crawl out of the water without drying out in the open air or sagging under their own weight. Thanks to their exoskeletons, arthropods became the world's first land animals some 300 million years ago.
This all-important exoskeleton is secreted by the outer layer of the arthropod's three-layer skin. The outer coating is a waxy protein. The middle horny layer and an inner flexible one are made of chitin, a tough polysaccharide, or complex sugar. The union of these three layers creates a tough covering that protects the arthropod's soft body like a suit of armor. For added protection, the exoskeleton of ocean-dwelling arthropods (the crustaceans) is strengthened by the addition of the mineral calcium carbonate. The result is a rock-hard outer shell, familiar to anyone who has ever cracked open a lobster or crab leg.
The arthropod's exoskeleton conveys still another great advantage: its inner surface provides an anchoring place for muscles. This enables the complex movements that power the arthropod's many jointed appendages.
For all of its wonders, the arthropod's exoskeleton comes with one rather serious drawback: it does not stretch or expand. Arthropods must therefore periodically replace their shells as they grow or change shape. They do this through a process called molting. When it molts, the arthropod literally bursts its old shell at the seams and backs out of it. This reveals a new, better-fitting exoskeleton that must then thicken and harden. No doubt this process requires a lot of energy. In addition, molting leaves the arthropod temporarily soft and vulnerable, until its new covering hardens.
All species of arthropods also share a basic body plan. Their bodies are segmented, or divided into distinct parts. The exact number of these segments varies from species to species; the segments tend to be fused together in groups to create distinct body regions such as head, thorax, and abdomen.
Arthropods have a large number of segmented appendages, or limbs; a pair of each can arise from each body segment. Evolution has fashioned these appendages into a brilliant array of forms. In addition to walking legs, arthropod appendages include antennae, mandibles (mouthparts), claws, swimming paddles, wings, and reproductive structures.
A typical arthropod has a simple brain, an amazing set of sense organs, and a nervous system that connects the two. Colorless blood flows through the arthropod's body to nourish these as well as the other organ systems. In the simplest arthropod species, this fluid simply squishes to and fro. Larger crustaceans such as lobsters and crabs have a true heart and a few simple blood vessels.
Like all animals, arthropods need oxygen to survive. Some small arthropods simply absorb oxygen through their thin body coverings. Larger aquatic species breathe through feathery, fishlike gills. Insects and some other land arthropods breathe through a system of tiny body tubes called tracheae. Others, such as spiders and scorpions, breathe through "book lungs" — air-filled cavities that contain thin tissues resembling the pages of a book. Both tracheae and book lungs are unique to the arthropods.
Typically, arthropods have reproductive systems that are either distinctly male or female, and therefore reproduce sexually. Exceptions include the barnacles, many of which have both male and female parts in each individual.
Groups of Arthropods
The arthropods are often classified, or divided, into four distinct groups, or subphyla. They are Chelicerata, Crustacea, Uniramia, and Trilobita. These subphyla in turn are divided into several classes. The largest of these classes are the crustaceans (within Crustacea), the insects (within Uniramia), and the arachnids (within Chelicerata). Within these classes are an almost infinite number of species. The members of Trilobita, the trilobites, are all extinct.
The chelicerates include horseshoe crabs, sea spiders, and the arachnids. The most familiar arachnids are the spiders, scorpions, and ticks. All these animals share a common body plan. The head and mid-body are fused together to form the cephalothorax. Behind it is a distinct abdomen, which has a tail-like appearance in many species.
Unlike other arthropods, cheliceratids lack antennae and jaws. Instead, the first pair of appendages is a pair of pincers, or fangs, called chelicerae. Behind these are five more pairs of appendages: a set of touching organs called pedipalps and four pairs of walking legs.
The most abundant and familiar chelicerates are the arachnids, a class of 30,000 species that includes spiders, scorpions, mites, ticks, and daddy longlegs. In spiders, the sharp chelicerae connect to a pair of poison glands used to subdue prey and ward off attackers. Spiders also bear a set of special abdominal limbs that they use to spin silk.
Horseshoe crabs are unique among the chelicerates. The last surviving members of the class Merostomata, they appear much as they did 400 million years ago. The horseshoe crab's body is divided into three parts. The prosoma, or head, bears chelicerae and five pairs of walking legs. The trunk bears breathing appendages called opisthosoma. Last comes the horseshoe crab's unique spikelike tail, or telson. There are as few as five species alive today.
Sea spiders also evolved from an ancient class. Their bodies are divided into a head, a cylinder-shaped thorax with legs, and a small abdomen. The 600 species of sea spiders that still exist today are especially common in cold, polar waters, although they occur in all seas except the Caspian. Some live an astonishing 21,500 feet (6,550 meters) beneath the surface!
Crustaceans are found primarily in water. Unlike the two-part body of the chelicerate, the crustacean body is divided into three parts: head, thorax, and a tail-like abdomen. Covering the entire body is a "crusty" exoskeleton reinforced with calcium extracted from the water. The body appendages of the crustacean are jointed and branched. This can be seen best in the large claws of lobsters and crabs. When one of these appendages is damaged or lost, the crustacean has the remarkable ability to simply grow a replacement.
Instead of pincers, or chelicerae, the crustacean's mouth is equipped with three pairs of mandibles. These jawlike appendages move side to side, like a pair of tongs, rather than up and down as do human jaws. As an adult, the crustacean also sports two pairs of antennae in front of the mouth. These antennae serve as sense organs for touching, tasting, smelling, and sometimes hearing. The crustacean's eyes are compound, which means that each one contains many small lenses.
Crustaceans make up an extremely large class. With more than 25,000 species, they rival the arachnids in abundance. Aquatic forms include lobsters, crabs, and shrimp, as well as the many tiny species that make up oceanic plankton. Terrestrial crustaceans include the land crabs as well as the pill bugs, or sow bugs, found in damp basements and gardens.
The Uniramia take their name from the Latin word for "unbranched," which refers to the fact that their appendages — legs, antennae, and the like — are not branched and jointed as are those of other arthropods. This group includes five classes of animals: insects, centipedes, millipedes, symphylids, and pauropods; the latter four are collectively known as myriapods, or "many-legged" animals. All have well-formed heads and long, flexible bodies, typically with 30 or more legs. All live on land, usually in damp, dark places such as the underside of rocks and logs.
By far the single largest group of arthropods is class Insecta, with an estimated 700,000 to 1 million species. Adult insects have one pair of antennae, one set of mandibles, and two compound eyes. Their compound eyes are often so large as to cover most of the head. These enormous structures often are accompanied by a pair of simple eyes called ocelli.
The insect thorax has just three segments, each bearing a pair of legs. (Most other arthropods have four pairs.) In addition, most insects bear one or two pairs of wings — making them the only flying arthropods. The insect's abdomen is devoid of limbs, but can bear a structure such as a stinger on the tip.
Despite this common body plan, insects are found in a bewildering array of forms, from the primitive silverfish to the sophisticated bees and butterflies. Like some crustaceans, most insects undergo metamorphosis. That is, they pass through one or more complete body changes as they mature from larva to adult. The classic example of metamorphosis is the caterpillar that changes into a butterfly or moth.
We know the fascinating trilobites only by their abundant fossils. They appear to have evolved in the early Cambrian period, some 545 million years ago. During their heyday, trilobites dominated the world's oceans. Some were as long as 18 inches (45 centimeters) and may have weighed 10 pounds (4.5 kilograms). Their bodies were divided into three side-to-side lobes as well as a head, a thorax, and a tail region (pygidium). They swam and crawled with an abundance of jointed appendages. Although successful for many centuries, the trilobites had entirely vanished by the Jurassic period, some 250 million years ago. Their extinction remains one of the great mysteries of paleontology.
Many biologists suspect that the arthropods evolved from the annelids, a phylum of segmented worms that includes the familiar earthworm. From their ancient annelid ancestor, the early arthropods inherited the segmented body plan that all arthropods share today. Some annelids also bear pairs of appendages on each of their segments, a trait shared by adult arthropods. And like annelids, arthropods are bilaterally symmetrical. That is, their right and left halves are mirror images of each other.
Fossils of these first arthropods can be found in rock dating to the Cambrian period, which began 545 million years ago. They included trilobites, horseshoe crabs, and crustaceans. Centipedes, millipedes, and scorpions were among the first arthropods to reach dry land. They appear in the fossil record about 450 million years ago. The insects arrived later, about 350 million years ago.
If sheer numbers be the judge, arthropods are by far the most successful group of animals ever to swim, fly, or crawl across the face of Earth. The secret to their success? The arthropod's tough, waterproof exoskeleton is no doubt part of the reason. It afforded the early arthropods unprecedented protection from predators. It also enabled the first terrestrial arthropods to survive on dry land without themselves drying out in the process.
Those who study insects, the most successful arthropods of all, also point out the great advantage of each species' specialized diet. This has enabled an abundance of different insect species to evolve and survive in small areas, some no larger than one cubic foot of soil. The insect's short life span and amazing egg-laying capacity also enable it to adapt to environmental changes that would drive other species to extinction.
Whatever the reason for arthropod success, the rest of the Animal Kingdom should no doubt be thankful. Insects form the broad base of the food pyramid on which all land animals depend for nourishment. A similar story occurs in the world's seas, where swarms of tiny crustaceans provide food for larger arthropods, small fish, and gigantic whales.