Arthropod

An arthropod is an invertebrate (an animal without a backbone) that has jointed legs and a segmented body. Arthropods are the world's oldest creatures and the most successful invertebrate group. The phylum Arthropoda contains nearly 1,000,000 species and is therefore the largest in the kingdom Animalia. It is estimated that arthropods account for 75 percent of all the animals on Earth. They range from dust mites to huge crabs.

EXTERNAL STRUCTURE OF ARTHROPODS

The name arthropod is translated as "jointed foot" but it is really a jointed or segmented body that is most characteristic of an arthropod. Since all arthropods have an exoskeleton, which is a hard covering that surrounds the outside of an animal's body, their outer skeleton cannot consist of a single solid piece or they would never be able to move about with any flexibility or speed. A tough covering that is composed of over-lapping plates (linked together by tough but flexible hinge joints) allows the arthropod to bend, twist, and move about with great freedom. The same applies to any appendages (legs, arms, tails, pincers) an arthropod may have; these too are made up of tough but flexible joints that allow them to maneuver easily over most surfaces. Whether a lobster or a beetle, most of the arthropod body is covered by an exoskeleton called a cuticle. This protects its soft tissues from predators and disease and supports its entire body. The cuticle is made of a protein called anthropodin, and a carbohydrate called chitin that together produce a tough and flexible covering. This exoskeleton can vary immensely. It may consist of the delicate and flexible wing of an insect or to the heavy and thick shell of a lobster. The major drawback of an exoskeleton is that it makes growth or physical expansion difficult. Since chitin is not living tissue, it cannot expand, and must instead be shed and regrown when an animal's body gets larger. This periodic shedding is called molting and occurs when an arthropod splits its exoskeleton and walks out of it to later form another. During this stage an arthropod is especially vulnerable to attack. The soft-shelled crabs that people eat and enjoy are actually hard-shelled crabs that are caught while molting.

INTERNAL STRUCTURE OF ARTHROPODS

Inside their suit of armor, all arthropods are basically the same. All have a nervous system made up of a brain, simple eyes, and nerves that connect to a long nerve cord running the length of their body. This allows them to perceive and react to their environment. Arthropods that live on land breathe through a tracheal system rather than through lungs. This system consists of narrow, air-filled tubes in their outer skeletons called tracheae that branch into smaller tubes inside the body and directly supply each cell with the oxygen it needs. Water-living arthropods breath with gills, located sometimes on their bodies and other times on their legs. Their circulatory system is an open one, meaning that blood flows freely throughout their body, pumped by a simple heart. Digestion always begins with a mouth and continues into a single long gut that runs the length of their body. Excretion of waste takes place from a separate opening. Most arthropods reproduce sexually (through the union of male sperm and female eggs), although a few species have both male and female organs. Most females lay their eggs in a protected place where they eventually develop into larvae.

TYPES OF ARTHROPODS

Crustaceans. Arthropods are classified by biologists according to the number of legs, antennae, and body regions they have. There are therefore five main groupings: crustaceans, arachnids, insects, centipedes, and millipedes. A crustacean has compound eyes, several pairs of legs (four or more), and two pairs of antennae, with a body divided into two main parts. There are about 32,000 species of crustaceans (lobsters, crayfish, crabs) who get their name from the hard case or "crust" they wear. Crustaceans eat other invertebrates, almost always live in water, and vary greatly in size. A few crustaceans, like the wood louse (also called pill bug), live on land.

Arachnids. The arachnids, which include spiders, mites, ticks, scorpions, and horseshoe crabs, have only four pairs of walking legs and no compound eyes. Their bodies have two main regions. Most live on land and feed on insects and other small animals. Most are harmless although some species are poisonous.

Insects. Insects are the most successful invertebrates on land and make up the largest class of arthropods. Well-known examples are bees, ants, grasshoppers, butterflies, moths, and the housefly. Insects live in nearly every habitat on Earth and total at least 800,000 species. All insects have a segmented body that is divided into three regions (head, thorax, and abdomen). The head contains the mouthparts and sense organs (often compound eyes); the thorax (the part of an arthropod's body where the legs are attached) has three pairs of legs or one or two pairs of wings.

Centipedes and Millipedes. The last two arthropod groups, centipedes and millipedes, both have cylindrical segmented bodies with many joined legs and antennae. The main difference between the two is that centipedes are poisonous and have one pair of legs attached to each segment. Millipedes have two pairs on each body segment. Both live in dark, damp places, but centipedes capture and eat other invertebrates, while millipedes feed mainly on decaying plant material.

BENEFICIAL AND HARMFUL

Arthropods are an extremely diverse group of invertebrates. They crawl, swim, run, and fly. They produce honey, silk, and other valuable products and provide the main meal for many of the fish that humans eat. They pollinate flowers and crops and recycle soil nutrients. Although beneficial, they can also be harmful. They can cause illness and even death to humans with their poison, and they can destroy our crops. Overall, however, arthropods are an integral part of many ecosystems (an area in which living things interact with each other and the environment), most of which would collapse without them.

[See alsoArachnids; Crustaceans; Insects ]