Arthropods (Arthropoda) are a highly-successful group of invertebrate animals that includes centipedes, millipedes, spiders, mites, horseshoe crabs, scorpions, insects and crustaceans. There are more than one million known arthropods species and many millions more that have yet to be named. Scientists estimate there may be as many as 30 million species of arthropods, most of which are insects. Arthropods evolved more than 500 million years ago and are still going strong. They have colonized a vast variety of ecological niches around the globe and have evolved into a multitude of forms.
Arthropods are highly diverse. The group accounts for over three-quarters of all currently known living and fossil organisms. The exact number of living arthropod species is unknown since many have yet to be identified, especially those inhabiting tropical rainforests.
The exoskeleton of an arthropod is a hard external structure made of chitin that protects the arthropod, prevents dessication and provides structural support. Since the exoskeleton is rigid, it cannot grow with the arthropod and must be molted periodically to allow for increases in size. After molting, a new exoskeleton is secreted by the epidermis. Muscles connect to the exoskeleton and enable the animal to control the movement of its joints.
The muscle structure of arthropods is more complex than that of most other invertebrates. Arthropods have longitudinal and circular bands of muscle tissue as well as isolated musles that together enable a wide range of movements. Additionally, arthropods have a well-developed nervous system. In more advanced arthropods, the nervous system consists of a brain and a double nerve cord. More primitive arthropods do not have a brain but instead have ganglia either located in each segment of their body or in a ganglionic mass near the head.
Arthropods are worldwide in their distribution and occupy a wide range of habitats including the deep sea, coastal waters, terrestrial habitats, freshwater rivers and streams, forests, deserts, scrublands, and grasslands.
The first arthropods appeared during the early Cambrian Period. They are thought to have evolved from annelids between 500 and 600 million years ago. Perhaps the best known early arthropods were the Triolobites, creatures that lived on the muddy bottoms of shallow marine environments. Trilobites disappeared about 225 million years ago during the Permian Period. The reason for their extinction is not known. The first land animal was a myriapod arthropod. Based on fossil evidence, these arthropods took to a life on land between 490 and 443 million years ago.
The main characteristics of arthropods include:
- bilateral symmetry
- segmented body
- jointed appendages
- numerous pairs of limbs
Arthropods are classified within the following taxonomic hierarchy:
Arthropods are divided into the following taxonomic groups:
- Centipedes and millipedes (Myriapoda) - There are more than 13,000 species of centipedes and millipedes alive today. Members of this group include centipedes, millipedes, symphylans, and pauropods. Centipedes and millipedes are terrestrial insects best known for the fact that they possess many pairs of legs (from five pairs to several hundred pairs).
- Chelicerates (Chelicerata) - There are about 77,000 species of chelicerates alive today. Members of this group include spiders, mites, scorpions, horseshoe crabs, harvestmen, and ticks. Chelicerates have specialized mouth parts known as chelicerae, which enabled early members of the group to feed on other animals. Modern chelicerates have diversified much since then and now include not only predators, but also parasites, scavengers, herbivores, and detritivores.
- Crustaceans (Crustacea) - There are about 67,000 species of crustaceans alive today. Members of this group include krill, shrimp, barnacles, crabs, lobsters, and crayfish. Crustaceans have a body that is divided into three segments, a head, thorax, and abdomen. Most crustaceans are aquatic animals, but there are also some terrestrial species and even a few perasitic species.
- Hexapods (Hexapoda) - There are more than 1,000,000 described species of hexapods alive today, most of which are insects. Members of this group include insects and springtails. Hexapods have a body made up of three sections (head, thorax, and abdomen) and three pairs of legs.
Ruppert E, Fox R, Barnes R. Invertebrate Zoology: A Functional Evolutionary Approach. 7th ed. Belmont CA: Brooks/Cole; 2004. 963 p.