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Facts About Arthropods

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Lobster - Nephropidae

Lobster - Nephropidae

Photo © Shutterstock.

In this article, you'll learn interesting facts about arthropods and find out about their unique characteristics, their life cycle and their evolutionary history.

FACT: Living arthropods are divided into four subgroups.

Chelicerates (Subphylum Chelicerata) are a group of arthropods that include spiders, mites, horseshoe crabs, and scorpions. The first evolved about 445 million years ago during the Late Ordovician Period. Chelicerate bodies are divided into two regions, the cephalothorax and the opisthosoma.

Crustaceans (Subphylum Crustacea) include about 50,000 species of lobsters, crabs, shrimp, barnacles, brine shrimp and others. Most crustaceans live in marine or freshwater environments but there are also some terrestrial species.

Hexapods (Subphylum Hexapoda) are a group of arthropods that includes the insects. The body of a hexapod is divided into three sections, a head, thorax, and abdomen.

Myriapods (Subphylum Myriapoda) include about 13,000 species of millipedes and centipedes. Myriapods are terrestrial arthropods that are most abundant in forest habitats. They are notable for their long bodies and for having many pairs of legs.

FACT: Ancestral arthropods were the first land animals.

The oldest known animal to have lived on land is the arthropod Pneumodesmus newmani. Fossil evidence of this species consists of a single specimen that was discovered in 2004 in Scotland. Pneumodesmus newmani is classified as a millipede and is thought to have lived 428 million years ago, during the Late Silurian Period.

FACT: Arthropods have segmented bodies.

The body of an arthropod is divided into a series of semi-repetitive segments. Each segment bears a pair of appendages such as antennae, legs, wings, or mouthparts. Segmentation enables the arthropod body to develop specializations in each region of the body for specific functions such as feeding, sensory perception, locomotion, and visceral functions. The specialization of the various segments is known as tagmatization and each area of speciality is called a tagmata.

FACT: Arthropods have an exoskeleton.

The arthropod body is encased in a hard structure called an exoskeleton. The exoskeleton acts as an external skeleton and provides the animal with structural support and protection. The exoskeleton consists of cuticle, a multi-layered substance secreted by the epidermis. Cuticle consists of chitin, proteins and lipids. In many aquatic crustaceans, the exoskeleton is mineralized with calcium carbonate that is acquired from the surrounding water. This produces a tougher, more rigid structure. Since the exoskeleton is rigid, it must be jointed to allow the arthropod to move and acts as a framework to which the animal's muscles are attached.

FACT: Arthropods are a highly successful group of animals—they account for over three quarters of all currently known living and fossil organisms.

Of all animal groups, arthropods are the most diverse and their diversity is due in no small part to the diversity of one subgroup, the insects. Scientists approximate that there may be as many as 30 million species of insects alive today. To date, over one million have been identified. The success of arthropods has been attributed to their versatile exoskeleton, the process of metamorphosis, and their metameric body structure.

FACT: During their life cycle, arthropods undergo a transformation called metamorphosis.

Metamorphosis is a biological process that involves a radical transformation of body form and physiology. The immature form of an organism, called a larva, undergoes metamorphosis to become an adult. Since the larva and adult forms differ greatly in the way they live and what they eat, metamorphosis enables organisms to reduce competition for resources that otherwise would occur between the immature and adult forms.

FACT: Arthropods employ a variety of reproductive methods.

All terrestrial arthropods undergo internal fertilization and sperm is usually transferred to the female indirectly. Aquatic arthropods vary in their reproductive methods, with some species using internal fertilization and others external fertilization. Most arthropods lay eggs. Scorpions are the exception to this rule, they incubate their eggs internally and give birth to live young.

FACT: Arthropods have an open circulatory system.

Arthropods have an internal cavity called a hemocoel. The hemocoel provides a cavity in which the animal's internal organs are bathed in body fluids. This bathing enables the exchange of nutrients, waste material, and gases.

FACT: When arthropods grow, they must molt their exoskeleton.

Since the exoskeleton of an arthropod is a rigid structure, it must be shed and replaced periodically to enable growth. This shedding process is known as molting or ecdysis and consists of a series of four stages. First, enzymes are secreted that loosen the old exoskeleton layers. Second, the new exoskeleton layers are secreted. Third, the old exoskeleton splits and falls off. Finally, the new exoskeleton hardens and sets.

FACT: The Subphylum Trilobitomorpha is an extinct subgroup of arthropods.

The Subphylum Trilobitomorpha is a group of arthropods that were a dominant marine animal between 345 and 600 million years ago. Trilobitomorphs had two furrows that ran the length of their body and divided it into three regions: a left lobe, an axial lobe, and a right lobe. The most familiar of the trilobitomorphs were the trilobites. Trilobites (Class Trilobita) had three body sections, a cephalon, thorax, and pygidium. Trilobites are present in the fossil record from the Early Cambrian through the Late Permian. The lifestyles of triolobites were varied. Some species lived on the sea floor and others swam in the water column.

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