Arthropods

Myriapods | Chelicerates | Crustaceans | Hexapods

What is an Arthropod?

An Arthropod is a segmented animal that can be characterized as having an exoskeleton.

Arthropods are a group of invertebrates that form the largest Phyla of all our planets living organisms. Arthropods account for 3 out 4 species of animal known to man. The biomass of arthropods far outweighs that of the vertebrate group of animals. Arthropods have several features in common. The main common feature is that they all have bilaterally symmetrical body shapes which are divided into segments. Over time, evolution has fused the segments into functional groups. There are 4 main groups of arthropods:

Myriapods (including centipedes and millipedes)

Centipede
Centipedes and millipedes have similar structured bodies whereby their form consists of a head and a body trunk made of similar shaped segments. These terrestrial creatures have biting mandibles, a single pair of antennae and they live in moist habitats as they do not have a waterproof cuticle.

Centipedes are fast moving carnivores with an elongated, slightly flattened segmented body. Each trunk segment consists of a pair of single legs. The first pair of legs on a centipedes trunk are modified as poisonous claws which they use to subdue their prey.

Millipede
Millipedes are mostly herbivores or scavengers and have an elongated cylindrical or flattened body. The first 3 segments of a millipedes trunk contain no legs at all. The remaining segments are fused into pairs known as diplosegments and each diplosegment has 2 pairs of legs attached to it.

Despite the millipedes common name, they never have a thousand legs.

Chelicerates (Arachnids - including spiders, scorpions, pseudoscorpions harvestmen, ticks and mites)

Arachnids have a head which is fused to their thorax forming a cephalothorax. The cephalothorax has sensory organs, mouthparts, limbs and stomach. The remaining body segments are fused to form an abdomen. The abdomen contains the heart, lungs, gut, reproductive organs and anus. This means that the body of an arachnid is only divided into 2 sections whereas insects have 3 body sections.

Arachnids use their sensory hairs, sensory organs and simple eyes to detect and analyse their surroundings. Arachnids are cold-blooded animals that get warmth from their environment.

Arachnids are diverse in size ranging from a few millimetres to 20 centimetres in length. They have a strong exoskeleton made of calcium and carbohydrates which protects them from predators. Arachnids do not possess teeth and most cannot digest solid foods, this is why they suck fluids from their preys body.

Scorpion
A scorpion has a tail that extends from their abdomen bearing a painful stinger on the end. At the front are 2 pincer-like chelicerae which are used to consume prey and a pair of limb-like pedipalps. They have 4 pairs of walking legs. Psuedoscorpions are not really scorpions although they have a similar appearance.

Pseudoscorpions do not have the extended tail with the stinging end, instead they produce venom from their pincer-like pedipalps. Pseudoscorpions also produce silk from silk glands which are located on the jaws or chelicerae. The silk is used to construct cocoons in which they molt and live during the winter. They usually range from 2 to 8 millimetres in length. The largest known species is Garypus titanius of Ascension Island at up to 12 millimetres.

Ticks are parasites that feed upon the blood of mammals such as humans, dogs, cows and sheep. Ticks are tiny arachnids, about the size of a grain of rice. They climb onto their host, gripping tightly with their legs and feed by sinking their mouthparts into the hosts skin and gorges itself on blood. Once the ticks body is full, it swells like a tiny balloon and falls off its host.

Most spiders are predatory arachnids that inject venom to catch their prey. Some spiders spin webs in order to catch their prey, others like the Brazilian Wandering spider walks the jungle floors in search of prey instead of residing in a lair or maintaining a web.

Crustaceans (including: Decapods (crabs), Eucaridans (krill) and Malacostracans (woodlice and relatives)

The word crustacean means 'with a crust'. This group of arthropods is an extremely diverse group who range greatly in size from minute copepods which are only just visible to the naked eye to large lobsters and crabs such as the Giant Spider crab who has a leg span of up to 4 metres (13 feet). A vast majority of crustaceans live in the sea while others can survive in fresh water environments and on land. The most successful crustaceans live in the sea. One of the reasons they are more successful is that the sea contains essential minerals, such as calcium carbonate, which are needed for shell building.

One crustacean that has adapted to life on land is the Woodlice. However, they do still live in damp places and can be found in rotting damp wood, under stones and in damp areas of houses.

Crustacean Anatomy

Crustaceans have compound eyes which are usually on the end of stalks. They have 2 pairs of antennae and a cuticle which is strengthened by calcium carbonate. A crustaceans head and thorax is most often covered by a carapace (shell) which extends forward to form a projection called a rostrum (a beaklike extension). Some crustaceans have additional mouthparts called maxillipeds. Unlike the internal skeletons of vertebrates, a crustaceans carapace or exoskeleton does not grow as the animal develops. Crustaceans have to molt periodically. During the molting process, every part of the body covering is shed including the covering of the eyes and fine hairs. Molting can be an exhausting ordeal and for some, things can go wrong. Some can become stuck in their old skin and the only option of escaping is to lose a claw or other appendage which unfortunately does not grow back.

A crustaceans thoracic appendages are called thoracopods and these are two branched and developed to perform various functions. These functions include feeding, moving, sensing the environment and for respiration by basal gills. The first pair of legs may be enlarged to form chelipads and are equipped with strong claws used for picking things up and defence.

Crustacean Life Cycle

Crustaceans develop from eggs to adults by going through a series of larval stages or instars. These stages are interspersed by full molts whereby they shed their exoskeleton and emerge a slightly larger and more complicated version of their previous form. Early larvae are plankton-dwelling creatures called nauplii that gradually develop various appendages. With each molt, the nauplii gains body segments and/or appendages gradually becoming more like its parent as it develops. Nauplii larvae do not feed, but utilize their internal yolk reserves from the egg for energy. Nauplii are the most abundant form of multicelluar life form on the planet.

Hexapods (Insects - Class: Insecta) The insect group includes many different species including grasshoppers, beetles, crickets, earwigs, wasps, bees, ladybirds, praying mantis, dragonflies, lacewings, flys, fleas, louse, termites and butterflies.

Hexapod is the most diverse and abundant animal group on Earth. Each species has evolved to live very varied lifestyles. Many insects are terrestrial, however, there is a significant number that are aquatic. Although different species of insect live varied lifestyles, they have several features in common. Each insect has 3 main body sections, head, thorax and abdomen. Their head parts contain external mouthparts, bear one pair of antennae and a pair of compound eyes. Three legs are attached to their thorax and they typically have 2 wings.

The abdomen of an insect contains the major organs for digestion and reproduction. Some immature aquatic species have gills, however, all adult insects breathe air and have a well developed tracheal system joined to the outside through small holes called spiracles. A strong exoskeleton, sometimes called the elytra, covers the insects body and protects it from damage.

Insects have a small brain which is a collection of nerve cells fused together. The nerve cells send signals to control all other organs. An insects compound eyes are made from many tiny lenses and are very good at detecting movement. Their single eyes only reflect light and dark. An insects antennae functions as an organ of touch, taste and smell. Insects are cold-blooded animals and their body temperature changes with that of their surroundings. Their growth and development depends highly on how hot or cold the weather is.

Some insects cannot fly and have lost their wings through evolution. Most insects are the only species of invertebrates that are capable of powered flight and together with their small size and waterproof cuticles they have been allowed to colonize a vast range of habitats.

Some interesting questions and answers about insects:

Which is the strongest insect on Earth?

The Rhinoceros Beetle (Family: Scarabaeidae) is the strongest insect as it can lift 850 times its own body weight.

Which insect provides the most young during reproduction?

The Cabbage Aphid (brevicoryne brassicae) can provide the most numerous offspring with numbers being in their billions.

Which insect is the fastest flyer?

A Dragon Fly (austrophlebia coastalis) is the fastest flying insect and can reach speeds of 58 kilometres per hour.

Which is the fastest running insect?

The Cockroach (Order: Blattaria) is the fastest running insect reaching speeds of 5.4 kilometres per hour.

Which insect lays the largest eggs?

The insect that lays the largest eggs is the Malaysian Stick Insect (Phobaeticus serratipes) whose eggs measure 1.3 centimetres long.

Which is the longest insect?

The longest insect is the Giant Stick Insect which measures 33 centimetres in length. The giant stick insect evolved before the dinosaurs and was thought to have been extinct for 80 years. However, in 2001 a species was found on a remote island off the Australian coast.

Which is the shortest insect?

The shortest insect is a species of parasitic wasp (dicopomorpha echmepterygis). The males of this species is also the smallest of all known insects. They are blind and wingless and measure no more than 0.139 millimetres in length.

You can find lots more information about individual insects and myriapods in our Insect and Bug section.

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