Arthropods 2BIbDF09 10/11

What is an Arthropod?

Arthropods are eukaryotic multicellular heterotrophic organisms that obtain nutrients by ingestion. Arthropods are members of the most diverse phylum in the animal kingdom. They are named for their jointed appendages, which enable these animals to move despite their hard outer covering, the exoskeleton and they have a segmented body with a main architecture constituted, from top to bottom, of a head, a thorax and an abdomen.

See the Appendix Arthropods 1

What are Arthropod's jointed appendages?

Limbs are a prominent feature of species in many phyla. They are particularly important in two major lineages: the ecdysozoans and the vertebrates. Some members of the Ecdysozoa, which is a Superphylum of Arthropods, the definition of which is "a clade of moulting animals that encompasses primarily the arthropods and the nematodes", such as arthropods, have more complex, jointed limbs, which make fast, precise walking, running, jumping movements possible and are a prominent limb type in vertebrates and arthropods. Arthropod's appendages of different segments are variously adapted for walking, feeding, sensory reception, reproduction, swimming and defense. Thanks to the evolutionary flexibility of their segments and appendages, it contributes to the great diversification of arthropods.

See the Appendix Arthropods 2

What is the role of their exoskeleton?

The body of a typical arthropod is covered by a hard cuticle, a rigid external skeleton, or exoskeleton, made of layers of chitin and protein.
In any arthropod, the skeleton provides antagonism, a protection, a point of attachment for muscles, all of which are internal, which support the body and allows then the arthropod to move, and a protection against physical forces. It is impermeable to water, a feature that may have enabled many arthropods to conserve water and colonize land, in much the same way as a tough seed coat allowed plants to colonize land.
But there is a problem, because the arthropod grows into the exoskeleton, but the exoskeleton doesn't grow then it becomes too small for this animal. The ecdysis allows arthropods, by changing periodically their cuticle (called exuvia after the refusal of this last one), to grow up in size (ecdysis of growth) or to acquire new organs, or even to change shape (ecdysis of metamorphosis), because their exoskeleton having become too small for them, arthropods abandon it. So, to many insects,three particular ecdysis allow the metamorphosis of the embryonic to the grown-up (Holometabolism, Hemimetabolism and Ametabolism (See 2.7: How Insects metamorphose?)). An arthropod is more vulnerable during all the molt, not only during the ecdysis, where it has no more the possibility of running away and where the risks of wound are frequent, but also in the pre and the post-moult, where the old cuticle becomes soft and where the new is not still enough hardened.

See the Appendix Arthropods 3

Are Arthropods divided into several groups?

Arthropods have two subphyla:

• Chelicerata who are composed of two classes themselves:

- Merostomata
- Arachnids

• Mandibulates who are composed of three classes:

- Crustaceans
- Myriapoda containing: Millipedes and Centipedes
- Insects

Features:

• Merostomata are Chelicerata sailors large-sized less numerous, in branchiale breath.

• Arachnids usually have four pairs of walking legs and specialized pair of feeding appendages.

• Crustaceans are almost all marine organisms but some live in fresh water or in a terrestrial environment. They all have a crustacean hallmark of multiple pairs of specialized appendages.

• Millipedes eat decaying plant matter.

• Centipedes are carnivores are the only arthropods without antenna . They have a pair of poison claws and a single pair of long legs. Both (millipeds and centipedes)have segmented body and jointed legs.

• Insects have six legs and mostly have two pairs of wings.

The estimation about the total arthropod population numbers is a billion billion individuals(10¹⁸), but only a million species has identified, mostly insects. They must be regarded as the most successful animal phylum. The major groups of Phylum Arthropoda are Chelicerata, with some 57'000 species includes spiders or scorpions; Crustacea comprise some 35'000 species of largely marine organisms, like crabs or shrimps; Myriapoda includes Millipedes, with 3000 species are known or Centipedes, with more than 12'000 species; and Hexapoda, like Insects, which has been estimated that approximately a billion billion individual insects are alive at any time.

See the Appendix Arthropods 4

What are the main characteristics of Arthropods?

We use a table to represent the different characteristics of every groups of the Phylum Arthropoda:

Class	Characteristics	Members
Chelicerata	Mouthparts are chelicarae (pincers or fangs)	Spiders, Mites, Scorpions, Horseshoe crabs, Ticks, Daddy long-legs
Crustacea	Mouthparts are mandibles; appendages are biramous ("two-branched"); the head has two pairs of antennae	Crabs, Shrimps, Isopods , Barnacles, Lobsters
Hexapoda	Mouthparts are mandibles; the body consists of three regions: a head with one pair of antennae, a thorax. and an abdomen; appendages are uniramous ("single-branched")	Insects ( Beetles, Bees, Flies, Butterflies, True bugs, Crickets, Dragonflies, Fleas, Termites, Sucking lice), Springtails
Myriapoda	Mouthparts are mandibles; the body consists of a head with one pair of antennae, and numerous segments, each bearing paired uniramous appendages.	Centipedes, Millipedes

Where do they live?

Arthropods can live almost everywhere: on earth, in water, at high height, in deep caves, in deserts and even in glaciers.

How do they feed?

Arthropods feed on different way: of alive or dead organic matter, sometimes live as a parasite on other dead animals. According to their way of food, the structure of the digestive tract changes.Only the central part of the digestive tract is used for the extreme digestion, the extreme parts are covered with cuticle.

• Chelicerata: In the class Chelicarata, the most are carnivorous, but mites are largely herbivorous. They paralyze them predators thanks to their poison and They can store the food in a cavity. They can ingest small particles but most can't consume solid food. They subsist on liquids, including solid food that they liquefy by injecting with digestive enzymes and then suck up with the muscular pharynx.

• Crustaceans: Crustaceans use their appendix to create a current which brings them the food to the mouth. They are filtreurs (zooplankton) or détritivores (crawfish, lobster).

• Insects: Insects are the most important herbivores in terrestrial ecosystem. Every kind of plants is eaten by one or more species.

• Centipedes: Centipedes are carnivorous, feeding mainly on insects. Their appendages of the first trunk segment are modified into a pair of poison fangs.

• Millipedes: In contrast to centipedes, most millipedes are herbivorous, feeding mainly on decaying vegetation such as leaves litter and rotting logs (which are typical habitats for the animals).

How do they reproduce?

• Arachnids: The male weaves a web on which it is going to deposit its sperm. It leaves then in search of a partner. After the coupling, the female eats generally its partner (source of food).

• Crustaceans: Females spawn. Larvas ( called nauplius ) moult several times before reaching the adult stage. This process occurs most of the times, but some crustaceans are however hermaphrodites.

• Millipedes, Centipedes, and Insects: Millipedes, Centipedes and Insects spawn, with extern fertilization. As crustaceans, insects pass by several embryonic stages before reaching the adult stage. In certain species, larvas look like the adults (ametaboles insects). Others undergo a metamorphosis. The metamorphosis is incomplete by hemimétaboles (3 stages: egg, larva, adult. The larva looks like the adult but has no wings) and complete by holometaboles (4 stages: egg, larva, nymph, adult. The larva is completely different from the adult).

How do they move?

Flight is obviously one key of mowing that an arthropod, more exactly an Insect, can make. An animal that can fly that can escape many predators, find food and mates, and disperse to new habitats much faster that an animal that must crawl about on the ground. Because their wings are extensions of the exoskeleton and not true appendages, insects can fly without sacrificing any walking legs. Unlike the flying vertebrates, like birds and bats, have one of their two pairs of walking legs modified for wings, which explains why these vertebrates aren't generally very swift on the ground.

Since their joints haven't exocuticule, Arthropods can move their members and fold a segment over the other one. Their movements are produced by the contraction and the slackening of the fibers of striated muscles. The majority of Arthropods use their members as locomotive means to move, for example the aquatic species use it as paddle and the species ground as legs, to settle, by means of hooks or of flattened lobes serving as suction cups and for "fouisser" (presence of peaks).

How do they breathe?

Arthropods living in two different environment, aquatic and ground, the means of breathings will not thus be the same: either the oxygen is levied in the water under dissolved shape, or in the air in gas form.

But there exists an inconvenience about these breathings: arthropods such as crabs, living in zones of balance of tides or still the larvae passing from an aquatic state to a ground state at the adulthood, what leads at a return in the middle aquatic on some of cycle of development, must be able to do in the face of these changes of breathing.

• The aquatic strict arthropods:

The oxygen, in the water, is present and dissolved shape. It stems from the production by autotrophes and of the distribution since the surface. The most effective structures to get the oxygen are gills. It exists three types of breathing: 1. The pallial breathing 2. The breathing appendicular 3. The branchial breathing: a) The gills b) The branchial cavity and the ventilation

• Zones of balance of tides:

The fact that the water level varies according to the tide, that it is increasing or by decreasing, the aquatic arthropods are then subjected to two sorts of different breaths. For this either they resist the anoxia and take place anaerobe by keeping some water in their mouth so their gills are continously in the water allowing gas exchanges, or they develop specialized structures allowing them to consume the oxygen of the air. This last way is found at decapods, as the crab, present a respiratory system allowing the consumption of dissolved or gaseous oxygen. This respiratory system presents three evolutionary stages: breath branchiale, breath integumentary, pulmonary respiration.

• The ground arthropods:

These ground arthropods find in the air some oxygen in unlimited quantity. The surfaces of exchange however have to remain wet what does not facilitate the gaseous exchanges. The respiratory system of integumentary origin and the trachea appliance are used to struggle against the desiccation.

• Return in a middle aquatic on some of cycle of development:

Even if numerous arthropods by evolving left their aquatic environment, certain number of species however stayed there. They are especially insects with aquatic larva becoming volatile adults, for example, odonates (dragonfly), short-lived, beetles, dytiscidae. In all cases, the adult remains however close to the aquatic environment. Other species are going to be all the time in the water. In every case, according to the importance of the aquatic phase, the modifications of the respiratory system will be more or less important: there is necessarily a resupplying on the surface. Certain species have neoformed structures allowing them the consumption of dissolved oxygen: it is then a real return in the aquatic life. 1. Use of the atmospheric oxygen a) Respiratory siphons b) Immersed reserves 2. Use of the dissolved oxygen a) Tracheal-gills b) Breath branchial or cutaneous

How is their internal anatomy?

Nervous system

Living arthropods have a relatively simple central nervous system with a dorsal brain linked to a ventral nerve cord that consists of paired segmental ganglia running along the ventral midline of the thorax and abdomen.

The brain is a complex of generally six fused ganglia (three pairs) located dorsally within the head capsule, encircling and mainly above the esophagus.

In thorax and abdomen, ganglia within each segment are linked to one another by a short medial nerve (commissure) and also joined by intersegmental connectives to ganglia in adjacent body segments which give arthropod nervous systems a characteristic "ladder-like" appearance.

In comparison to vertebrates, an arthropod's nervous system is far more decentralized. Most behaviour (e.g. feeding, locomotion, mating, etc.) is integrated and controlled by segmental ganglia instead of the brain. In some cases, the brain may stimulate or inhibit activity in segmental ganglia but these signals are not essential for survival.

Circulatory system

Arthropods have open circulatory systems, in which a fluid (called hemolymph) bathes the organs directly with oxygen and nutrients and there is no distinction between blood and interstitial fluid.

A dorsal vessel is the major structural component of an arthropod’s circulatory system. This tube runs longitudinally through the thorax and abdomen, along the inside of the dorsal body wall. In the abdomen, the dorsal vessel is called the heart. It is divided segmentally into chambers that are separated by valves (ostia) to ensure one-way flow of hemolymph. Peristaltic contractions of the muscles force the hemolymph forward from chamber to chamber. During each relaxation, the ostia open to allow inflow of hemolymph from the body cavity.

In front of the heart, the dorsal vessel lacks valves or musculature. It is a simple tube (called the aorta) which continues forward to the head and empties near the brain. Hemolymph bathes the organs and muscles of the head as it emerges from the aorta, and then haphazardly percolates back over the alimentary canal and through the body until it reaches the abdomen and re-enters the heart.

About 90% of hemolymph is plasma. It fills the entire interior of the body and surrounds all cells. Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

How Insects metamorphose?

The biological diversity of insects (ten different main orders) is thus related to environmental diversity, in this case, the variety of food that insects eat. All insects have separate sexes, and fertilization is internal. There are three sorts different of metamorphosis for insects: Complete Metamorphosis, Incomplete Metamorphosis and Division of labor in insect societies.

• During development, the majority of insects (~85%), like butterfly, undergo a change in body form known as Complete Metamorphosis, also called Holometabolism, which has four stages: egg, larva, pupa, and adult. In these species, the larval stage is often spent in an entirely different habitat from that of the adult, and larval and imago(adult holometabolous insects usually have wings)forms utilize diffirent food sources, but they thus do not compete directly for the same ressources. The dramatic body transformation from larva to adult occurs in the pupa stage.
• The remaining insects, like grasshopper, undergo Incomplete Metamorphosis, also called Hemimetabolism, in which change is more gradual, but it has only three stages: egg, nymph, and adult. Young insects, called nymphs, look like miniature adults without wings when they hatch from their eggs, As they grow and feed, they shed their skin seversl times each times entering a new instar, or stage of growth.
• Some insects, like bees, wasps, ants or termites, have developed complex social behaviour and live cooperatively in underground or aboverground nests. Such colonies exhibit a Division of labor or Ametabolism, in that some individuals forage for food and care for the brood (workers), others protect the nest, and some only reproduce (the queen and one or two males).

see the appendix arthropods 5

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