............................ KINGDOM ANIMALIA (METAZOAS) II. The Arthropods

Introduction. Among all the groups of multicellular organisms, the arthropods are the biological group that has had more biological outcome on our planet, both in terms of number of species and number of individuals, as in number of ecosystems conquered (see the graph below).

There is more than a million of species, which means that 80% of all known animal species are arthropods and live at sea (crustaceans), in fresh water (crustaceans and insect larvae) as the terrestrial environment (insects, arachnids and millipedes). Moreover, its outcome is not new but it is a of very old group, there are fossils of arthropods from a month to 600 million years old. The first mammals appeared 200 million years ago. The reason for its outcome is due primarily to its skeleton of chitin, a waterproof substance, tough and very light, which allows them to live outside of the water without the danger of desiccation. In this aspect they are much more efficient than terrestrial vertebrates, it can by seen through the comparation of the ease of the flight of an insect with a bird or a bat. They are the real flying animals. But it is not only benefits, the external skeleton implies that they are unable to achieve large sizes, because the transportation of air from the outside to the deeper cells of tracheal tubes requires an excessive effort. Below the different groups of arthropods and the general characteristics of the true kings of the animal world are presented.

Activities to do. Read the explanations on the general characteristics of arthropods, arachnids and millipedes and perform Multiple Choice Test 17.1 and Relate pictures with names 17.1. Then read the text on crustaceans and insects and perform Multiple Choice Tests 17.2, 17.3, 17.4 and 17.5 and Crossword 17.

1. The Arthropods. Are the invertebrates that have an external skeleton made of chitin with mobile articulated appendices. They have asegmented body, it is why they experience the molting during growth and sometimes shepe changings (metamorphosis). They have open circulation, which means its internal environment (hemolymph) is driven by the heart to the vessels from which it goes to a large internal gaps and from these is passed back to the heart through its side openings. The arthropods are classified in Arachnidas, Myriapodas, Crustaceas and Insects.

2 . The Arachnidas. Are the arthropods that instead of having two jaws in the mouth, one on top of another, have two appendices, one beside the other, called Cheliceraes, whose bodies are divided into two parts (prosome and opistosoma), do not have antennas and have 8 legs in the prosoma. They breathe through branched ducts (trachea) that carry air from the outside to each of the cells (tracheal breathing). Some of them also show tracheas that converge in cavities and act as lungs since, inside of them, the oxygen passes to an internal liquid which distributes it (plumonary breathing).

3. The Myriapodas. Are the terrestrial arthropods that present a head with 2 antennas and jaws and a trunk formed by numerous segments (10 to 200). They present the tracheal breathing.

They are classified into two groups called Diplopoda and Chilopoda

Classification of the Myriapodas
	Diplopodas. They have a cylindrical trunk and two pairs of legs on each of the segments. They are called millipedes and have the ability of rolling up around themselves (not to be confused with woodlouses which are the crustacean isopods of terrestrial environment).
	Chilopodas. They have flattened trunk and only one pair of legs on each segment. They are called the centipede centipedes. For example scolopendra whose first pair of legs was transformed into two poisonous nails (claws).

Multiple Choice Test about arachnidas and myriapodas

4. The Crustaceans. Are the mandible arthropods with two pairs of antennae, respiration through gills and body usually divided into two parts called cephalothorax and abdomen (a few, as insects, have three parts called head, thorax and abdomen). In many of them the chitin is impregnated with salts (calcium carbonate) to become more resistant. In these, the part of the skeleton corresponding to cefalotòrax is called the carapace. The appendages of the crustaceans have two branches (biramous) but usually one of them is very reduced or transformed into an internal gill. Near the mouth there are two appendages called maxillipeds that serve to approximate the food. The last two abdominal appendages are called uropods. They generally presente the metamorphosis.

The most notable groups of crustaceans are Decapods, Copepods, the Cirripedias and Isopods.

Classification of the crustaceans
Decapods. Are characterized by having to 10 legs. For example prawns, lobsters, crayfishes and sea crabs.	Copepods. Are the crustaceans that constitute the plankton. They are generally microscopic.	Cirripedias. Are the crustaceans that are sessiles (stationary). For example barnacle and balanus or sea urchin.	Isopods. Are generally aquatic. Some are terrestrial, as woodlices.

5. The Insects. Are the arthropods that have three body parts called: head, thorax and abdomen, and which have one pair of antennae, jaws, three pairs of legs and tracheal breathing. More than a million species is known, making it the dominant group on Earth. They live on land and in fresh water but not in the sea.

A) External morphology. The head has two compound eyes and two or three small simple eyes. The two antennas have olfactory function. The mouth can be the bitinig-sucking type like mosquitoes, the chewing type like grasshopper, the sucking type like "proboscis" of butterflies, or the chewing-licking type like bees. In the thorax three segments are differentiated each one with a pair of legs, therefore they have six legs. Most of insects have a pair of wings in the second and third segment, therefore they have four wings.

In the Dipteras, such as flies, the second pair of wings is reduced to a couple of small clubs (rocker) with flight stabilizing function. The abdomen has 11 segments which have a pair of side holes called stigmata, which are the inlet air ducts, called tracheas, distributing the air through all cells. In 8th female and 9th male segments are genital holes and copulatory appendages. In females of some species there are special organs forlaying the eggs (like grasshoppers ovipositor) or for the defense (like the sting of bees).

B) Internal anatomy. They have a tracheal breathing, a tube-shaped dorsal heart, nervous system with the brain and ventral nerve chain and the tubular digestive system.

C) Reproduction. The fertilization is internal meanig through intercourse. From the egg comes a larva that experience the metamorphosis (changes in form) until becoming an adult. There can be distinguished the simple metamorphosis when only small changes of proportion happen, like the grasshopper, and complex metamorphosis when the egg passes into caterpillar (mobile), then into pupa (inmobile and with a protective cover), which in butterflies is called a chrysalis, and finally into an adult, as in butterflies.