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Vocabulary |
There are at least a million different species of arthropod, at least 90% of all animals, and some scientists think there may be ten times that many. When an organism is that common, it must be doing something right because it is surviving so well.
The arthropods are successful partly because their exoskeletons protect them, keeping water in and predators out. Exoskeletons also help the animal move more easily. However, one problem with the exoskeleton is that it does not grow along with the animal. When an arthropod gets too big for its exoskeleton, it must molt, or shed the skin. This makes it easy to kill until its new shell hardens. However, the exoskeleton helps arthropods live in many different places, including deserts.
The jointed legs of arthropods help them move easily and quickly. The legs are adapted to do many jobs, such as eating, grasping, walking, and biting. In fact, "arthropod" means "jointed foot."
Arthropods are an enormous phylum, and because there are so many they are broken into large groups. The main groups are arachnids, crustaceans, myriapods, and insects.
If an arthropod has two main body parts with all its legs attached to the front section, and it has no antennae, it is probably an arachnid. Arachnids include spiders, ticks, mites, and scorpions.
Spiders are extraordinary creatures. They have two body parts, four pairs of legs, four pairs of eyes, venom, and spinnerets which can make silk. Even spiders which do not make webs have spinnerets. Their silk is stronger than steel and very elastic. With it, they can make webs, draglines, parachutes, nests, and cocoons.
A little-known fact is that spiders cannot eat solid food. They must eat liquids. However, they are predators, which means they catch and eat other animals. How do they do it? They must digest their prey outside their bodies, with enzymes from their digestive systems..
Another type of arthropod is the crustacean. Crustaceans, unlike arachnids, have two pairs of antennae. They include crabs, lobsters, shrimp, krill, and crayfish. Most of them live in water. Crustaceans often have many different kinds of legs, with many different jobs.
The unusual red crab of Christmas Island lives most of its life on land. There are perhaps 120 million of them living in the shade of the forests. However, to fertilize and lay their eggs they must travel to the sea. They do this all at once, at the beginning of the wet season. The parts of the island on the way to the beaches are covered with migrating red crabs for about 18 days.
An arthropod with many segments and many legs, resembling a worm with an exoskeleton, is probably a myriapod (meaning "many feet"). These animals are more closely related to insects than to crustaceans or arachnids. The group includes the centipedes and the millipedes. Although "centi" means hundred and "milli" means thousand, a centipede does not have a hundred legs and a millipede does not have a thousand. However, they do have many legs. Centipedes are carnivores with a poisonous bite, and millipedes are herbivores which can give off an unpleasant stink.
Who rules the Earth? Not human beings. Insects rule. There are over a million species of insects, and they live everywhere. If you were to name an average animal, it would be an insect.
Insects have three body sections: A head, a thorax (THO-racks), and an abdomen (AB-doe-muhn). The head contains the sense organs. Three pairs of legs are attached to the thorax, which is in the middle, and if the insect has wings, they are attached to the thorax also. The abdomen contains the digestive organs and the reproductive organs.
By far the most common groups of insects are ones which contain the beetles (coleoptera), the bees and ants (hymenoptera), the butterflies and moths (the lepidoptera), and the mosquitos and flies (the diptera)
Most insects go through a complete change in form during their lives. This change is called metamorphosis. In many insects, the egg hatches a larva which is basically an eating machine. The butterfly larva is the caterpillar, for instance, and the beetle larva is the grub. When the larva has eaten enough, it constructs a cocoon or other shelter and becomes a pupa. During this phase, it moves all its organs around and changes its shape. When it comes out of the pupa, it is an adult whose form may be completely different from the larva.
Many insects, such as the cockroach, the mosquito, and the fly, are considered pests because they can spread diseases. Bees can sting, ants can bite, and moth larvae can eat holes in your clothes. Often people would like to get rid of them. Yet insects have been on Earth far longer than humans, they far outnumber us, and they may even out-last us. Though their brains are tiny and their bodies are small, they rule the Earth.
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