Arthropod

Arthropods are any of the invertebrates with a hard shell and jointed legs. They are assigned to the taxonomic phylum Arthropoda, which is the largest phyla of animals on earth, comprising over 80% of the world's known organisms with over a million modern species described. Their number is unknown but it is estimated that there are 1,097,289 described living arthropods. Their name "arthropoda" means jointed foot describing the appearance of their appendages.

They live in almost every habitat on earth, common throughout marine, freshwater, terrestrial, and even aerial environments. They are also ecologically diverse predators, herbivores, and there are even symbiotic and parasitic forms. Despite this unbelievable diversity, the basic body plan of arthropods is fairly constant. Arthropods all share the following in common: body segmentation, a hard exoskeleton (external skeleton), and jointed appendages. They include the crustaceans, arachnids, insects, and centipedes.

Anatomy
The Phylum Arthropoda covers a wide range of animals, but they all have common characteristics. The sizes of terrestrial arthropods can range from 0.25 mm to 3.5 oz. Aquatic arthropods can become even larger. Arthropods are classified by the number of body segments, mouth parts, and appendages.

All arthropods are bilaterally symmetrical and have segmented bodies. These segments are the head, thorax, and abdomen (the number of segments varies between each arthropod). In some groups, such as arachnids, the head and thorax are fused together to form what is called the cephalothorax. They each have an exoskeleton made out of the protein chitin. These exoskeletons do not grow with the animal, but rather, are shed periodically through a period of molting. Most arthropods molt several times during their lifetime. During these periods they become very vulnerable to predators because they no longer have a hard, protective shell.

Each body segment contains at least one pair of jointed appendages. The name arthropoda comes from the Greek word arthron meaning "joint", and podos meaning "foot". 

Reproduction
Arthropods are either male or female and almost always reproduce sexually. In a few cases there are no males in a species so females reproduce by parthenogenesis (reproduction without fertilization and a genetically identical offspring is produced)..

Terrestrial arthropods always have internal fertilization, but aquatic species can either reproduce internally or externally. Both sexes have paired reproductive organs and paired ducts. Males can either place a packet of sperm inside the female or the female can pick up a packet of sperm to fertilize her eggs. 

The eggs can develop as oviparous (the eggs hatch outside the mother's body), or as ovoviviparous (the eggs develop inside the mother's body and then are born live). Most arthropods develop and either go through a complete metamorphosis or an incomplete metamorphosis. An incomplete metamorphosis involves three different stages: egg, nymph, and adult. The nymph goes through several stages until it slowly develops into an adult with wings and reproductive organs. On the other hand, a complete metamorphosis involves four stages: egg, larva, pupa, and adult. The larva look like worms and continue to eat until they are large enough to develop into a pupa. The animal undergoes a complete transformation while in the pupal stage and comes out as an adult. 

Ecology
Since arthropods include 3/4 of living creatures, they are found in a variety of habitats. They live all over the world and can live in streams, deserts, the ocean, grasslands, rain forests, ponds, and your homes. Their possible habitats are endless because of the vast numbers in this phylum and their greatly differing forms.

Many predators of arthropods include: birds, frogs, bats, fish, and other arthropods. Humans also prey on arthropods; in some Asian and African cultures insects are a part of the diet. Arthropods themselves eat a variety of things. They are known to be omnivores, carnivores, herbivores, detritivores, and fungal feeders.

Human Benefits
Arthropods play an important part to human life. There are many beneficial arthropods that help remove pests like other insects or bacteria. These predatory creatures include ladybugs, beetles, spiders, mites, and wasps.

Bees help pollinate flowers as well as create honey. Honey is created when a bee carries nectar in his internal honey sac and brings it back to his hive. The house bee then spreads the nectar throughout out the comb. As the excess moisture evaporates and another layer of wax is put on the cells of the hive honey is created. Beeswax, another product of bees, is an ingredient in many things like cosmetics, waxes, polishes, and waterproofing.

Shrimp, crab, and lobster are all found at restaurants and on the table at homes. Crustaceans can be a huge food source for those who live by the sea or enjoy seafood.

Another fact is a silkworms' cocoon can be used to make hypoallergenic silk. Silk has the same durability as steel and is naturally flame retardant! 

Related References

 * Phylum Arthropoda by University of Michigan Museum of Zoology. Animal Diversity Web.
 * Biology, Miller/Levine, copyright 2008, published by Pearson Prentice Hall
 * Inquiry into life - tenth edition, Sylvia Mader, copyright 2003, published by McGraw-Hill
 * Veterinary Entomology: Arthropod Ectoparasites of Veterinary Importance by Richard Wall, David Shearer.
 * Wikipedia by various authors.
 * zoolab: Introduction to the Arthropods by University of Wisconsin. A website for animal biology.
 * Marvelous Metamorphosis by Arizona Science Standards.
 * Beneficial Insects and other Arthropodsby W.S. Cranshaw. Colorado State University Extension.
 * Honey Bees by Isaac Watts. Insecta Inspecta World.