Short answers, pulled from the story.
An arthropod is an invertebrate in the phylum Arthropoda, defined by an exoskeleton with a cuticle made of chitin, a body of differentiated segments, and paired jointed appendages. The word arthropod comes from Greek words meaning jointed leg. Arthropods account for over 80 percent of all known living animal species.
Estimates of arthropod species range from about 1,170,000 to 5 to 10 million. Arthropoda is the largest animal phylum, and its sub-group the insects includes more described species than any other taxonomic class. A 1992 study estimated 365,000 of the 500,000 animal and plant species in Costa Rica were arthropods.
Arthropods moult because their exoskeleton cannot stretch and therefore restricts growth. Through ecdysis they shed the old cuticle, the exuviae, after growing a new one beneath it. Moulting is dangerous and may be responsible for 80 to 90 percent of all arthropod deaths.
The four major groups of arthropods are Chelicerata, which includes sea spiders, horseshoe crabs, and arachnids; Myriapoda, which includes millipedes and centipedes; Pancrustacea, which includes copepods, malacostracans, branchiopods, and hexapods; and the extinct Trilobita. The living phylum is typically divided into four subphyla, one of them extinct.
The greatest contribution of arthropods to humans is pollination, valued by a 2008 study at 153 billion euros, or 9.5 percent of world agricultural production used for human food in 2005. Crustaceans, insects, and their grubs are eaten as food, bees produce honey, and horseshoe crab blood yields limulus amebocyte lysate used to test antibiotics and kidney machines for bacteria.
Blood-sucking insects spread severe diseases including malaria, carried by Anopheles mosquitoes, which causes 267 million cases and one to two million deaths a year. Aedes mosquitoes spread dengue fever and yellow fever, and Culex mosquitoes spread filariasis. Ticks can cause tick paralysis and parasite-borne diseases.
The evolutionary ancestry of arthropods dates back to the Cambrian period, and the earliest Cambrian trilobite fossils are about 520 million years old. Arthropods also provide the earliest identifiable fossils of land animals, from the Late Silurian, and the scorpion-like eurypterids grew as long as 2.5 m.