Arthropodes

Arthropodes, or arthropods, form the phylum Arthropoda, the largest group of animals. They include insects, arachnids, crustaceans, and myriapods. With hundreds of thousands of described species and many more yet to be discovered, they inhabit almost every ecosystem, from deep oceans to high mountains.

They share key features: an external skeleton of chitin, segmented bodies, jointed appendages, and an open circulatory system with a tubular heart. Most also have a ventral nerve cord and well-developed sensory organs. Growth occurs by molting, during which the exoskeleton is shed and replaced.

The major living groups are Chelicerata (arachnids such as spiders and scorpions, and a few related forms like horseshoe crabs), Crustacea (crabs, lobsters, shrimp, copepods and others), Myriapoda (centipedes and millipedes), and Hexapoda (primarily insects and their relatives). Trilobites are extinct, but illustrate early arthropod diversity.

Body plans vary: many species have a head, thorax, and abdomen; in arachnids the front body region is a combined cephalothorax. Insects typically have a three-part body and three pairs of legs. Reproduction is mainly sexual, with a variety of developmental pathways including complete and incomplete metamorphosis in insects.

Ecologically, arthropods occupy nearly every niche: pollinators, predators, decomposers, and prey for many animals. They also have large economic importance as pests, vectors of disease, and sources of food and materials.

The fossil record shows arthropods appearing in the early Cambrian. Their exoskeletons fossilize well, aiding study of their evolution. The group's diversity reflects a long evolutionary history and extensive adaptation to aquatic and terrestrial environments.