Arthropod

An arthropod is an invertebrate animal with a segmented body, a hard external skeleton and jointed legs. The phylum Arthropoda includes insects, spiders and scorpions, crustaceans and centipedes, and it holds well over a million described species, more than all other animal groups combined.

The exoskeleton is made of chitin and protein and protects the body, but it cannot grow, so arthropods molt it periodically. Their bodies are divided into segments, often fused into regions such as head, thorax and abdomen, and they have paired jointed appendages. They have an open circulatory system in which hemolymph bathes the organs, and they breathe through gills, book lungs or tracheae.

Insects, such as the monarch butterfly and the western honeybee, are by far the largest group. Chelicerates include spiders, like the European garden spider, scorpions like the deathstalker, and the Atlantic horseshoe crab. Crustaceans include the American lobster and the giant isopod, and myriapods include the house centipede.

Arthropods pollinate crops, decompose waste and form much of the food web. Some are harmful: mosquitoes such as Aedes aegypti, ticks and fleas carry diseases, which are studied in the field of vector biology. Their success may come from the efficient body plan and the capacity for flight in insects, and their diversity is a main topic of biodiversity research.

• Hemolymph — their circulating fluid
• Aedes aegypti — an arthropod that transmits disease
• Biodiversity — the wider diversity of life