Insects are a class of invertebrate animals within the arthropod phylum that have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals on the planet, with over a million described species and representing more than half of all known living organisms.
The study of insects is called entomology, and insects are widely considered to be important in many fields such as agriculture, medicine and ecology. Insects pollinate crops, provide food for other animals and break down dead plant and animal matter. They also can cause disease in humans and other animals; however, some insects also act as vectors to spread diseases between humans or from animals to humans. In addition, some insect groups are beneficial to humans because they prey on crop pests or help control weeds.