Arthropoda is the largest phylum of Animalia and includes the insects. Over two-thirds of all named species on earth are arthropods. They have an organ-system level of organisation and are bilaterally symmetrical, triploblastic, segmented and coelomate. The body is covered by a chitinous exoskeleton and consists of head, thorax and abdomen. They have jointed appendages (arthros: joint, poda: appendages). Respiratory organs are gills, book gills, book lungs or the tracheal system. The circulatory system is of open type. Sensory organs such as antennae, eyes (compound and simple) and statocysts or balancing organs are present. Excretion takes place through malpighian tubules. They are mostly dioecious, fertilisation is usually internal, they are mostly oviparous, and development may be direct or indirect. Examples include economically important insects such as Apis (honey bee), Bombyx (silkworm) and Laccifer (lac insect); vectors such as Anopheles, Culex and Aedes (mosquitoes); the gregarious pest Locusta (locust); and the living fossil Limulus (king crab). Arthropods are therefore best remembered for their jointed appendages, chitinous exoskeleton, open circulation and malpighian tubules, features that clearly separate them from annelids in comparison-based questions, and for the fact that they are the most successful and abundant group on earth.