one of the classes of arthropoda, including those that have one pair of antennae, three pairs of mouth organs, and breathe air by means of tracheae, opening by spiracles along the sides of the body. in this sense it includes the hexapoda, or six-legged insects and the myriapoda, with numerous legs. see insect, n.-in a more restricted sense, the hexapoda alone. see hexapoda.-in the most general sense, the hexapoda, myriapoda, and arachnoidea, combined.