Medical Definition of Insecta
: a class of Arthropoda comprising segmented animals that as adults have a well-defined head bearing a single pair of antennae, three pairs of mouthparts, and usually a pair of compound eyes, a 3-segmented thorax each segment of which bears a pair of legs ventrally with the second and third often bearing also a pair of wings, and an abdomen usually of 7 to 10 visible segments without true jointed legs but often with the last segments modified or fitted with specialized extensions (as claspers, stingers, or ovipositors), that breathe air usually through a ramifying system of tracheae which open externally through spiracles or gills, that exhibit a variety of life cycles often involving complex metamorphosis, and that include the greater part of all living and extinct animals
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