A paired sclerotized plate on the lateral (side) surface of an insect's or crustacean's thorax, located between the leg attachment point (coxa) and the back plate.
From Greek 'epi' (upon) + 'sternon' (breastbone/chest), literally 'upon the chest,' used since the 1800s as the standard anatomical term in entomology.
If you've ever looked at a detailed insect diagram, the episternum is one of those precisely-named parts that shows how obsessively scientists have catalogued the architecture of tiny creatures.
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