Hornets are the largest eusocial wasps, that reach up to 45 millimetres (1.8 inches) in length. The true hornets make up the genus Vespa, and are distinguished from other vespines by the width of the vertex (part of the head behind the eyes), which is proportionally larger in Vespa; and by the anteriorly rounded gasters (the section of the abdomen behind the wasp waist).