hornets are the largest of the social wasps. they can reach up to 55mm (5.5 cm, 2.2in) in length. the "true hornets" make up the genus vespa. true hornets can be distinguished from other vespines by the form of their head. hornets live in europe, asia, africa, and the americas. they live in warm and wet forests, and in desert climates. they do not occur in climates where it is cold all year round.