Hornets are the largest of the eusocial wasps, and are similar in appearance to yellowjackets, their close relatives. Some species can reach up to 5.5 cm (2.2 in) in length. They are distinguished from other vespine wasps by the relatively large top margin of the head. Worldwide, 22 species of Vespa are recognized. Most species only occur in the tropics of Asia, though the European hornet is widely distributed throughout Europe, Russia, North America, and north-eastern Asia. Wasps native to North America in the genus Dolichovespula are commonly referred to as hornets, but all of them are actually yellowjackets.