Hornets are the largest of the eusocial wasps, and are similar in appearance to their close relatives yellowjackets. 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 and by the rounded segment of the abdomen just behind the waist. Worldwide, there are 22 recognized species of Vespa, Most species only occur in the tropics of Asia, though the European hornet, is widely distributed throughout Europe, Russia, North America and Northeast Asia. Wasps native to North America in the genus Dolichovespula are commonly referred to as hornets, but are actually yellowjackets.
Read article on Wikipedia