Hornets (insects in the genera Vespa and Provespa) are the largest of the eusocial wasps, and are similar in appearance to their close relatives yellow jackets. 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,^[1] and three species of Provespa, which are unique amongst hornets in being nocturnal.^[2] Most species only occur in the tropics of Asia, though the European hornet (Vespa crabro), 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 (e.g. baldfaced hornets), but are actually yellowjackets.

A wasp is any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. The Apocrita have a common evolutionary ancestor and form a clade; wasps as a group do not form a clade, but are paraphyletic with respect to bees and ants.

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