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 yellow jackets.
Like other social wasps, hornets build communal nests by chewing wood to make a papery pulp. Each nest has one queen, who lays eggs and is attended by workers who, while genetically female, cannot lay fertile eggs. Most species make exposed nests in trees and shrubs, but some (like Vespa orientalis) build their nests underground or in other cavities. In the tropics, these nest may last year-round, but in temperate areas, the nest dies over the winter, with lone queens hibernating in leaf litter or other insulative material until the spring.
Hornets are often considered pests, as they aggressively guard their nesting sites when threatened. This is particularly true for hornets nesting close to human habitation, as their stings are more dangerous than those of bees.[3]
Hornets have stings used to kill prey and defend hives. Hornet stings are more painful to humans than typical wasp stings because hornet venom contains a large amount (pkp,5%) of acetylcholine.[6][7] Individual hornets can sting repeatedly; unlike honey bees, hornets and wasps do not die after stinging because their stingers are not barbed and are not pulled out of their bodies.
The toxicity of hornet stings varies according to hornet species; some deliver just a typical insect sting, while others are among the most venomous known insects.[8] Single hornet stings are not in themselves fatal, except sometimes to allergic victims.[8] Multiple stings by non-European hornets may be fatal because of highly toxic species-specific components of their venom.[9]
People who are allergic to wasp venom are also allergic to hornet stings. Allergic reactions are commonly treated with epinephrine (adrenaline) injection using a device such as an epinephrine auto injector, with prompt follow-up treatment in a hospital. In severe cases, allergic individuals may go into anaphylactic shock and die unless treated promptly.[11]
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https://en.wikipedia.org/wiki/Hornet