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Hornets are the largest eusocial wasps; some species can reach up to 5.5 cm (2.2 in) in length. The true hornets make up the genus Vespa and are distinguished from other vespines by the width of the vertex (part of the head behind the eyes), which is proportionally larger in Vespa and by the anteriorly rounded gasters (the section of the abdomen behind the wasp waist). 

Hornet Nests
In Vespa crabro, the nest is founded in spring by a fertilized female known as the queen. It generally selects sheltered places like dark hollow tree trunks. It first builds a series of cells (up to 50) out of chewed tree bark. The cells are arranged in horizontal layers named combs, each cell being vertical and closed at the top. An egg is then laid in each cell. After 5-8 days, the egg hatches, and in the next two weeks, the larva undergoes its five stages. During this time, the queen hornet feeds in a protein-rich diet of insects. Then, the larva spins a silk cap over the cells opening and, during the next two weeks, transforms into an adult, a process called metamorphosis. Then, the adult eats its way through the silk cap. This first generation of workers, invariably females, will now gradually undertake all the tasks that were formerly carried out by the queen (foraging, nest building, taking care of the brood, etc.) with one exception: egg-laying, which remains exclusive to the queen.

Hornet Life History
As the colony size grows, new combs are added, and an envelope is built around the cell layers until the nest is entirely covered with the exception of an entry hole. At the peak of its population, the colony can reach a size of 700 workers, which occurs in late summer.

At this time, the queen starts producing the first reproductive individuals. Fertilized eggs develop into females (called gynes by entomologists), and unfertilized ones develop into males (sometimes called drones�). Adult hornet males do not participate in nest maintenance, foraging, or caretaking of the larvae. In early to mid-autumn, they leave the nest and mate during nuptial flights Males die shortly after mating. The workers and queens survive at most until mid to late autumn; only the fertilized queens survive over winter.

Hornet Stings
A hornets sting is painful to humans, but the sting toxicity varies greatly by hornet species. Some deliver just a typical insect sting, while others are among the most venomous known insects. Allergic reactions, fatal in severe cases, can occur; an individual suffering from anaphylactic shock may die unless treated immediately via epinephrine (adrenaline�) injection using a device such as an EpiPen, with prompt followup treatment in a hospital.

All hornet stings* are an allergen for people with an allergy to wasp venom.* those allergic to wasp venom are not necessarily allergic to bee venom as they contain different chemicals.* are more painful than a typical wasps due to a large amount (5%) of acetylcholine.

As in all stinging wasps, hornets can sting multiple times; they do not die after stinging a human as is typical for a worker honey bee, as a hornets sting is not barbed (the honey bee stinger has evolved for defending the nest against vertebrates/mammals). They can also bite and sting at the same time.

Hornets, like many social wasps, can mobilize the entire nest to sting in defense, which is highly dangerous to humans. The hornet alarm pheromone is used to raise alarm of nest attack and to identify prey, such as bees. It is not advisable to kill a hornet anywhere near a nest, as the distress signal can trigger the entire nest to attack. Materials that come in contact with pheromone, such as clothes, skin, dead prey, or hornets, must be removed from the vicinity of the hornets nest. Perfumes and other volatile chemicals can be falsely identified as pheromone by the hornets and trigger an attack.

Hornets prey on many insects that are considered to be pests. They do also prey on bees, but unlike honey bees, hornet colonies die out every winter.