Hornets are the largest eusocial wasps, reaching up to 45 millimetres (1.8 inches) 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). See wasp and bee characteristics to help identify an insect.
A Hornet's 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 injection using a device such as an EpiPen, with prompt followup treatment in a hospital.
Hornets, like many social wasps, can mobilize the entire nest to sting in defense: this is highly dangerous to humans. The hornet alarm pheromone is used to raise alarm of nest attack, and to identify prey, for example 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 attack.
Hornets and yellowjackets prey on many insects that are considered to be pests, so are actually beneficial. They do also prey on bees, but unlike honey bees, hornet and yellowjacket colonies die out every winter.