Hornets are the largest eusocial wasps, that reach 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).

Life cycle

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. This occurs in late summer.

At this time the queen starts producing the first reproductive individuals. Fertilized eggs develop into females (called "gynes" by entomologists), unfertilized ones into males (sometimes called "drones"). Adult 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.

Other temperate species (e.g. the yellow hornet, V. simillima, or the Oriental hornet, V. orientalis) have similar cycles. In the case of tropical species (e.g., V. tropica), life histories may well differ; and in species with both tropical and temperate distributions (such as the Asian giant hornet, Vespa mandarinia), it is conceivable that the cycle depends on latitude.

Stings

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 ("adrenaline") injection using a device such as an EpiPen, with prompt followup treatment in a hospital.

European hornet sting

• in itself is not fatal except sometimes to allergic victims.
• multiple stings (several hundred) may be fatal due to the amount of venom (similar to wasps and bees).Fact: date=February 2007
• is similar to a wasp's.
• is less toxic than a bee sting.

Non-European hornet sting

• in itself is not fatal except sometimes to allergic victims.
• multiple stings (a nest full) can be fatal due to highly toxic species-specific additions in the venom.
• is more toxic than a wasp's or bee's sting
• from the Asian giant hornet (Vespa mandarinia japonica) is the most venomous known (per sting).