Sources : Hornet

Pliny the Elder [1st century CE] (Natural History, Book 11, 23-24): [Book 11, 23] ...wasps and hornets are brought to life from horses' bodies... [Book 11, 24] Wasps make their nests high up, of mud, and in them make cells of wax; hornets make them in caverns or underground; all of these have hexagonal cells, and make their combs of bark, like spiders' webs. The actual offspring are not uniform but vary - one flies out while another is in the pupa and another in the grub; and all of these stages are in the autumn, not the spring. They grow chiefly at full moon. ... The forest variety of hornets live in hollow trees, hibernating in winter like the rest of insects; they do not live beyond the age of two. Their sting is rarely not followed by fever. Some authorities state that twenty-seven hornet-stings will kill a human being. Another kind that seems less fierce has two classes - workers, smaller in size, which die in winter, and mothers, which last two years: these are not fierce at all. They make nests in spring, usually with four entrances, in which to breed the workers. When these have been reared, they then make other larger nests, in which they may now produce those who are to be mothers. Then the workers begin to function, and feed the mothers. The mothers are of a wider shape, and it is doubtful whether they possess stings, because they do not come out. These also have their drones. Some people hold the view that all these insects lose their stings towards winter. Neither the hornet nor the wasp kind have kings, nor do they swarm, but their numbers are continually renewed by offspring. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 8:5): The hornet [scabro, crabro] is named from cabo, that is, from the pack-horse (caballus), because it is created from them. ... hornets are born from the rotting flesh (caro) of horses. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.17): [Thomas repeats some of this in his description of the wasp, and uses some the same sources there.] Hornets [crabrones], as Pliny says, live in caves or under the ground. The cells of these are hexagonal, and the waxes are of the bark [cortice], like spider webs. The offspring are not uniform and uncivilized, one flies, another is in the water, another remains a worm. They eat meat. They grow in the full moon. They are hidden in winter. Authorities say the a child will be fatally stung by nine hornets. Hornets and wasps do not have kings, as bees have, but are ruled by their own government, from which it happens that they are exposed to many dangers. They make horrible noises and murmurs, and more so in hollow places. Their honey is useless for human use, and any who wishes to steal it exposes himself to foolish danger, for he is hurt with their stings. - [Badke translation/paraphrase]