Spanish Fly (Blister Beetle)

Family Meloidae — the blister beetles — contains about 7,500 species worldwide and is famous for two unrelated extraordinary biological traits. First, all blister beetles produce cantharidin — a terpenoid vesicant that causes severe painful blistering on contact with mucous membranes or skin (the source of the family name). Cantharidin from the European Lytta vesicatoria (the 'Spanish fly,' actually a beetle) was used since ancient Roman times as an alleged aphrodisiac, despite causing severe poisoning, kidney failure, and death in even modest doses. The compound is still used clinically today as a topical wart treatment (Cantharone, Canthacur). Cantharidin is also famous as the historical murder weapon of choice in late Renaissance European intrigue. Second, blister beetles undergo HYPERMETAMORPHOSIS — multiple distinct larval body plans across larval development, in contrast to the simple larva-pupa-adult progression of most insects. The first instar (called the 'triungulin') is a small, highly mobile, three-clawed wandering larva that climbs flowering plants and hitchhikes on visiting solitary bees, riding the bee back to the bee's nest. Once in the nest, the triungulin moults into a sedentary grub that consumes the bee's eggs and then the pollen-and-nectar provisions. Subsequent instars include a 'coarctate' diapause stage and a final pre-pupal grub. This bee-parasitism life cycle makes blister beetles significant pests of native solitary bee populations, and infested orchards lose substantial managed mason bee, leafcutter bee, and ground-nesting bee broods to blister beetle larvae each year.