Treasure of the monthA fat hummer that does not fly

20 January 2020

Photo: Heiko Bellmann/Frank Hecker

Quite bloated: the female mayworm carries up to 9,500 eggs in her abdomen.

Our January treasure has been part of our culture for 4,000 years and has now been chosen as the Insect of the Year 2020: the black-blue oil beetle (Meloe proscarabaeus) or also popularly known as the "May worm". Its cover wings cover only a small part of the abdomen and are unfortunately unsuitable for flying. Instead, it produces the irritant toxin cantharidin, which has found many uses throughout human history. The LIB collection includes several specimens of the species Meloe proscarabaeus, which have been found mainly in Hamburg and the surrounding area. The oldest finds were collected around the year 1900.

Love potions, poison murders and labor patches
Oil beetles are a diverse family of beetles with 7500 species; one species within the oil beetles is the "Spanish fly". The poison of this oil beetle is already mentioned in an ancient Egyptian papyrus from 1550 BC. Processed into a plaster it should produce contractions and facilitate births thereby. Dissolved in honey, the poison of the "Spanish fly" is said to act as an aphrodisiac, i.e. to increase potency. However, if mixed incorrectly, this tonic is more likely to lead to certain death. The poison of a single beetle is enough to send an adult human being to eternal sleep. This potential was already recognized by the ancient Greeks, who used it for executions.

Not only humans want the "oil" of the beetle
The oil beetle uses cantharidin for self-defense against ants and ground beetles: when threatened, it secretes a yellow liquid from pores on its leg joints that strongly resembles oil droplets. This is how the beetle group got its pithy name. However, certain predators, such as hedgehogs or birds, are immune to the venom. Some species of beetles, bugs and midges are even attracted to it and use it for their own defense. Gnats are often found between the beetle's abdominal rings trying to get at the coveted defensive fluid. The hind legs are the only weapon in protection against these pesky parasites - which, however, usually remains ineffective.

No offspring without a host
A single female can produce up to 9,500 eggs at intervals of up to two weeks and carry them around in her abdomen. She can repeat this process up to six times. Despite this enormous reproductive power, the oil beetle is on the red list of endangered species in Germany. This is because the larvae depend on ground-nesting wild bees to grow out. They climb onto flowers and wait there for the right flight opportunity. If everything works out, a wild bee brings the oil beetle larva into its nest, where it feasts on the eggs and pollen stores of its host. After overwintering in the soil, the beetles hatch between March and May. Unfortunately, something often goes wrong during this process, so experts estimate that only one in a thousand larvae becomes an adult oil beetle.

Threatened with extinction
It takes a total of two years for an egg to become a finished insect. Fully developed, the life span is only about one month. Today, more than 30 species of the oil beetle family live in Central Europe. The May worm is the most common. NABU attributes the population decline primarily to habitat loss and road traffic. Because development from egg to adult is so prone to disturbance, even small changes can lead to major losses. Marianna Simoes, curator in LIB's Entomology Department, welcomes the selection as Insect of the Year of the Senckenberg German Entomological Institute (SDEI) in Müncheberg: "The oil beetle shows that not only we humans are highly dependent on bees. With the extinction of bees, not only we humans, but numerous fascinating animals are in great danger."