Thunderworms (Mermis spp) are pale, thread-like worms, up to about 12cm long, that seem to appear from nowhere after heavy downpours in the spring and early summer.
Yesterday morning, after two days of persistent rain and warm temperatures, this thunderworm was draped across the flowers of a thyme bush.
Thunderworms are parasites, but not of humans – they’re completely harmless to us and pets*: they parasitise insects.
For most of their lives these thread-like worms live underground, but they need to surface to lay their eggs. They can only do that during very damp conditions so that they don’t dry out – hence they’re only found after rain (or irrigation). The adult female worm climbs as high as it can to lay its eggs on plant material in the hope that the eggs will be eaten by an insect munching on the vegetation. The eggs will then begin their development within that insect, before migrating into the soil to spend a couple of years growing to maturity there.
The name thunderworm came about because the deluges that the worms need to emerge are often accompanied by thunderstorms. Once upon a time it was thought that the worms actually came down from the sky with the rain or the lightning.