The fairy wasp, called Caraphractus cinctus, is a member of the Mymaridae family of parasitoid wasps. It might be small and dainty like a fairy, but it is far from nice – especially if you are a diving beetle.
Fairy wasps belong to a particularly dastardly supergroup of parasitoid wasps called chalcids, which target other invertebrates.
Their secret weapon is that they are extraordinarily small, which means they fit into impossibly small places. From their larvae to their pupae, they find their niche. Some famously parasitise butterfly chrysalises, causing the caterpillars to burst forth with a wriggling brood of maggots, something even Ridley Scott might deem implausibly violent.
The Mymaridae take things a step further: they parasitise insect eggs. After all, an egg contains all the raw resources to support a developing insect’s life, so why not steal them?
Fairy wasps are an evolutionary exercise in extreme miniaturisation. Because they are so small, very little is known about them. While many adults have been discovered – they were popular subjects of Victorian microscopists, who sought them out for their mounted slides – the specific details of many life-cycles and hosts remain a mystery.
Of the 87 British species, Caraphractus cinctus is one of the best studied, and while at around 0.25mm in length it isn’t the smallest of the fairy wasps, this parasitoid predator of water beetle eggs is truly remarkable. For starters, underwater is the last place you would expect to find any winged insect, but a species so minute and with delicate hair-fringed, spoon-shaped wings just doesn’t make sense.
The main focus in life for a Caraphractus cinctus is finding water beetle broods. The eggs – lozenges of developing life – are already well hidden from view, inserted into the stems of submerged water plants. But find them it does. The females scamper around the verdant underwater jungles, moving through any open water using their legs and paddle-shaped wings. When they locate an egg, presumably by some chemical trace, they will first scrutinise it for suitability. Any egg that’s already been parasitised will be ignored in favour of one that is uncontaminated by malignant life within.
Once the perfect vessel is alighted upon, the wasps penetrate its shell with a needle-like ovipositor (egg-laying tube). This micro-syringe is, in effect, the sting, and is loaded with potent venom. What it does is unclear: it isn’t needed to subdue the prey, but it is thought to advertise the egg as ‘taken’ to other wasps that might stumble upon it. When satisfied, they inject their eggs.
Weighing just 0.0002mg each, these are the lightest eggs known in nature. They are so tiny that up to 100 can be deposited in a single beetle egg. Here, the larvae hatch and complete their development through to adulthood. They will even mate within the claustrophobic confines of the egg before emerging to start their own devious and miraculous life-cycle.
To think that a single beetle egg, less than a millimetre across, contains enough nutrients to support an entire life is surprising enough, but to believe that it might support up to 100 lives starts to boggle the mind. Yet they do, and this is happening all around us – but because it is at such a small scale, we rarely notice it.
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