Nick Baker’s Hidden Britain
The popular naturalist, author and TV presenter reveals a secret world of overlooked wildlife
Baby love
MICRODON HOVERFLY
Clever adaptations allow the larvae of this parasitic hoverfly to happily munch on ant grubs undetected
A FLESHY DISC WITH THE APPEARANCE of a rubbery drop-scone slides slowly forward. Around it, ants dash in a panic, seeming to tend to the imposter’s every need.
Parting the grasses on this ant hill, I had expected to see the usual brood: specks of eggs; crescents of larvae; and the lozenges of cocoons. But this was the oddest thing. Was it an animal, a plant or a fungus? Well, it had to be an animal as it was moving.
What I had found was the rarely seen larva of a hoverfly called Microdon myrmicae. The adult insect is rather unassuming – abig-eyed hoverfly with charismatic thickened antennae and a truncated body. Quite cute, by hoverfly standards.
But its benign appearance belies its dastardly intent. This hoverfly is a social parasite of various species of ants, particularly those of Myrmica scabrinodis, a red ant species that is widespread in grassland habitats throughout the UK.
Back at the height of summer, fired up by the warmth of the sun, the adult hoverflies mated. The female then sought out the nest of these ants, depositing her eggs at the entrance of the colony.
After that, not much is known about their life-cycle. Do the ants take the eggs into the nest thinking they are their own brood? Or do they sit there ignored until they hatch and then make their own way into the dark confines of the brood chambers? Howsoever it happens, that is where they end up, eating ant larvae and pupae right out of their cradles.
The curious larval form of Microdon is adaptated to this way of life. The hemispherical disc doesn’t give much purchase to the ants’ jaws, while the tough rubbery cuticle acts like armour. A fringe of fleshy tassels skirts the edge of the Microdon’s body. This, and its low profile, create a challenge for any ants wanting to get underneath. Even if they did succeed, they wouldn’t find any handy legs to grab as the larva doesn’t have any. Instead, the underside is covered in microscopic hairs on which it moves using muscular waves – peristaltic pulses that are a little bit like those of a slug. It also produces a mucus that helps it slide around.
“When it reaches its victim, it saws into it with a pair of tiny, toothed blades”
When on the move and hunting for prey, the tiny head (or pseudocephalon) can be seen swinging from side to side. When it reaches its victim, it lifts the front edge of its body over the ant larvae or pupae and saws into it with a pair of tiny, toothed blades. When the prey has been violated, its liquid contents are sucked up. The larva will graze on the ant babies like this for two years.
It’s thought that Microdon gets away with it by inheriting the chemical scent of the ant colony from its own parents. There is evidence, indeed, that the hoverfly is only successful in completing its life-cycle if it finds itself in a nest closely related to the one from which its parents emerged – if not the exact same nest.
Scientists have shown that moving the larvae to more distant nests results in them being attacked by their hosts. It’s good news for the ants as the Microdon’s reliance on an invisible and essential chemical signature might keep the odds balanced in their favour, going someway to explain the patchy distribution of a hoverfly whose hosts are highly widespread.
There are two species of Microdon in the UK: Microdon myrmicae and M. mutabilis. Almost identical, they can only be told apart by differences in their larvae and pupae.
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No place like home
Hoverflies have eclectic lifestyle choices. There is the drone-fly (Eristalis), whose rat-tailed maggots snorkel around in a soup of rotting vegetation; the hornet mimic (Volucella zonaria), whose larvae grow up inside wasps nests; and the bumblebee mimic (Volucella bombylans), whose larvae develop within bumblebee nests. But the prize for the grossest hatching location goes to the common snout (Rhingia campestris), whose maggots spend their larval days in cowpats.