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Snow flea facts

Sep 01, 2023

Nick Baker takes a look at one of our toughest and fascinating winter insects, the snow flea

There I was, in December – I had pulled over in a patch of heathy woodland to answer a call of nature when I caught sight of movement on the ground by my feet. It was winter – what could it be?

On closer inspection, I realised I had found my first (and, to date, only) snow fleas, Boreus hyemalis. The frigid winter months are not known for entomological exuberance. Wings are a ‘no-no’, unless you’re the toughest of midges or moths. That's probably why the snow flea, one of our most bizarre insects, is wingless.

Despite its name, it isn't a flea or a fly. It belongs to the scorpion flies, a group of insects, mostly denizens of summer hedgerows. Britain has three other species of scorpion fly, all bedecked with picture-frame wings. The males possess a curious swollen genital capsule, which is heldat the tip of their slim abdomen and curls backwards, producing a threatening look, reminiscent of a scorpion's tail.

The snow flea lacks pretty wings or outwardly distinctive genitals. But it has the other diagnostic feature of this family of insects – a long ‘face’. In reality, this is an elongated rostrum, or beak, with mouthparts at the tip.

A fly that can't fly sounds like an oxymoron. Yet it doesn't stop the snow flea getting about, which it does by walking or by jumping up to 5cm in a single bound. Though it has no obvious external adaptations for the saltatorial task of leaping, the snow flea does have long legs and the use of an elastic protein called resilin.

This acts like an internal catapult and is the same stuff that gives actual fleas their bounce – together with snow fleas’ small size (around 5mm), it explains at least part of the common name.

The rest of the moniker probably refers to the fact that the species is usually noticed against snow. These insects live mainly in mossy subnivean (below the snow) habitats, occasionally gathering on the surface. They don't need snow but they do best in regions with harsh winters.

As insects, they are ectothermic (cold-blooded), so cannot generate body heat. Rather than be at the mercy of the cold and its cell-rupturing ice crystals, they have the ability to supercool. They lower the freezing point of fluids in their cells, using special proteins and polyhydric alcohols, so that ice crystals don't form. Being active in cold conditions is worth it – there is less riskof meeting predatory insects.

The pair I had disturbed were pretty much the only things moving in the frozen landscape, so got my full attention, but I noticed something odd. The female – easily recognised by her long ovipositor (egg-laying tube), which gives her abdomen a sharp point – was on the backof the male.

It's not strictly true that male snow fleas have no wings – they are reduced to strange, spine-like projections – and this male was busy using his like a large pair of sexual tongs. He had rotated them behind his thorax, and clamped them about the head capsule of the female, holding her in position for mating.

These amazing little insects are an entomological highlight of winter, for those determined enough to look.

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