The fly that lets winter in

Go as far south as land goes and the insects very nearly run out. In maritime Antarctica, in mats of wet moss near the shore, there is a wrinkled grub several millimetres long. Belgica antarctica is a non-biting midge, a true fly in the order Diptera, and its larva does nearly all the living on this page.Head as far south as there is land, and insects almost run out. But in clumps of wet moss near the shore in Antarctica there is a wrinkled little grub, about as long as a grain of rice. Its name is Belgica antarctica. It is a midge -- a kind of small fly -- and it does not bite. Almost everything on this page happens while it is still a grub. Scientists call that stage a larva.

It is the southernmost recorded free-living insect, and the only free-living insect endemic to maritime Antarctica. Both words are doing work. Free-living means it makes its own way rather than riding on a host, and parasitic lice and fleas can occur farther south than it does. Endemic means it lives here and nowhere else, which rules in this midge but not Parochlus steinenii, a midge native to the same region but not only there.It holds two records. It is the furthest-south insect that lives on its own, and it is the only insect that lives on its own in this part of Antarctica and nowhere else on Earth. Both of those careful bits matter. On its own means it looks after itself instead of hitching a ride on another animal -- lice and fleas do get further south, but only stuck to a host. Nowhere else rules out a neighbour, Parochlus steinenii, another midge that lives here but lives in other places too.

It is a fly that, in practice, does not fly. The adult's wings are reduced to stubs. It is also missing the halteres that other flies use as gyroscopes to stay balanced in the air. It walks across wet ground instead of taking to the air. The larvae live in wet coastal moss, algae, and rotting organic matter, often 10 to 20 m from the shore.It is a fly that cannot really fly. The grown-up's wings are shrunk down to little stubs, and it is also missing the tiny built-in balancers other flies use to stay steady in the air. So it walks. The grubs live in wet moss, algae, and rotting plants, usually about 10 to 20 metres back from the sea.

On Cormorant Island in January 2018, one good plot held a measured maximum of 38,850 larvae per square metre. Lay a sheet of A4 paper on moss that crowded and you are covering roughly 2,400 of them. The number belongs to the plot that was sampled, not to all patches of moss. It still changes what you picture: the single wrinkled larva under a moss stem is one of a crowd living beneath your boots.On Cormorant Island in January 2018, scientists counted one patch of moss with 38,850 grubs in a single square metre. Put a sheet of paper down on moss that crowded and you would be covering about 2,400 of them. That was the busiest patch they measured, not every patch. Still, it changes the picture in your head: that one wrinkled grub under a moss stem has thousands of neighbours right under your boots.

Most of its life is spent as a larva, and much of that is spent frozen. A two-year cycle is common, though not the only one. Like other insects, the larva grows in stages called instars, each one ending in a moult; this midge passes through four of them. In the common pattern it meets its first winter around the second instar and its second winter as a fourth-instar larva. The back half of that fourth instar, labelled L4-2, enters obligate diapause: a set pause in its development the larva cannot skip, even if conditions turn mild.It spends nearly its whole life as a grub, and much of that frozen. Two years is the usual plan. Like other insects, it grows in steps, wriggling out of its old skin at the end of each one. Scientists call each step an instar, and this midge goes through four. So it usually meets its first winter in step two, and its second winter in step four. Halfway through step four it switches on a built-in pause button called diapause. It cannot skip that pause, even if the weather turns nice.

Even the good season is cold. Southern summer arrives while the northern half of the world is in winter. Across five sites watched from mid-December into late March, the moss and soil averaged 3.0 to 5.6 C. That is the summer figure, not a winter one, and it covers the midge's best months. When the moss and soil rise above freezing, the larvae move and feed.Even the good season is cold. Summer down there happens while the top half of the world is having winter. Scientists watched five spots from mid-December to late March, and the moss and soil averaged only 3 to 5.6 C -- about the temperature inside a fridge. And that is the warm half of the year. Whenever the moss climbs above freezing, the grubs get moving and start eating.

Then, in the common pattern, the adults come out together, all at once. They walk, they mate, they lay eggs in a blob of jelly, and they never eat. An adult lives roughly 7 to 14 days. Two winters of getting ready, spent in a week or two, and nearly the whole life story belongs instead to the larva and to the wet ground around it.Then, usually, the adults all appear at once, together. They walk, they find a mate, they lay their eggs in a blob of jelly, and they never eat -- not once. A grown-up lives about 7 to 14 days. Two whole winters of getting ready, spent in a week or two. Nearly the entire story belongs to the grub instead, and to the wet ground it lives in.

The larva freezes solid, and then it is fine. It is freeze-tolerant in summer and in winter alike, not only in some prepared season. Ice forms in the extracellular spaces, the gaps around and between its cells, while the cells themselves stay whole. When the thaw comes, liquid water returns to the moss and movement returns to the larva, and it carries on from where it stopped.The grub freezes rock solid, and then it is fine. It can do this in summer just as well as in winter -- it does not need to get ready first. Ice forms in the little gaps around and between its cells, and the cells themselves stay whole. When the thaw comes, the water in the moss goes runny again, the grub starts moving again, and it carries straight on from where it stopped.

It is not a brief freeze either. A temperature logger buried in occupied moss on Torgersen Island recorded one winter from mid-to-late April through mid-November. The ground stayed frozen without a break for roughly seven to eight months, mostly sitting near -1 to -3 C. A larva in that moss would have spent the better part of a year frozen solid.And it is not a quick freeze. Scientists buried a thermometer in moss where these grubs actually live, on Torgersen Island. It stayed frozen without a single break from around late April to mid-November -- seven to eight months -- mostly sitting at about -1 to -3 C. A grub in that moss spent most of a year as a solid lump.

A second record, taken 1 cm down, found 0 to -2 C for more than 300 days, with -7 C reached on only two occasions. Dull, in other words, and nowhere near as savage as the word Antarctica suggests. The wet moss is what buffers it. These two records come from different places and are not readings of the same winter, but together they show what a larva's winter can be: long, frozen, and buffered.A second thermometer, 1 cm down, recorded 0 to -2 C for more than 300 days, and only dropped to -7 C twice. That is boring, and far less brutal than the word "Antarctica" makes you expect. The wet moss works like a blanket. These two thermometers were in different places and not in the same winter, so they are not one measurement -- but together they show what a grub's winter is like: long, frozen, and gentler than you would guess.

Drying a larva out under controlled conditions can leave it better at surviving a freeze later, which is a strange thing for drought to do for you. In one test, larvae collected in summer spent 48 hours at 98.2 percent relative humidity and +4 C. That is damp air by any normal standard. It still pulled water out of them slowly: they lost about 30 percent of their osmotically active water, the part free to move in and out rather than bound up inside the tissue.Here is the odd bit. If you dry a grub out first, it gets better at surviving a freeze afterwards. Drying out should not help you cope with ice -- but it does. In one test, grubs collected in summer sat for 48 hours in air at 98.2 percent humidity and +4 C. That is very damp air, like a bathroom after a hot shower. Even so, it slowly pulled water out of them. They lost about 30 percent of the water that can move freely in and out of their bodies.

Then came the cold. All of the slowly dried larvae survived three days at -10 C. Of the larvae that had not been dried, fewer than one in ten survived even two days at -10 C -- a shorter test, and still far worse results. The obvious answer is wrong, too. Drying left the supercooling point near -9 C in both groups, so the dried larvae were not holding ice off any better than the others. The protection did not come from holding ice off. What did produce it, the study did not establish.Then came the cold. Every single slowly-dried grub survived three days at -10 C. Of the grubs that had not been dried, fewer than one in ten survived even two days -- a shorter test, and they still did far worse. And the obvious explanation turns out to be wrong. In both groups ice still started at about -9 C, so the dried grubs were not keeping ice away any better than the others. Something else was protecting them. The scientists could not work out what.

Three different things in this chapter all involve losing water, and they are worth keeping apart. Plain desiccation is simply drying out. Prior drying is the rehearsal above, where losing water first leaves a larva better able to survive a freeze later. Cryoprotective dehydration is the third and the strangest: a cold larva sitting near ice loses water to that ice, bit by bit, and by giving up the water it never freezes at all. Only that last one is offered as a whole-winter route, and it is the one this page ends by questioning.Three different things in this chapter involve losing water, and they are worth keeping straight. Drying out is just drying out. Drying out first is the practice run above, where losing water helps you survive a freeze later. The third is the strangest: giving your water away to ice. A cold grub sitting near ice slowly leaks its water into that ice, and because it has handed the water over, it never freezes at all. Only that third one is offered as a plan for a whole winter -- and it is the one this page ends up doubting.

When Kelley's team sequenced this midge, they found a haploid genome of about 99 Mb, roughly 99 million letters of DNA. For an insect that is very small. What it was missing is the interesting part: hardly any repeating stretches, few of the parasitic sequences that copy or move themselves around a genome, called transposable elements, and unusually short introns, the non-coding gaps that sit inside genes. Their 2014 paper described it as the smallest sequenced insect genome reported in 2014.Every living thing carries an instruction book written in DNA. When Kelley's team read this midge's book, it turned out to be only about 99 million letters long. For an insect, that is tiny. What was missing is the interesting part. Most animals' instruction books are stuffed with repeated bits, with freeloading passages that copy and paste themselves all over the place, and with long gaps sitting inside the useful parts. This midge had hardly any of that. In 2014 their paper called it the smallest insect instruction book anyone had read so far.

That record has since fallen. A 72.1 Mb assembly for Xenos peckii, a twisted-wing parasite, was published in 2024. It is tempting to link the tidy genome to the hard life, and that temptation should be resisted. A stripped-down genome and freeze tolerance turn up in the same insect here, but nothing in the work shows that one produced the other.That record has since been beaten. In 2024 a twisted-wing parasite called Xenos peckii came in smaller still. And it is very tempting to say the midge has a short, tidy instruction book because its life is so hard. Do not. Both things are true of this insect, but nobody has shown that one caused the other.

The way the larva handles stress is spare in its own way. Heat-shock proteins are a cell's repair kit for trouble, normally ramped up sharply when something goes wrong. In the larvae Rinehart's team tested, hsp70, hsp90, and a small heat-shock protein were being made all the time. The alarm was already sounding, and the heat and cold shocks they tried did not make it louder. That was true of the larvae in those particular tests; the adults did not behave the same way.The way it handles trouble is stripped-down too. Cells carry a repair kit -- proteins that most animals switch on hard the moment something goes wrong. In the grubs Rinehart's team tested, three of those repair proteins were being made all the time. The alarm was already ringing. Heating the grubs up or chilling them down did not make it ring any louder. That was what happened to grubs in those particular tests. The grown-ups did not behave the same way.

Cryoprotective dehydration is real, and this midge can do it. In the founding laboratory experiment, fourth-instar larvae held near a physically separate piece of ice gave up their water to it, never froze, and lived. The same paper found something that muddies the story: in wetter soil, ice touching a larva simply seeded ice inside it, a process called inoculative freezing.Giving your water away to ice is a real thing, and this midge really can do it. In the first lab experiment, grubs were put near a lump of ice they were not touching. They slowly handed their water over to it, never froze, and lived. But the same paper found something awkward: in wetter soil, ice touching a grub simply started ice off inside the grub.

So which route is even available turns on moisture. In the wet moss where these larvae really do spend the winter, ice presses against the larva from all sides and seeds freezing directly, leaving little room for the dehydration route. Dry conditions with the ice held physically apart, of the kind a laboratory can arrange, leave that route open. Nobody has published a moisture line where one takes over from the other -- there is no published universal cutoff -- and rather than invent one, the laboratory below refuses to answer in the middle.So which route is even possible comes down to how wet it is. In the soggy moss where these grubs really do spend the winter, ice presses in from every side and starts them freezing straight away, leaving little room for the drying-out route. Dry conditions with the ice kept apart -- the sort of thing you can set up in a lab -- leave that route open. Nobody has ever published the exact wetness where one takes over from the other. So rather than make a number up, the machine below refuses to answer in the middle.

Someone finally ran the two side by side. Yoshida's team took second-year diapausing fourth-instar larvae, 55 per treatment, and held them at -5 C for six months: one group frozen, one group cryoprotectively dehydrated. The frozen larvae won, and not by a little. Of the frozen group, 41.8 percent were alive at six months and 23.6 percent went on to become adults during the following 50-day scoring window. Of the dehydrated group, 9.1 percent were alive, and not one of them became an adult.Finally, someone tested the two side by side. Yoshida's team took 55 grubs for each route -- all of them second-year grubs on their built-in pause -- and kept them at -5 C for six months. One group was frozen. The other was dried out next to ice. The frozen ones won, and not by a little. Of the frozen group, 41.8 percent were still alive after six months, and 23.6 percent went on to become grown-up flies. Of the dried-out group, 9.1 percent were alive -- and not one ever became an adult.

Read those numbers carefully before spending them. The treatments were held at one constant temperature, and the dehydrated one was severe. The two groups were also cooled in different ways, and becoming an adult depends on breaking diapause as well as on surviving. So these are not field survival rates, and they are not a verdict on all ways of drying a larva out. What they do rule out is the tidy story -- that the clever move is to dry out so that nothing can freeze. Given six months of it, the larvae that surrendered to the ice were the ones still alive at the end.Now read those numbers carefully before you spend them. All of this happened in a lab, held at one steady temperature, and the drying was harsh. The two groups were also cooled in different ways. And becoming an adult needs the grub to wake up from its pause, not just to stay alive. So these are not the numbers for real grubs out on real ice, and they do not settle every way of drying a grub out. What they do knock down is the neat story -- the idea that the clever move is to dry yourself out so nothing can freeze. Given six months of it, the grubs that gave in to the ice were the ones still alive at the end.