[♪ INTRO].

You might say fairies aren’t real,  but fairyflies beg to differ. They’re flying around us all the time, but they largely go unnoticed thanks to  their own kind of magic: miniaturization!

This group includes the  smallest insects on the planet! And scientists are eagerly  studying them in the hopes of learning more of their teeny tiny secrets. If you haven’t heard of fairyflies,  it’s not because they’re rare.

There are around 1400 species  of these delicate little wasps. That’s right, they're wasps,  not flies. Poor choice of name.

Anyhow, they live practically everywhere. It’s just that you don’t see  them, because...they’re tiny. Most species are about a millimeter long.

And the smallest ones are around a  sixth or a seventh of a millimeter — roughly the size of a single-celled paramecium! What makes this especially  mind-blowing is that, as creatures go, flying insects are actually pretty complex. They’ve got most of the same organ systems we do.

Plus wings and cute little articulated legs. That fairyflies can pack all of that into a body that’s literally microscopic feels supernatural. Now it’s not, of course — it’s  just some extreme adaptation.

In fact, experts think fairyflies may  be about as small as insects can get. And they got some pretty neat tricks  to reach such minuscule proportions. For starters, their bodies are a little  simpler than those of their larger kin.

Like, they have most of the normal parts, but structures like legs and  antennae tend to have fewer segments. Their circulatory and respiratory  systems can be a lot simpler too. Since gases and nutrients don’t have  as far to go, fairyflies can rely more on diffusion.

And the smallest species  forgo blood vessels and even hearts. But not everything can be simplified. In some cases, being small  means making compromises.

Take sight, for instance. Since space  is so limited on their tiny heads, they’ve cut down the size and number  of facets in their compound eyes. Each one is now so small that it’s near  the limit of what’s physically needed to bend and absorb light, so it  doesn't capture the clearest images.

And fewer of them means they have  lower visual acuity than most insects. So, they rely more on smell to get around. The males of some species have  actually ditched their eyes altogether, along with other bits of their bodies.  Like their wings.

And their mouthparts. Because if you live just a few  days, you don’t need to eat! That gets them down to just  one-seventh of a millimeter — making them the smallest adult insects period.   It’s not just the super-micro  males that have ditched wings.

Some whole species do without  them, especially ones that live in windy habitats, like on oceanic islands. The wings of the ones that do fly are much  smaller and cuter than other insect wings. They’re oar-shaped, with a frill around the edge that looks like something between  a row of eyelashes and a feather.

These bizarre wings likely work  because, when you’re that small, the air feels thicker — more like a fluid. So experts think those frills act like a paddle, allowing them to have effectively  bigger wings while sparing weight. They also probably reduce turbulence and drag.

Mostly, though, fairyflies get small  by having fewer and littler cells. For instance, some of their cells  super-condense their DNA to shrink their nuclei. But the nervous system poses  a particular challenge.

Fairyflies have already ditched  as many neurons as they can. The tiniest species have just 7400 neurons  total—which is already pretty minimal, when you consider that flies and  bees have hundreds of thousands. But that alone is not enough to make their brains small enough to fit in their tiny heads.

And they can’t lose any more  neurons because the ones remaining have essential jobs that can’t  be eliminated or combined. I mean, you can’t have just  one nerve that does everything. They also can’t shrink their  neurons down any further.

They’ve already squeezed out any extra cytoplasm. And the long, skinny axons of each neuron  are as thin as they can be, because axons only can get so small before they  stop being able to reliably send signals. So, to get even more compact, they’ve  done something kind of drastic: most of the neurons have jettisoned  their space-hogging nuclei.

Yeah, I said nuclei. They ditch them just before they mature. And that means the adults rely on  the proteins they made as juveniles to sustain them for the rest of their lives!

Which is only like a week, so it works out. Fairyflies’ final big miniaturization trick lets them down-size their reproductive systems. You see, it takes a lot of  energy to grow an embryo.

So most insects make eggs with  big, nutritious stores of yolk. But when you’re tiny, this can be a huge  burden. Like, in the smallest beetle, each egg is more than half the  length of the mom’s entire body!

Fairyflies have evolved a macabre  solution to this: a parasitic lifestyle! Well, technically, they are parasitoids,  because ultimately they kill their hosts. They lay their eggs inside  the eggs of other insects so their offspring can mooch nutrients from  the other egg’s yolk and eat the embryo.

And that ultimately lets the  fairies lay much smaller eggs. It’s not so great for the other insects, of  course. Though, it’s kind of useful to us.

You see, fairyflies parasitize many different  hosts, including agricultural pests. So some are already used by humans  as natural forms of pest control! And further research on them  could reveal other useful species.

That’s just one reason scientists  are eagerly studying fairyflies. Others hope that learning more of  their tricks for scaled-down living will ultimately help us understand the  lower bounds of multicellular life. And they could show us how  to miniaturize our world.

Like, since their brains pack maximal  complexity into a super tiny size, scientists think they could teach  us a lot about brain circuitry, or even inspire designs for smaller computers! So the next time someone says they don’t  believe in fairies, let them know that they’ve been around fairies their whole  life. They’ve just overlooked them.

Because they’ve been laying their  eggs inside of other insect eggs. Thanks for watching this episode of  SciShow! We hope you enjoyed learning about the magical, miniature world  of these real-life fairies.

I definitely did! That’s very weird,  they eject the nuclei from their neurons! If you did enjoy it, I have a feeling  that you will also like our episode on weird phenomena that science can  explain.

So maybe watch that one next! Plus, there’s always that little subscribe button if you need more awesome  science in your life in general. [♪ OUTRO].