When you think of insects first taking to the  air, you might imagine flowers luring them there.

After all, the story of plants evolving  to attract pollinators is a familiar one.

But there’s one problem – insects  developed powered flight more   than 100 million years before  flowering plants even evolved.

So if it wasn’t about finding  food, why did they do it?

Well, it might have been a different  relationship, millions of years older,   that pushed them into the air: escaping  from a particular kind of predator.

It looks like insects first  flew to avoid becoming food.

See, spiders and their ancestors have been  driving an arms race that began before either   they or insects stepped foot onto land and  resulted in the first powered flight on Earth.

But how did this competition of webs versus   wings drive such a massive evolutionary  adaptation into an entirely new realm?

For the same reason –it turns out – that  spiders today are abandoning their aerial   webs and returning to the ground  as a new type of wings take over.

Spiders belong to the group Chelicerata,   which got its start in the Cambrian  seas over 500 million years ago.

The early chelicerates were predators, preying  on crustaceans – the ancestors of insects.

Eventually, the pressure from these  predators likely drove some crustaceans   into the intertidal zone, seeking refuge  and stashing their eggs to protect them.

But these ancient spider ancestors  had some traits that made it easy   to follow their prey as they  made their way toward land.

Their segmented bodies and exoskeletons helped  them move around without the aid of water,   and they had flappy appendages called book gills  that allowed them to breathe through gas exchange.

For example, marine chelicerate relatives that  are alive today – like horseshoe crabs – can   spend long stretches of time on land, as  long as their book gills remain moist.

Spider ancestors could probably do this too.

These spider ancestors also likely  excreted a precursor to silk,   which may have acted as a protective bubble,   like a ‘space suit’ that kept their book  gills wet – easing their transition to land.

At first, their crustacean prey probably  stuck to intertidal and damp environments   because they also needed the water to breathe.

But eventually, both the ancestors of insects   and the chelicerate predators chasing them  began co-radiating into a wide range of forms.

We know that they were radiating at  the same time because scientists see   a statistical relationship in the  relative abundance of each group.

Using statistical methods to track  the groups helps us understand   the complex relationships between  predator and prey in the deep past.

And when spider ancestors diversify  in ways that tightly correlate with   crustacean diversification, this points to a  real world relationship – telling scientists   that these two groups were likely  radiating in response to each other.

Methods like these can help paint a picture  of what was happening when well preserved   fossils of these little creatures are rare.

Luckily, we do have some fossil remains, too.

The first fossils that most likely  represent insects are from Scotland,   dating to the Early Devonian Period  between 400 and 412 million years ago.

And the first truly spider-like relative  followed, with the oldest known fossil,   Attercopus, coming from New York and  dating to about 385 million years ago.

This land-dweller was making silk  – and we know this because the   fossil shows that Attercopus had silk  excreting organs, known as spigots.

But they weren’t making webs just yet.

They  seem to have lacked the appendages that spigots   need to sit on – known as spinnerets –  to help pull that silk into a thread.

Instead, Attercopus was likely using  silk to line its burrow, supporting   its structure – or maybe to help navigate  with safety lines, or to protect its eggs.

And the presence of Attercopus on  land and its ability to make silk   meant a new stage was unfolding for  the race between predator and prey.

Having now driven insect ancestors to land,   the predators began exploring new ways to  pursue their prey, looking toward the air.

Now, there’s a gap in the fossil record for  both spiders and insects that makes it hard   to trace the exact timing of when powered  flight took off and web development began.

But the next true insect appears  in the early Carboniferous Period,   around 320 million years ago, in the form  of the dragonfly-like Delitzschala.

And within the next ten million years,   flying insects became common – with  spiders’ abundance keeping pace.

It’s around this time that the fossil record  reveals the first true spider, Arthrolycosa.

And while we haven’t yet found any  fossilized webs from this time,   Arthrolycosa had the anatomy  necessary for weaving.

The spigots and spinnerets were both present and  arranged to make the silk thread and weave it.

So scientists think that simple,  funnel webs were likely being built,   probably to capture jumping insects,  as well as weak and fallen fliers.

And with the continued expansion  of the forests after the Devonian,   many of these plant structures provided  the scaffolding that spiders needed   to start attaching their webs to  things in more vertical positions.

This allowed spiders to begin making increasingly   complex webs to trap their  increasingly complex prey.

Because flight was so radically successful,   flying insects had become abundant by the  Permian Period, 298 million years ago,   with the fossil assemblages including both  insects and spiders beginning to appear.

And this, some scientists argue, is where  the arms race began to really pick up.

Insects in the Permian had to develop more   sophisticated ways of avoiding  the increasingly complex webs.

By the Triassic Period, 252 million years  ago, both the mechanics of insect flight and   the weaving abilities of spiders continued  to radiate into new and different forms.

Around this time, the spiders that weave  vertical orb webs – those classic, spiral,   wheel-shaped webs – began to emerge.

These spiders are known as araneomorphs.

Unlike funnel webs that capture  whatever falls into them,   orb webs are built to catch insects in flight.

And this period also records  the earliest mygalomorphs,   the group that would later give rise  to tarantulas and trap door spiders.

By the middle Jurassic, roughly 165  million years ago, the earliest member   of another spider family appears - and it also  happens to be the largest fossil spider known!

But it’s not until the Early Cretaceous Period,   140 million years ago, that we see the  first fossil web, preserved in amber.

One amber specimen dating to  about 110 million years ago,   even contains a fly and a mite trapped in silk.

So amber fossils provide hard evidence  of spiders building webs and using them   to capture flying insects, rather than  inferring this from fossil spider anatomy.

It’s only at this point that the co-radiation  of flowering plants and flying insects finally   takes off, driving diversity well after  spiders drove insects into the air.

This marks the height of web-building spiders   dominating the air – their  "peak web era," if you will.

But something else was happening to the spiders  at the end of the Cretaceous Period, too.

Their winged adversaries in the ‘webs vs  wings’ arms race had taken on a different form.

See, during the time of the dinosaurs,  the battlefield had become more crowded,   with birds and their ancestors, as well as  mammals hunting both insects and spiders.

These spider-predators, like birds,   probably cued in on the web as a  kind of flag pointing out a meal.

So with more arms in the race, spiders  begin evolving in a new direction,   adapting away from the web.

See, webs are costly to make and maintain,   given the energy needed to  create and build and repair them.

So by the Late Cretaceous, some  spiders began returning to the ground,   seeking insect prey with fewer  risks and lower energy costs.

And non-web-based spiders had begun  radiating by the mass extinction that   ended the Cretaceous Period, 66 million years ago.

So these are non-web-based spiders,   they're analog.

They're like, your  old-fashioned brick and mortar... ....spiders that you had to, like, meet  in person, cause they're not online.

This group started speciating faster than  the orb-weavers, indicating that this was   the direction the group was, and still is,  evolving– a trend that continues to this day.

Now, non-web-based spiders are still  users of silk, but instead of webs,   they use it to set up other kinds of traps –  like nets or dropping lines to ‘fish’ for prey.

And many, like wolf spiders and tarantulas,   are going back to the ancestral use  of egg brooding and burrow lining.

Today, spiders are in all environments  from land to water to air, making them   important predators in almost every  environment that provides a meal.

And while they largely seem to be abandoning their  web-based aerial combat, leaving it to the birds,   the arms race is still going on…just  using different battlegrounds and tactics.

So the story of spiders and insects  reminds us that co-evolution can be