Explaining Giant Caterpillars that Chase Prey

Question

The giant caterpillars in question are carnivorous, about the size of a barrel in width and length (ie. big enough to swallow an adult male whole), and have elongated limbs and mandibles to aid in running and catching prey.

The idea is the exact opposite of a regular caterpillar; small, relatively slow, moves by undulating body (and gripping/pulling/pushing with legs), eats plants-in that these things are large, speedy, don't utilize their bodies as much for motion (instead focusing on legs), and eat meat.

Synopsis:

These giant caterpillars are the result of magically enhanced mutation, and through natural selection, have become fast and aggressive hunters that chase after prey, catch it with their powerful mandibles (which are essentially arms at this point), and shove it into their roughly diamond-shaped bodies.

I know carnivorous caterpillars already exist, it's weird but plausible enough. Giant caterpillars, as giant insects, are a known Acceptable Break From Reality. But giant, carnivorous caterpillars that will chase down and eat people are something else entirely.

Under what conditions would giant, carnivorous caterpillars that chase down and swallow prey whole come to pass?

Additional Information:

1. The answer is asking why giant, carnivorous caterpillars would evolve to chase down and eat prey instead of camouflaging and ambushing said prey. Greater speed makes sense, given that otherwise giant caterpillars could find themselves vulnerable to predation by wolves and such, but there are armored caterpillars, so I'm not sure what conditions would lead to speeding (rather than tanking) caterpillars.

2. Chasers (that's what these are called) are Uncommon monsters; you'll see them all the time in a requisite Spooky Dark Forest, while most people will have only heard rumors of them. Basically, they're like the R.O.U.S. in The Princess Bride. They don't encounter humans all that often, and when they do, they generally target them, as humans are registered as 'weak but worth it' by Chaos Sense. Why does this matter? Because going off of the scenario I hope to create, this monster must be:

• Fast enough a human with pumping adrenaline can't quite outrun it (ie. they can stay just ahead of it, but one misstep and they're caught, it's over)

• Strong enough to overpower and devour a teenage girl (I know it sounds sketchy, but this is to make a point about us underestimating common fantasy creatures when IRL said creatures, like giant spiders and goblins, would likely kill us, or in other words, this is to apply reality to video-game mobs)

• Capable of tanking (taking with negligible or non-existent damage resulting) hits from a dagger

So, assuming Chaos Energy makes it feasible for there to be giant, carnivorous caterpillars that tend to swallow prey whole, and that these things are biologically feasible, under what conditions would they evolve?

I'm asking because even with magic involved, natural selection is still a thing, and in order for these things to chase down prey, that has to be a viable hunting strategy.

Answer 1

For all the same reasons Centipedes exist in addition to pressures that lead to meat-eating:

• Animals provide a different sent of nutrients and in different proportions than plants.
• Caterpillars start eating small animals such as aphids, which allow them to acquire more resources, which allows them to get bigger and therefore better at being a caterpillar ^{[citation needed]}, which then allows them to hunt larger and larger prey, which then keeps going...
• Now we have giant caterpillars which actively hunt people!

This assumes a bigger caterpillar is actually better at being a caterpillar! Maybe this magic happens to accelerate this process?

Answer 2

A plant eating caterpillar won't adapt itself for predating anything, however carnivorous ones just might do.

There's just no way for a slow prey animal to turn into a chasing predator. If it's not fast enough at the beginning it won't catch anything and so nothing will encourage it to be faster. Changing an animal from herbivory to carnivory is also tricky. The most likely place to start is an ambush predator. More like big felines ready that pounce and less like wolves chasing a deer. Here's how it would go:

Consider the Hawaiian Eupithecia a species of carnivorous caterpillar. Its actual legs are used to catch prey while its fake legs anchor it to a surface. It does move like a geometer moth caterpillar making it seem like it's measuring its journey. The advantage with this body plan is that it can lash out like a snake to take its prey by surprise due to its deceptively long range.

This doesn't mean a gigantic version won't be able to run. No, on the contrary it will be extremely fast. Consider the cheetah, which bends and extends its spine each stride.

Now you have giant caterpillars with explosive speed and deceptively long range. Good luck!