the title is pretty self explanatory why can’t bugs be big
In: Biology
The biggest issues are bugs don’t have lungs and bugs don’t have a skeleton. If a bug got too big they couldn’t get oxygen into the deepest parts of themselves so even a big bug needs to be a skinny bug. The lack of a skeleton means they use their exoskeleton to hold themselves up and frankly it’s just not as efficient as bones are. Back in Ye olden dinosaur times there were larger bugs when the oxygen concentration was higher.
Bugs dont breathe. They absorb oxygen through skin. When earth had more oxygen in atmosphere, bugs also were larger (when dinos were roaming around). Also another thing is, that at some point exoskeleton just cant handle stuff as well as bones can.
Several limitations:
First is that they don’t have lungs like you do, they rely on pores and passive gas diffusion to breathe. Oxygen gas only diffuses so far so fast, and so they’re limited in maximum size by the oxygen concentration of the atmosphere. They were once much larger in an oxygen-rich primordial Earth.
The second is weight. Bugs don’t have bones, they’re shaped and supported by an armored exoskeleton. It’s incredibly strong and provides excellent protection from slashing and puncturing, but it’s also very heavy. Bigger insects require exponentially more musculature to actually move this suit of armor around, and the math quickly becomes impossible. An ant the size of a man wouldn’t even be able to lift its head, much less several times its own mass.
A couple house cleaning things first.
A “Bug” is a specific term but often used colloquially to mean itty bitty insects and the like. Beatles, spiders, flies, are not actually bugs but we still can call them that because the English language is fun.
Second “big” is subjective. A big spider might be an inch in size. While the largest ones can get nearly a foot in size. But when you say big I think you mean large like a dog.
It has to do with most bugs anatomy.
For example spiders don’t have muscles in their legs. How do they move? Their heart pumps fluid into their legs. This fluid builds up in pressure and can extend their leg or contract their leg depending on the pressure.
Ants don’t have lungs. How do they breath? They are so small and require so little oxygen that it enters through pours in their “skin”.
As you start to scale up small bugs into larger creatures, their anatomy stops working. They would need muscles to move instead of fluid pumped into their legs. They would need lungs instead of pours in their ‘skin’. Their carapaces would be so heavy that they wouldn’t be able to lift them, etc.
But what if the bugs could change so that they acquired the things they needed to survive being big? Well those already exist. We call them animals. Bears, dogs, cats, cows, birds, etc. Their biology fits their size. If scaled down to a bugs size you’d find that their biology also fails in the same way as if bugs were scaled up.
Bugs have weird guts.
The biggest factor is they don’t have lungs and their blood doesn’t carry oxygen like a lot of other animals. Instead, the parts of their body that need oxygen have their own little breathing tubes and absorb what they need from the air. So it’s kind of like they breathe all over their bodies.
That only works if the tissue is relatively tiny. The oxygen can only move so far this way. If their muscles get too big, parts of them won’t be able to get enough oxygen and will die off.
Now, they can be BIGGER. We do know ancient insects could be a lot larger. Like, some ancient dragonflies were as big as small hawks. But we think as more and more birds evolved and started preying on insects, being smaller and more maneuverable helped them survive. So the big ones died out because the smaller ones survived better.
But there’s some size of insect where if they don’t evolve lungs and a different circulatory system, they simply won’t be able to survive. That would be such a radical evolution biologists might not even call the creature “an insect” anymore. They might make a new category and say “this evolved from insects”.
There are also some theories that maybe exoskeletons can’t be strong enough to support larger creatures. This is tougher. We do know that to some extent Physics presented a lot of challenges to massive dinosaurs and one of those big challenges is as the “mass” of the creature gets bigger, the size of the bones needed to support the body gets bigger *faster*. In theory there’s a dinosaur size where the bones needed to support its weight won’t fit inside its body. In practice that doesn’t exist because, well, it’d die very fast if it did.
This is kind of a “What if…” topic though. We can theorize and think about biological exoskeleton materials that MAY support larger insects, and coming up with what the maximum size might be is a fun project.
So the much bigger concern is oxygen delivery. Like I said, for them to get VERY large like horses, they’d need to make so many evolutions we wouldn’t call them “insects” anymore.
“Bugs” breathe through something called spiracles, basically small holes in the side of the body, movement of the body can increase the rate of gas exchange, but the gas only penetrates a small way into the body limiting the size. https://youtu.be/a7OPV3QZWfs
Bugs have been bigger in the past. Like, massively big. The main problem with bugs not being quite as big are the rise of the other orders of animals and the decrease in Oxygen levels compared to hundreds of millions of years ago when insects ruled the world.
Insects being insects and not having lungs like most other animals and instead breathe directly through their bodies means the more oxygen in the atmosphere the more they can take in at any one point. The larger they are then the larger amount of oxygen they can also take in to support their daily living needs. Conversely the lower the oxygen levels then the smaller insects can grow.
Hence why insects are the size they are now. But as I said, in the past things were different.
Given the right amount of oxygen, you might get a really big bug, if you can call an 81/2 centipede”big”: https://www.britannica.com/animal/Arthropleura
Insects don’t have a way to move oxygen through their bodies. Unlike humans, oxygen passively travels into the body through tiny holes (spiracles) and passageways in their bodies. Since there is no mechanism to pump air in and out like lungs, this limits the amount of oxygen that gets to their core.
Vertebrates, OTOH, usually have a circulatory system that carries oxygen in blood through the body. This is usually the limiting factor in any activity, and a good circulatory system allows them to grow very large. The blue whale, for example, has a heart the size of a small car.
Insects do have a circulatory system, but it is only designed to carry water and nutrients, not oxygen, so again, that limits their size
Because the big bugs can’t breathe.
Bugs don’t have lungs, so they can’t pull air in deep enough into their bodies if they’re too big.
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