Eli5 why no big Insects?


Okay so I’m sure if you’ve read in history that sometime around Jurassic age that insects were like stupidly big due to O2 saturated environments. why can’t we manufacture an enclosed space with gradually increasing oxygen saturation over a few generations and see how big we can get insects?

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In: 42

Insects passively breathe.

Instead of having organs like lungs which pump air in-and-out, they just have a series of tubes all along their sides which allow air to get in and that is how they perform gas exchange.

This is very efficient on the small scale, and it allows them to breathe without using any energy at all. But as the insect gets bigger it’s demand for oxygen increases quicker than the limited amount of surface area in these tubes can supply.

Having an external exoskeleton also limits how much an organism can grow, but since coconut crabs are dramatically larger than the largest insect alive today that is clearly not the biggest limiting factor on insects.

And the truly gigantic insects were before the dinosaurs evolved, long before the Jurassic. They were around when the oxygen levels were higher, so the passive breathing was still able to get more oxygen into their bodies.

Evolution isn’t a short process and size is determined by genetics moreso than environment. It would (and did) take millions of years for insects to evolve to new size. You might see some slightly larger variants after a couple of thousand of generstions, but only by a centimeter or two.

Insects don’t have lungs, but absorb oxygen through holes in their exoskeletons. Their blood doesn’t carry oxygen, the air has to get to each cell from the holes only. It doesn’t scale well for large volume-to-surface area.

Insects respire in a more passive way than fish, reptiles or mammals, they have holes in their exoskeleton and air channels inside them which circulate oxygen through them so they can live and consume energy. It’s a perfect system if you have the right volume to surface area/volume and oxygen percentage ratio. Millions of years ago, there was a good deal more oxygen in the air, I believe about 35 percent around it’s peak, and insects could grow much larger before their size became a limiting factor in whether or not they could efficiently perform respiration.

Experiments have been done, with surprising results. Dragonflies do in fact grow larger if raised in a high oxygen environment, without any genetic change.
[Science Daily article ](https://www.sciencedaily.com/releases/2010/10/101029132924.htm)

[Wired article](https://www.wired.com/2010/11/huge-dragonflies-oxygen/)