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Long post, but I couldn’t resist an opportunity to talk about LEGS: how and why?

One reason animals evolve bipedalism is because it’s very energetically efficient. Humans are champs at endurance walking partly because we’re letting gravity do half the work, falling forward with every step. In comparison, an animal like a beetle or a newt with splayed legs has a very stable standing position, but in all parts of their walk cycle, they’re actively pushing their body up against gravity. This is no biggie when you weigh a fraction of an ounce, but once you’re pushing 50 pounds it becomes a chronic problem. In the evolution of land vertebrates, you can see limb attachments starting at the sides of the body (i.e. ancient amphibians) where the fins used to be, but in later animals, the limbs moved underneath the body and became struts to passively support their body weight against gravity. (i.e. a horse)

But why move onto only two legs? Well, feet are also heavy, and in order to walk you have to lift them up. This is the motivation for many animals evolving digitigrade or even unguligrade legs: if there’s less heavy bones and complex joints at the end of your leg, that’s less weight to pull up against gravity, which means you can run slightly faster and walk slightly more efficiently.

And for animals with only two walking legs, that’s even LESS weight that they have to lift around in order to get up and go places... at the price of having fewer backups in case of injury.

So, that’s great. What about arthropods though? Well, I think the main reason we don’t see any bipedal arthropods on our planet right now is because they don’t have any of the evolutionary pressures that cause it. First of all, they aren’t big and heavy-- arthropods are limited by their passive diffusion respiratory systems, and their need to wriggle out of their entire skeleton and be mushy for a while after in order to grow. Second, they’re not being pushed into the long distance walking lifestyle of humans or horses-- arthropods with distance to cover just fly over it, because they weigh nothing, so flying is super easy. But if you muck around and change some of those evolutionary factors, how would the leg structure of (for example) a beetle change to accommodate them?

I’d imagine it would look strangely familiar. Legs straightening out, moving underneath the body, simplifying and strengthening the hinges, we’ve been here before. What’s new is that there’s more leg to work with. For walking efficiency at a larger size, I’d expect one of the pairs may reduce their role in locomotion, somewhat like the forelimbs in mantids or butterflies. If you’re aiming to make a bipedal bug, forelimb specialization is a good excuse to remove four of them from the ground. As for the legs on the ground, another familiar phenomenon may happen over time...

With similar motivation, mass would move away from the foot and towards the body in cursorial species just like for vertebrates. As for structure itself in the exoskeleton, I recommend looking at the larger arthropods of real life... terrestrial crustaceans like coconut crabs come to mind. For a truly huge arthropod, a mix of endoskeleton and exoskeleton and/or a water-dependent molt might be necessary to get away with the high body weight. As for posture, the world is your oyster! Humanoid or not, just make sure your handsome lad has a good center of balance!

That’s all folks!