Many different books and novels contain creatures that have human upper body. But have the legs and thorax of spider. They are usually called by different names but they're sort of like centaurs but instead of a cross between a human and a horse they're a cross between a human and a spider. What biology would this creature need in order to exist without the help of magic?
1$\begingroup$ Bear in mind that many such depictions don't have a human upper body, but a humanoid upper body that still uses spider-like biology. In these cases, your requirements will be more relaxed. $\endgroup$– MichaelSMar 4, 2016 at 5:29
$\begingroup$ @MichaeIS good point $\endgroup$– Bryan McClureMar 4, 2016 at 5:55
$\begingroup$ Any specific examples? I've never heard of such a thing. $\endgroup$– JDługoszMar 4, 2016 at 10:28
1$\begingroup$ An example would be the Drider from the Forgotten Realms (not a good example for answering this question due to the in universe rationale behind them but could give a clue as to what they look like) $\endgroup$– Mac CooperMar 4, 2016 at 11:47
$\begingroup$ @JDługosz Think a centaur, but with a spider. $\endgroup$– Xandar The ZenonMar 4, 2016 at 14:30
So brushing away the problems with making a spider big enough to be "half human": What kind of world changes are necessary to make giant spiders feasible?
Biology wise It could still have the same organs in the same general areas:
And get the "half human" effect by simply having a differently shaped cephalothorax, where the head and thorax are fused like in a normal spiders, except that the "head" is shaped to resemble a human upper body (in low light).
The pedipalps could act like the "arms" for instance.
1$\begingroup$ Pelican spiders might give some ideas about just what kind of wild shapes the cephalothorax of a spider can already take... it's not that big of a stretch to me to think of one with a more human-like appearance. $\endgroup$ Dec 1, 2021 at 23:47
1$\begingroup$ @YoungJohn Thanks, I hate it! Seriously though, that's a pretty cool example. And by example, I mean an example of something I don't want crawling on me. $\endgroup$– AndyD273Dec 3, 2021 at 18:12
Without magic, I would use evolution. Specifically create it in a world that would foster a creature like that and allow it to evolve. Appearance and biology would be more realistic. Then transfer the creature to your world. It wouldn't be the classic spiderling, but more believable than a lab created one (imo)
If lab or similar is the only option and relative realism is a focus, I would go with another creature; biology would seem trivial in that context
I already addressed some of this in this question here:
All you need to do is take the process there and apply it to spiders and then extend it. If it starts gathering food from trees then over time the front section of the creature would rise up and the front pincers or maybe even smaller limbs from around the mouth becoming adapted for fine manipulation rather than combat. Over time it could create what seemed like a humanoid body at the front, a process which would accelerate as intelligence increases and tool usage begins giving a real advantage to fine manipulation ability.