So is the arachnoid body coming from her backside, or is she just having some sort of 'shadowy' simulacrum behind her, telling that she is that spider.

Also, is that sum penis in her hand?

It's jeffr's work, so yes, it's got to be either a penis or some other important dismembered organ.

I was just wondering since it hadn't been tagged as such.
Now the question about her spiderback is more interesting to say the least.

It's probably just some web and a spider leg.

If you click the source, the image shows up at pixiv even when you're logged out, so he probably considered this worksafe.

It does look a lot like a cum-dripping tentacle though.

Considering the drippy effect is also in the webs behind her I'd guess it's a spinneret.

That is a stinger