Oh, that one seemed pretty obvious actually. There was no record of demons with mouths on their stomach either, yet Behemolt suddenly sprouted one after spending some time in Baal's captivity. Add to this the tidbits from other chapters, like the Fingers being interested in data from Levialon and the glimpses of lab equipment, and you can infer that Baal dabbles in unethical experimentation on live demons, specifically "enhancing" their bodies, probably to make them more primordial or somesuch. Bestowing the fruits of such experimentation upon your enforcers seems logical.There’s no record of a spider demon so who is that guy?