Walter thought on that for a moment. He could compare the blood effect to that of a vampire, but there wasn't any guarantee the man would know the intricacies of what he was talking about, and a lengthy lesson would not necessarily be practical at this point. "Ah, but the venom is only venom. It does not continue to act as part of the spider once it leaves the spider. But, I daresay you're right about the general purpose."

That being to put the food source in a state where they are incapable of warding off the advances, even as they cross into predation and eventually endanger the prey's life.