Scarlet Spider, Ben Reilly

If any permanent has an ability that triggers when the permanent you return leaves the battlefield, that ability will be put on the stack after you finish casting the spell, and it will resolve before that spell.

Use the mana value of the creature as it last existed on the battlefield to determine the value of X. If it had X generic mana in its mana cost, X is 0 when determining its mana value.

The creature you return must be tapped at the time you pay costs, but doesn't necessarily need to be tapped before you start casting the spell. For example, if you tap a creature with a mana ability to pay part of the spell's web-slinging cost, you could also return that creature to your hand to pay the cost.