OOC:
Saving against Mal's Animal Friendship.
After an initial fearful reaction, the spider calms down and take the offered food. It kinda squeals, trying to express the discomfort of its situation, before you untie it (unless someone stops you from doing that).
As you are done casting the spell, the giant spider -an impressive beast!- seems keen to stay around you and probably obey basic order. It approach and pushes your bag with its head, looking for more food.
OOC:
GM hint: your request might be a bit complex for a creature that basically is less intelligent than a dog ;)
And you would probably realize that after asking.