Upon scouting 2 factories, you put a spine and spore in each mineral line. The purpose of the spore is to prevent them from picking up hellbats and dropping them small chokes and crevices. The purpose of the spine is to just deal extra damage and fuck with the AI of the hellbat so that it makes it harder for them to control the unit and it forces them to target drones. The spine is only so much though, so you should be bringing your zerglings / units to the base where the drop is going in order to fully defend it.