"And they grow into ... ?"
"They won' grow inter nuthin' ...I got 'em ter feed ter Aragog."
-- Ron and Hagrid (HBP11)
A giant grub is the white, foot-long larvae of an unknown creature.
- Hagrid kept a writhing barrel-full of the hideous slimy creatures to feed to the ailing Acromantula, Aragog (HBP11).
A Grubby Death: The same year that Hagrid was saving up giant grubs for Aragog, the House-elf Kreacher sent his master Harry a package full of maggots for Christmas (HBP16). Maggots and grubs are associated with carrion-eating flies and beetles, and at Christmas dinner, Ginny pulls a maggot from Harry's own head. Harry associates Kreacher with the death of his godfather Sirius Black, and it's possible Kreacher is sending him a death wish for Christmas. In HBP, Aragog dies, followed by the traumatic death of Albus Dumbledore, so all the grubs and maggots can be seen as foreshadowing. Harry suddenly thinks of Hagrid and the giant grubs as he is about to walk to his own death in "The Forest Again" chapter of Deathly Hallows, perhaps once again comparing himself to Aragog (DH34).