Fred said, "Lumos!", giving George light by which to aim. George obliged with "Tarantallegra!" and hit a giant spider right in the middle of its eight eyes. It did not even flinch. By the light of Fred's wand, they saw clearly that they were surrounded by enormous spiders, black and hairy and fanged, and evidently rather resistant to jinxes. They were thirteen and did not know any more dangerous magic than jinxes, which meant that they were not well-equipped to defend themselves with any effectiveness from this hazard.