Ralph looks over and sees a group of boys heave an enormous boulder in the sea below, and he calls for them to stop:
His voice struck a silnce among them.
A strange thing happened in his head. Something flittered there in front of his mind like a bat's wing, obscuring his idea.
At once the ideas were back, and the anger" (107-108).
This scene is extremely significant, because it reveals Ralph's struggle to maintain his focus on what he deems is the most important act on the island--maintaining the signal fire. He knows that he has to gather the boys to make their way from Castle Rock to the mountain where Samneric spotted the beast, but for a moment, Ralph completely loses his focus and grip on his priorities. He sees the enthusiasm of the boys pushing the rocks all around him, and Ralph's train of thought completely slips away. Golding uses the simile "like a bat's wing" to describe the way that Ralph's thought process completely shut down, even if momentarily. He and his leadership faltered, and he could not remember the importance of the smoke signal. Golding's simile is an apt one for the scene, as he described the cave and the pink rocks being streaked with guano, like icing earlier; his diction also seems like a play on words, as if to suggest that Ralph is going 'batty.'
A strange thing does happen on Castle Rock; Ralph loses his grip on reality and begins to go a little batty. Does he actually see anything? Well, he sees the other boys acting like idiots, but no, the "strange thing" occurs solely in Ralph's head. The "bat's wing" is Golding using figurative language.