Chapter 8 in The Hobbit is titled "Flies and Spiders." It is about the first half of Bilbo and the dwarves' adventure in Mirkwood forest. Mirkwood is an extremely dense, dark, and foreboding forest that Beorn has warned them is very dangerous. He told them to not stray from the path through the forest for any reason. Even though they stay on the path in the early part of the journey, they still encounter several animals along the way. When they try to light fires at night, moths by the hundreds are attracted to the light and they decide to forget about campfires. The light from the campfires also reveals to them numerous bats flying in the dense tree canopy. Later, when the group is trying to cross a dark colored river without getting wet from the water (Beorn had warned them about the water), a deer leaps over them, causing Bombur to fall in the water and immediately fall into a deep sleep. The group also sees a family of white deer along the path and tries to shoot them with arrows for food, but to no avail. Finally, at one point Bilbo climbs a tree to get a bird's eye view of the forest from above the canopy. Along the climb he sees some small spiders in the tree but when he peeks through the canopy and into the sunny sky above, he sees hundreds of beautiful butterflies sitting on the treetops.
At this point, the company next encounters the wood elves and ultimately the giant spiders, the most dangerous creatures in Mirkwood.