I see lots of Philodromus spiders with missing legs, some of them with up four legs missing. They are - incredibly - able to walk, and presumably find enough food to stay alive. Many spider species have weak points in their legs, when in danger of being predated or otherwise attacked, they shed them, a phenomenom called autotomy. Spider autotomy has actually been shown to be a voluntary act, as anesthetised spiders would not shed their legs. For a spider, it is costly to lose a leg, as it might impair reproductive or fighting ability, or success at hunting, but it is a small price to pay when the alternative is to be eaten. And the best thing is that sometimes legs are not lost forever, as immature spiders maintain the ability to regenerate lost limbs in the next moult (although long lived spiders that moult when adult, like tarantulas, do keep that ability for life). The spider on the top shot, enjoying a juicy aphid a few days ago, has regenerated the middle pair of legs on the foreground: notice that they are thinner and shorter than the rest, and lack the colour pattern seen on the equivalent legs on the other side. Even in the Philodromus below, which has lost four legs and a palp, the apparent stumps of the missing legs are actually the miniature folded regenerated legs.