Dashed lines depict the trajectories of the Body Center of Masses (BCM) following the trajectories of pendulums. A) Hanging spiders: normal pendulums. The vertical distance between the highest and the lowest point of a pendulum (h) determines the amount of potential energy at the highest point which is converted into kinetic energy at the lowest point. The maximum force, (and maximum speed of the BCM) occurs at the lowest (middle) point of a pendulum stride. Therefore, if spiders move like pendulums during suspensory locomotion, natural selection should favour longer legs. B) Standing spiders: inverted pendulums. When spiders move as inverted pendulums, they have to change the direction of the BCM between steps. Here, the maximum force occurs at the end of each step and this force depends on body mass. Thus, the larger the spider the larger the forces ( and ) necessary to change the direction of force to . As a consequence, as body size increases the maximum attainable leg length is constrained in order to keep and sufficiently large to re-direct . In contrast, in (A), most of the force for the next stride comes from gravity.