1. We measured the body sizes (weights or lengths) of animal species found in the food webs of natural communities. In c. 90% of the feeding links among the animal species with known sizes, a larger predator consumes a smaller prey. 2. Larger predators eat prey with a wider range of body sizes than do smaller predators. The geometric mean predator size increases with the size of prey. The increase in geometric mean predator size is less than proportional to the increase in prey size (i.e. has a slope less than 1 on log-log coordinates). 3. The geometric mean sizes of prey and predators increase as the habitat of webs changes from aquatic to terrestrial to coastal to marine. Within each type of habitat, mean prey sizes are always less than mean predator sizes, and prey and predator sizes are always positively correlated. 4. Feeding relations order the metabolic types of organisms from invertebrate to vertebrate …