As long as natural enemies have some potential to be omnivorous, plants can provide food resources to encourage their retention and increase the impact they have on herbivore populations. This potential, however, can hinge on a number of the insect's traits. For example, hemipteran predators can use their sucking mouthparts to make use of leaves, stems, and fruits, but spiders with chelicerae cannot. Still, insects widely considered to be purely carnivorous have been observed to diverge from expected feeding behavior. Some plants simply tolerate a low level of herbivory by natural enemies for the service they provide in ridding the plant of more serious herbivores. Others, however, have structures thought to serve no purpose other than attracting and provisioning natural enemies. These structures derive from a long history of coevolution between the first and third trophic levels. A good example is the extrafloral nectaries that many myrmecophytes and other angiosperms sport on leaves, bracts, stems, and fruits. Nutritionally, extrafloral nectaries are similar to floral nectaries, but they do not lead the visiting insect to come into contact with pollen. Their existence is therefore not the product of a pollinator–plant mutualism, but rather a tritrophic, defensive interaction.