Just as some plants avoid giving a reward, some animals take the reward but don't provide any pollination services. Think back to the 10-inch-long flower. Many flowers have deep (though not quite so extreme) nectar sources, which make them inaccessible to insects with short proboscises.
214 Part lll: What Evo|ution Does
Some insects have developed a clever solution to this problem: They just hold the base of the flower, chew a hole, and slurp out the nectar, never getting anywhere near the pollen. In this case, the insect is a parasite of the flower. As the species co-evolve, the plant species is selected to have less-chewable flowers, and the insect is selected to be better able to chew its way to the nectar.
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