Consider two families of host plants, the Umbellifers (fennel, cumin, parsley, hemlock) and the Gramineae (grasses). The umbellifers have many aromatic species, and are chemically diverse and pharmaceutically interesting. Their resins and oils are defences against herbivory and parasitism. On the other hand, grasses have one big defence to herbivores: their stony inclusions in their cells wear down the teeth of herbivorous mammals, but they have few chemical defences. Both families are attacked by leaf-miner flies. There are four times as many grass species as there are umbellifers, but there are twice as many leaf-miner species on the umbellifers. 