Toxicity of angular furanocoumarins to swallowtail butterflies: escalation in a coevolutionary arms race?

Xanthotoxin, a linear furanocoumarin occurring in many plants of the family Umbelliferae, is not appreciably toxic to the umbellifer-feeding larvae of Papilio polyxenes (Lepidoptera; Papilionidae), whereas angelicin, an angular furanocoumarin found only in a few relatively advanced tribes of the Umbelliferae, reduces growth rate and fecundity. The biosynthetic pathway leading to angular attachment of the furan ring may thus have been a response within the Umbelliferae to selective pressures exerted by specialized herbivores that had adapted to feeding on linear furanocoumarins.