Butterflies, with their bright colors and delicate wings, are more discerning diners than you might think. Female swallowtails (Papilio machaon), for instance, use a fascinating skill when choosing a plant for their eggs—they taste the leaf with their feet! Their offspring’s survival depends on this extraordinary ability.
The Swallowtail Caterpillar: A Colorful Stage
Both butterfly and caterpillar stages of the swallowtail are striking. Young caterpillars appear black and red with a white spot, later turning green with black-red stripes and orange dots. You might find them munching on umbelliferous garden plants such as carrot, parsley, or caraway.
Defense Tactics
When threatened, the caterpillar displays a remarkable defense. It extends a brightly colored organ called the osmeterium from behind its head, releasing an unpleasant scent to deter predators.
Tasting with Their Feet: How It Works
Female butterflies are unable to bite leaves like caterpillars; instead, they tap their feet on the surface. Their feet have special sensory hairs (chemosensilla) that absorb and analyze chemical compounds. If the plant smells right, the butterfly lays her egg; if not, she flies on.
Scientific Insights
Japanese swallowtail (Papilio xuthus) responds only to a specific mix of flavonoids, nucleosides, alkaloids, and cyclitol found in certain rutaceae, like the citrus plant Citrus unshiu (Nature Communications, Ozaki et al., 2011). If the butterfly detects only one compound on a test leaf, she won’t lay an egg—multiple chemicals must be present.
The Right Plant: Ensuring Offspring Success
By tasting this chemical cocktail, butterflies can even assess the age or health of the host plant—selecting only the freshest, healthiest leaves for their young.
Summary
Specialized taste organs on butterflies’ feet ensure they pick the optimal plant for their future caterpillars—a delicate, but highly effective evolutionary adaptation.
