Butterflies Taste With Their Feet

A butterfly's long coiled tongue is for drinking, and that is all it does. The tasting happens somewhere far less dignified: the feet. Butterflies carry chemical receptors on their tarsi, the last segment of each leg, so the moment one settles on a leaf it already knows what that leaf is made of.

This matters most to the females, who face the single biggest decision in a butterfly's life — where to leave the eggs. A caterpillar cannot travel far, and most species can eat the leaves of only a small handful of plants. So the mother lands, and drums. Small spines on her front legs scratch the surface, sap seeps out, the receptors read it, and within a second or two she knows whether this plant is a nursery or a mistake. If it is the wrong one she lifts off and tries again, sometimes dozens of times before she is satisfied.

The equipment is startlingly good. Several species can pick out sugar in solutions so weak that a human tongue registers nothing at all — a butterfly standing in the same puddle would call it syrup. Those same feet recognise salt, bitterness, and the particular mustard oils that tell a cabbage apart from everything else growing beside it.

Which explains the slightly deflating truth about the butterfly that lands on your arm on a warm day. It has not come to admire you. You are damp, you are salty, and it is standing there tasting your sweat and quietly deciding whether you are worth a second sip.