Which body part do butterflies mainly use to taste?

Answer

Butterflies have taste receptors on their feet. Landing on a leaf lets them test it, often to check that it is a safe plant for their eggs.

Context

Insect taste organs are hair-like structures called sensilla, each with a single pore at the tip and usually four receptor cells inside. Many insects carry them on the undersides of the feet (the tarsi), so the animal tastes a surface just by standing on it. Nerve fibres from these hairs run to the nearest nerve cluster in the same body segment; an insect has no single taste centre.

For butterflies the foot receptors serve mostly egg-laying decisions. Britannica notes that adult butterflies have tarsal cells tuned to chemicals specific to their host plants, for example glucosinolates for cabbage-family feeders. These signal that a plant is the right species, not that it is nutritious.

Cornell researchers describing monarchs explain that the legs are full of chemoreceptors. A female drums her legs and antennae on a leaf and may scratch it, which can release compounds that her feet, antennae or proboscis then sense, before she decides to lay an egg. Long-distance finding of milkweed is less well understood and probably relies on smell through the antennae, and possibly colour vision.

So feet are not the only taste route. The proboscis tastes nectar and other food, and the feet mainly sample surfaces. Flies and some other plant-feeding insects have comparable foot receptors, so the trait is common among insects rather than special to butterflies.