Many animals dramatically change their appearance and way of life as they reach adulthood. The fish-like tadpole of a frog, for instance, loses its tail and on coming out of water takes to hopping around on land. But the transformation that has most captured human imagination is that of an ugly, hairy or warty, green or black caterpillar into a colourful butterfly or moth. This transformation is accompanied by a radical change in the insect's equation with the plant world.

Caterpillars are enemies of plants, being little more than a pair of jaws that munch ceaselessly on plant matter and a gut enclosed in a bag of skin that digests the food so that the caterpillar may grow fatter and fatter. But having reached the limit of growth they make an about turn, for many adult moths and all butterflies are great friends of plants. In return for a sip of nectar they transfer the pollen from flower to flower, enabling plants to set seed.

Indeed, it is hard to imagine a more sensational example of what may aptly be termed a love-hate relationship.