A penultimate instar Cloudless Sulphur butterfly caterpillar feeds on Wild Senna, one of its host plants. Many caterpillar species are finicky feeders, and only forage on a small suite of plants. In this case, a few members of the pea family. Caterpillars grow through a series of molts - often five - and each growth phase is called an instar. The penultimate instar is the next to last. Caterpillars often change appearance markedly between instars, and this caterpillar will look quite different in its last instar.