Metamorphosis, changing from one life stage to another, is taken to the extreme by insects. The study of the metamorphosis process is fascinating. The changes that occur are substantial and rapid. Many of the pupae of moths and butterflies are at least partially clear. We can see through the clear cuticle, a window on the change that is occurring inside.
A caterpillar has no compound eyes. Caterpillars don’t see much and have limited visual receptors. A few spots of photosensitive cells on the caterpillar head (called stemmata) are the only windows a caterpillar has on the world. Once the pupa forms, we can see the compound eyes of the soon to emerge adult clearly through the cuticle.
A caterpillar has chewing mouthparts: mandibles for cutting bite sized chunks of leaves, a labium to collect the bits of leaf, and maxillae to shove the bits into the food canal. The adult will have straw like sucking mouthparts that are formed by extended growth of the larval maxillae into a long sucking straw for drinking nectar. Each maxilla forms half of the long tube that is then “zipped” and glued together. These mouthparts are clearly visible through the cuticle of the pupa.
The wings of the adult moth form in plain sight on the dorsum (back) of the pupa. The wings are plastered tightly to the body of the adult that is forming inside the pupa. The adult moth will fully develop (in a crinkled form inside its cramped pupa). When it is time to emerge, the pupa will split open, the moth will emerge, then expand its wings and body to its full size.
Those who rear caterpillars will get an opportunity to observe these changes as caterpillars undergo metamorphosis to an adult moth.