The caterpillar of the butterfly is actually its larva, which is the first stage in the biological cycle of these insects, and looks completely different from the adult stage (butterfly).
Due to this great difference, in the past it was assumed that caterpillars and butterflies had nothing in common and were considered as two different animals. But this is not the case!
A series of fascinating transformations allows the caterpillar to become a butterfly and take on the more common and charismatic appearance we all know. This biological process of transformation is called metamorphosis.
Butterflies are holometabolous insects, meaning they have a complete metamorphosis in which larva and adults have completely different appearances and ecological roles.
The whole process of butterflies' metamorphosis has four different phases:
- Egg: after mating, adult female butterflies lay their eggs on the leaves of specific host plants, which will provide food for the caterpillars that will emerge from the eggs. Each female butterfly may lay a different number of eggs and these are generally laid on plant stems or on the top or bottom of leaves, where the female butterfly ensures they are firmly fixed to the substrate by means of a viscous substance secreted by glands in the insect's genital apparatus.
- Caterpillar: following maturation, the egg hatches and a small larva emerges from it. After devouring what is left of the egg, the larva begins to eat the plant on which it stands thanks to its strong jaws and highly developed mandibles. The caterpillar then starts to grow until, having increased in size, it has to get rid of the cuticle, the outermost part of its outer envelope, as it is not extensible and cannot grow together with the caterpillar itself. At this point, the caterpillar changes "skin" and molts, a process that occurs 2 to 10 times during the larval stage of Lepidoptera (butterflies and moths). Often, before molting, the caterpillar sticks to a substrate by means of silk threads and starts to ingest plenty of air to increase its internal pressure. At this point, by means of some torsional movements, it begins to shed the old skin which, once extracted, is called exuviae.
- Chrysalis: once the caterpillar has reached its maximum growth stage, it stops feeding and starts looking for a suitable place to carry out its final molt, which will transform it into a chrysalis. From Greek, chrysalis literally means "small golden object", a particularly apt description in the case of the Inachis io chrysalis. Inside the chrysalis, the caterpillar undergoes a complete metamorphosis called nymphosis, which can take place on a plant or underground, safe from danger. During nymphosis the chrysalis sheds the larval exuvia, which often remains attached to the bottom of the chrysalis. This liberation process takes great effort on the part of the chrysalis and occasionally quite a long time. Once the process is done, the chrysalis begins its nymphal rest and in most species it will not move anymore, while in some cases, such as Papilio Machaon and Pieris brassicae, it is able to move its abdomen in case of danger or when approaching the flickering phase. The duration of this phase can span from two weeks to more than a year, depending on the species, and can be prolonged due to the insect overwintering in a chrysalis state with subsequent flickering in the following spring.
- Butterfly: shortly before flickering, the structure of the now formed butterfly and the colours of the wings folded around the body can be seen from the walls of the chrysalis. At this point the hatching, known as butterfly fluttering, takes place. The butterfly tears open the chrysalis tegument and emerges with its head, inhaling a lot of air, while helping itself with its legs and body movements to get out. Fatigued and still completely wet, with its wings crumpled and shapeless, the butterfly remains anchored to the nymphal husk. The wings slowly dry out and stretch so that, within one minute to one hour, the butterfly is ready to take flight.