Pupal Stage or the Chrysalis

Pupal stage is the third stage of the life cycle of a holometabolous insect. This stage starts at the pupation and ends with the eclosion of imago aka the adult. In the case of butterflies, this stage is called a Chrysalis. This stage is highly immobile (but can flex some abdominal segments) and in most cases well camouflaged.

When the 5th instar larva is done eating and is ready to pupate, it is called pre-pupatory larva.

After completing the task of feeding, final instar larva wanders around and find a suitable spot to pupate. It may be a dead twig, underside of a leaf, inside a fruit or even inside an ant nest depending on the species. There it lay a pad of silk as a substrate to hold onto. This pad is usually held onto by the hooks of the anal prolegs. Some species use a second thread of silk , a girdle around their thorax.

Nymphalids usually pupate head-down and hanging while Papilionids pupate vertical, head-up using the second silk girdle to support their weight. Lycaenids, Riodinids, Hespirids and Pierids also use the silk girdle but usually they remain in a neutral angle.

When the stage is set, hormones such as ecdyosone, juvenile hormone and prothoracicotropic hormone and enzymes come to play. A pupal skin is formed under the larval skin and enzymes digest the cells in-between the layers. When the process is complete it can shed the dead larval skin and reveal the underlying pupal skin thus becoming the pupa. At first it is a delicate cover of chitin but given some time it hardens with sclerotin to form a rigid cover.

Inside the pupa almost all the previous contents except the tracheal system, imaginal discs and some critical organs become a soup of nutrients, digested by enzymes. Imaginal discs are groups of cells that are destined to become certain parts of the adult from the beginning (from the egg). Under the control of hormones they start to develop, divide and differentiate forming all the organs of the adult inside the pupal case. Nutrients are taken from the predigested contents inside the pupa.

Parts that will become antennae, proboscis, legs, wings and eyes can be identified by looking at a new pupa at the very moment of pupation. This explains that some structures have already been developed deep within the larva well before pupation.

After it sheds the larval skin, hook-like structures called crochets at the end of the cremaster keep on holding the silk thus keeping the pupa in place. Cremaster is a structure at the hind end of a pupa which has hooks (crochets) to take hold of the previously laid silk pad.

in the pupal stage head, thorax and abdomen are present as they were on the larva and eyes, labial palpi, proboscis, legs, wings, spiracles, anus can be identified on the cuticle.

Labelled image of Pachliopta aristolochiae (Common Rose) pupa [up-ventral view, down- lateral view]

Butterfly pupae are diverse in shapes and colours unlike moths and many distinct features are shared within closely related species. Some pupae can be readily distinguished to the species level but there are many closely resembling pupae of different species which are at a glance looks almost the same.

There are many things that can go wrong in the process; some die in the process of pupation, some die being unable to develop fully and some fail at eclosion.

Because this stage is immobile, extreme measures has to be taken for survival. Predators, parasites, pathogens, environmental factors and many more test these guys to their limits. A study on Papilio demoleus (Lime Butterfly) showed a mortality rate of 32% (Suwarno 2012).  But just as we discussed in the larval stage, pupal stage is also not helpless at all.

Pupae may bear many peculiar features such as horns, filaments, spines, cones, setae and processes and are found in a diverse range of colours and shapes. Some of them are extremely camouflaged and some are extremely bright and showy (usually the bright colours are aposematic). There are some who pupate amidst ants in their nests (some Lycaenids) and some who pupate inside narrow holes in dead twigs too.

Some peculiar pupae of Sri Lankan butterflies

In many species eggs and sperm mature in pupal stage and thus they are ready to mate just after eclosion.

If everything goes according to plan wings will become visible through the pupal skin some days before emergence and the colours will show on the night before eclosion. Usually butterflies emerge at dawn or at the early hours of light. Environmental conditions can induce diapause even if development is complete. Because of that, timespan of pupal stage is highly variable. Even though there aren't extreme climatic changes in Sri Lanka, rainfall/humidity has been documented to be a governing factor.

Pupae of some common butterflies in Sri Lanka