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The butterfly lifecycle

The Lepidoptera ( butterflies, skippers and moths ), belong to a group of insects called Endopterygotes, all of which go through 4 distinct phases in their lifecycles :

Section 1 - EGG                ( ovum / ova )
Section 2 - CATERPILLAR   ( larva / larvae )
Section 3 - CHRYSALIS      ( pupa / pupae )
Section 4 - ADULT             ( imago )
 
Pupa  NEXT >> ( adult butterflies and moths )
 
pupation | anatomy | diapause | metamorphosis
 
Pupation
 
When a caterpillar becomes full grown, it undertakes a final moult to become a pupa. For a day or two prior to pupation the caterpillar goes through a wandering phase when it usually leaves its foodplant and may walk up to a kilometre before finding a suitable place to undertake the transformation into a pupa. During this phase it is particularly prone to predation, but the act of dispersal probably reduces overall predation of pupae by spreading them over a wider area - if they were concentrated on or around the foodplant it would be easier for birds to home in on them and wipe out the whole brood.
 
When a butterfly larva has located a pupation site it usually spins a button of silk on a leaf or stem, and anchors itself to it by its cremaster ( anal claspers ). In the case of Papilionidae and Pieridae it also spins a silken girdle around it's waist to secure it to a twig or leaf. Lycaenidae and Riodinidae do not possess cremasters - they either pupate loosely on the ground, or attach themselves by a silk girdle to a leaf or stem. Hesperiidae pupate loosely, usually within a flimsy silk cocoon. Moths pupate either loosely in a chamber just below the surface of the ground, or within a silk cocoon spun on the leaves, stems or branches of their foodplants.
 
The pre-pupal caterpillar hangs motionless for a day or two, preparing itself for its final moult, and during this time the prolegs start to shrink, the thoracic segments become enlarged, and the larva adopts a curled hump-backed position.
 

Gonepteryx rhamni, fully grown larva ( pre-pupa ) just prior to the final moult

When the final moult takes place, the skin splits behind the head, but instead of a caterpillar walking out of the old skin, what emerges is quite different in nature - a legless, wriggling, non-eating entity called a pupa or chrysalis.

At first the pupa is soft, limp, and very vulnerable to attack by parasitoid wasps and flies, but within a few hours the skin forms into a hard outer shell that will protect the insect until it eventually emerges as an adult butterfly.

Gonepteryx rhamni, pupa suspended from buckthorn leaf

Anatomy

Caterpillars show no external evidence of sexuality, but pupae can be distinguished as male or female. A male pupa will have 2 tiny bumps close to it's tail, corresponding with the anal claspers of the adult butterfly or moth. It is also usually slimmer and lighter than a female pupa. Many other anatomical details of the adult butterfly can be seen on the pupa - e.g. antennae, legs, eyes, wing cases and palpi.
 
Most Skipper larvae ( Hesperiidae ), and those of moth families including Lasiocampidae, Arctiidae, Saturniidae, Notodontidae and Zygaenidae pupate within cocoons. These can range from flimsy affairs composed of little more than a few strands of silk, to hardened shells made of dozens of layers of silk interwoven with bits of chewed bark, such as that of the Puss moth Cerura vinula.
 
The pupae within these cocoons are generally dark in colour, smooth and devoid of protrusions. The same applies to species which pupate beneath the soil or amongst leaf litter - Sphingidae, Noctuidae, Geometridae, and most Lycaenidae; and those which pupate within stems of plants or trunks of trees ( Cossidae, Sesiidae and Pyralidae ).
 

Anthocharis cardamines, pupa attached to garlic mustard stem
 
Butterflies in the families Nymphalidae, Papilionidae and Pieridae usually pupate openly, attached to leaves, stems or twigs. These unprotected pupae are highly vulnerable to predation and parasitism. They are usually cryptically coloured and patterned to resemble foliage, lichens, bird droppings or other common natural objects. This protective resemblance makes it much harder for birds and small mammals to locate them, and increases their chances of survival.
 
Many butterfly pupae in addition are adorned with spines, keels, knobs and other protrusions. They are also frequently contorted into strange shapes. The overall effect is that they often very strongly resemble twisted and desiccated dead leaves or bits of wood. Other butterflies such as Ithomiinae and Danainae are smooth, bulbous and silvery, looking like glistening rain drops.
 
 Heliconius pupa, probably melpomene or sara, Rio Madre de Dios, Peru
 
The use of protective resemblance as described above is a form of passive defence, but there are a small number of pupae which use active defence - the pupa of the neotropical Brassoline Dynastor darius for instance is a well documented example of a pupa which resembles a snake's head. Another well known example is the pupa of the Deaths Head Hawkmoth Acherontia atropos, which wiggles and squeaks if molested. The squeaking sound is produced by forcing air in and out of the spiracles.

Diapause

The pupal stage of the lifecycle can last anything from 1 to 40 weeks depending on the species. In polyvoltine species ( those which have more than one generation per year ), the summer pupal stage will be short - often just a few days; but the 2nd brood may overwinter as a pupa from August or September until the following May or June.
 
In a few cases certain species will delay emergence and remain as pupae for 2 or even 3 winters. This is probably a natural insurance to safeguard the species from putting "all of its eggs in one basket" - in an unusually short or harsh summer a species might not be able to survive in viable numbers, so it spreads the risk and only emerges if it senses that a good summer lies ahead.

Metamorphosis

The popular belief that the bodily fluids within the pupa break down into a "soup" and later reform in the shape of a butterfly is largely untrue. The change from larva to adult butterfly is actually a very gradual process. Clusters of stem cells from which the wings develop are present in segments 2 and 3 of small larvae. They replicate and diversify during larval development. In the last few days prior to pupation the development accelerates, so that the wings are almost fully formed at the time of pupation. The same applies to the antennae, eyes and palpi, all of which are visible on the newly formed pupa.

Within the pupa the changes that take place are surprisingly minor. The wing scales develop as plate-like extensions from cells on the wing surface. The heart, brain, eyes, antennae and proboscis all develop from the fairly simple organs "hidden" within the larva, into the recognisable features of an adult butterfly.

Gonepteryx rhamni, pupa with wing patterns showing through, just prior to emergence

About 2 or 3 days before the butterfly is due to emerge it changes colour, and the colour and pattern of the wings can usually be clearly seen.
At this stage female pupae of many species exude pheromones which attract males even before the butterfly has emerged. In Costa Rica e.g. I have observed that female pupae of Heliconius erato, when close to emergence, often have several male adults in very close attendance. A frantic battle takes place the instant the female hatches, as all the males struggle to copulate with her, not even allowing her time to expand and dry her wings. The mated pair then have to endure the continuing aggravation of the remaining males, which are often extremely persistent, trying to prise the pair apart. Eventually, with the approach of dusk, the unsuccessful males disperse, allowing the pair to remain copulated until the next morning.
 
 
 

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