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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
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( adult butterflies
and moths )
pupation
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anatomy
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diapause
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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 ).
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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.
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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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( adult butterflies
and moths )
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