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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 )
Larva
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( more about caterpillars )
hatching
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feeding
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carnivores
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cannibals
|
anatomy
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moulting
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armature
Hatching
Most butterfly eggs undergo colour changes as the
young caterpillars develop within them. The eggs of the
Marsh Fritillary
Euphydryas aurinia for example are pale
yellow when first laid, but after a day or two turn pinkish-brown,
then deep crimson, and finally dark grey just before the larvae hatch.
The eggs of the Orange tip Anthocharis
cardamines are pure white when first laid, but turn orange
within 2-3 days, then become dull grey when hatching is imminent.
Anthocharis cardamines - the egg on the
right is freshly laid, that on the left is about 3 days old.
When
a caterpillar
is ready to hatch, it bites a tiny hole out of the top of the egg,
and over a period of an hour or so, nibbles away until the hole is
large enough to allow it to crawl out. Some species use a different
technique, nibbling a circle around the perimeter of the egg to
create a "lid" which is pushed upwards to allow the caterpillar to
make it's exit.
After hatching, some
caterpillars such as that of the
White-letter Hairstreak
Satyrium w-album
rush immediately away and eat their way into a young leaf bud or
flower. Most caterpillars however
stay long enough to devour most of their eggshell, which contains
vital nutrients, and will die if prevented from doing so.
Feeding
For the remainder of the larval stage they feed on the leaves,
stems, flowers or seeds of particular plants. Some species are
adapted to feed on a variety of different plants, but most are
limited to just one or two plant species, and will refuse to eat
alternatives.
Very
young larvae such as this 1st instar Buff Ermine
Spilosoma luteum feed by nibbling tiny
holes in the lower cuticle of leaves. At this stage the larva is
about 3mm in length.
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Odina hieroglyphica
( Hesperiidae : Pyrginae ), Singapore. 2nd instar larva
cutting a segment out of an Erycibe
leaf. Later it will roll this up to form a shelter within
which it will hide when not feeding.
Photo courtesy
© Horace Tan |
Larvae and adult
butterflies of any given species generally use different sources of
food. Marsh Fritillary caterpillars for example eat the leaves of
devil's bit scabious, but the adult butterflies feed on the nectar
of buttercups, milkworts and thistles. In temperate areas the larvae
and adults live at different seasons, but in the tropics the
2 stages often co-exist at the same time of year, so the dichotomy
between larval and adult feeding behaviour enables them to avoid
competing for food.
The chemical
make up of plants changes seasonally. Plants often accumulate toxins
in their leaves as a means of defence against caterpillars. Older
leaves are more strongly toxic and have less protein than younger
leaves. Towards the end of the season leaves may be so toxic that
they can kill any caterpillar that eats them. Therefore the larval
stage of many species, particularly oak feeders, tends to be
compressed into the early part of the season.
Some species
which breed on less nutritional plants need to spend several months
in the larval stage, so towards the end of the season they need to
search out the freshest and least toxic leaves if they are to
survive. Some species even switch to alternative host plants which
come into leaf later in the year.
Caterpillars often
feed on different parts of their foodplant at different stages in
their growth.
The balance
of proteins, carbohydrates, minerals, alkaloids and essential oils
in plant leaves varies considerably at different times of day.
Caterpillars therefore tend to be feed at specific times when the
foliage is most nutritional and least toxic. Some species therefore
feed at dawn, others in mid-afternoon, and others at night.
There are
further reasons why many caterpillars only feed nocturnally. Firstly
they are less likely to be preyed upon by insectivorous birds, which
need daylight in order to locate their prey. Secondly, larvae of
Satyrinae and Hesperiinae which feed on grasses - and species which
feed on herbaceous plants - are prone to be accidentally consumed by
grazing animals such as rabbits, sheep or cattle. To avoid this they
evolved to become nocturnal feeders, and spend the daylight hours
hidden away deep down in grass tussocks where grazing animals cannot
reach them.
The duration of the larval stage varies according to the the
nutritional value of the food.
Species which eat foods with high nutritional value ( flowers,
fruits ) grow quickly, going from egg to pupa in less than a month.
Species which feed on leaves take longer, up to 2 months, because
leaves are harder to digest, and often contain toxins which need to
be processed. Slower still are the species which feed on grasses, bamboos, palms
or roots, all of which are low in nutrients and particularly
difficult to digest. In these species the larval stage usually lasts
at least 3 months.
In the case of species from temperate regions
growth is often so slow that the larvae are unable to complete their
development during the summer, and have to hibernate and resume
feeding the following spring.
The evolutionary
battle between plants and caterpillars
Many
plant species attempt to protect themselves from being eaten by
caterpillars. They have evolved toxic anti-herbivore compounds
called alleochemics which kill or cause sickness. Plants in the
genus Senecio ( ragworts ) for instance
will kill cattle if consumed. Most caterpillars have found ways to
avoid being poisoned. One method they use is to bite through leaf
veins, stems or petioles, to allow the toxic juices to bleed out
before eating a leaf. Another method is to avoid eating the veins,
and to just nibble at the edges of leaves where the toxicity is
minimal.
Many
larvae have developed an immunity to the poisons, but store
them in their bodies, or convert them into even more toxic
substances which they use as a defence against predators. An example
of this is Isognathus leachi from
Peru, which advertises its toxicity by means of bold stripes.
the larva of the Hawkmoth
Isognathus leachi from Peru is
poisonous to birds
There is a
constant battle between plants and the caterpillars which eat them,
each evolving ways to try and stay ahead in the struggle for
survival. While most plants defend themselves passively by
accumulating toxins to discourage browsing caterpillars, other
plants such as Passiflora vines have
evolved different means of defence:
Passiflora vines ( passion flowers )
actively defend themselves against being eaten by the offspring of
Heliconius butterflies. The butterflies
normally only lay a single egg on each sprig of
Passiflora, to minimise competition
between siblings for food. Some Passiflora
species have evolved to make use of this fact by randomly producing
tiny structures on their leaves or stems which visually mimic
Heliconius eggs. When a
Heliconius butterfly detects the false
egg it avoids laying "further" eggs in the immediate vicinity,
so the vine effectively prevents the butterflies from ovipositing on
it.
Another vine
Passiflora adenopoda has evolved a
different trick - its leaves and stems are covered with a coating of
sharp microscopic hairs which puncture the skin of browsing
caterpillars, rendering them immobile, and killing them by
starvation.
Leaf-miners
The newly hatched
caterpillars of some moths, known as "leaf-miners" burrow into
leaves and spend their entire lives living and feeding between the
upper and lower membranes. Each species leaves it's own
characteristic trail as it weaves it's
way about between the membranes. When
the caterpillar is fully grown it emerges and pupates on the surface
of the leaf.
Leaf-mines produced by caterpillars of the moth
Stigmella aurella
Not all caterpillars feed on living flowers or leaves. Calycopis
and
Detritivora for example feed on dead vegetation on the forest floor,
while some moths such as
Cossus cossus
live within tunnels in the trunks of dying trees, chewing their way
through solid wood !
Cannibals and
carnivores
Not all caterpillars feed on exclusively on vegetation. Some species
such as the
Orange tip butterfly
Anthocharis cardamines
( Pieridae ) and the Dun-bar moth
Cosmia trapezina ( Noctuidae
) have strong cannibalistic tendencies. Both normally eat living vegetation but will attack and eat any
other larva that they encounter.
Cannibalism has two possible advantages for larvae - firstly by
eradicating competitors they ensure they have enough leaves for
themselves. Secondly, they grow faster because they save themselves
the time-consuming business of digesting and processing the
vegetation. There is also unpublished evidence ( Hoskins ) that
Orange tip larvae which are deprived of the opportunity of
cannibalism produce smaller and weaker adult butterflies. It is
perhaps surprising that cannibalism is not more prevalent.
Caterpillars of the Large Blue
Maculinea arion, feed in
their early instars on flowers or leaves. When older they produce
and disseminate pheromones similar to those of the ant
Myrmica sabuleti. The ants,
which under other circumstances would probably kill and eat the
caterpillars, are appeased by the pheromones, and fooled into
carrying them back to their underground nests, where
they feed on the ant grubs.
In Africa, caterpillars of
Euliphyra
hewitsoni ( Miletinae ),
live in the nests of
Oecophylla
tailor-ants, feeding on ant grubs. Their cousins the Woolly Leg
butterflies
Lachnocnena
and Harvester butterflies
Megalopalpus
lay their eggs amid colonies of aphids, coccids, psyllids or
membracids ( Homoptera ), and their caterpillars feed throughout
their lives by attacking and consuming the living insects.
In Borneo, another member of the Miletinae,
Allotinus
apries,
feeds on coccids when it is tiny, but during the 2nd instar it develops
protrusions which act as grapples used by a particular species of ant
Myrmecaria lutea,
to carry it to it's nest - where the larva proceeds to devour the ant grubs.
More odd feeding habits
Another
fascinating species is the moth
Ceratophaga vicinella,
which lays it's eggs on the empty shells of dead Florida Gopher
Tortoises Gopherus polyphemus.
The resulting caterpillars feed gregariously on the keratin shells,
constructing a mass of silk tubes which act as anchors, connecting
the outer shell to the sandy substrate. In Africa, the caterpillars
of other Ceratophaga
species feed in tunnels within the hooves and horns of antelopes and
cattle.
Clothes moths
Ceratophaga
species are members of the Tineidae - the family to which clothes
moths belong. The caterpillars of the Case-bearing Clothes moth
Tinea pellionella
are well known for their habit of eating holes in woollen fabrics,
but in the wild state live within bird's nests, feeding on the
keratin in hairs and feathers.
The caterpillars of the related Tapestry moth
Trichophaga tapetzella
feed on coarser fibres, particularly horse hair, and are also common
in owl pellets ( regurgitated fur and bone ).
Many moths have caterpillars that feed on dung. These include two
species Acrolophus pholetus
and Idia gopheri,
which both ( as you may have guessed from the scientific name of the
latter ) spend their lives feeding on dung within the burrows of our
friend the Florida Gopher Tortoise !
Caterpillar of Privet Hawkmoth Sphinx ligustri
(
Sphingidae ), Hampshire, England.
Anatomy
A
caterpillar has only 2 functions during it's life - to eat, and to
survive.
It is basically just an eating machine, with large powerful jaws (
mandibles ), a huge gut, legs to enable it to move around it's
foodplant, and highly elastic skin that
stretches to accommodate the huge amount of food consumed.
|
Head of 5th
instar Polyura hebe (
Nymphalidae ), Singapore
( photo courtesy
© Horace Tan ) |
All butterfly caterpillars have six true legs located
on the first 3 ( thoracic ) segments, which are used primarily for
holding and manipulating the leaves on which they feed. On the
abdominal segments they have 4 pairs of false legs called "prolegs" -
effectively suckers that enable them to cling to stems or leaves,
and to walk. There are also a pair of gripping "anal claspers" at
the tail end of the body, which are used to secure the caterpillar
while the prolegs are doing the walking.
Caterpillars breathe through oval spiracles, one per segment, as
shown in this close up of a Peruvian Saturniid larva,
Cerodirphia ( sp. unidentified ),
Pantiacolla, Rio Alto Madre de Dios, Peru.
The caterpillars of many butterflies and moths spin a
sticky silken trail as they walk, which enables them to grip the
stems and leaves more securely. If they lose their grip, most larvae
fall to the ground and are unable to find their way back again, but
the larvae of some moths spin a silk "bungee cord" from which they
dangle if jolted from their foodplant, and are able to use it to
pull themselves back up.
Moulting
As the larva
feeds and grows, it's elasticated skin eventually becomes too tight,
and has to be moulted. Moulting is triggered by nerve cells called
scolopidia, which detect stretch in the elastic skin between the
caterpillar's segments.
Each of the stages between moults is known as an instar. Caterpillars
of the Lycaenidae have 4 instars, but those of the Hesperiidae,
Nymphalidae, Papilionidae and Pieridae usually have 5 instars. The
Riodinidae have between 6 - 8 larval instars according to species.
Prior to
moulting, caterpillars anchor themselves either to a small pad of silk that
they have spun on a leaf or twig; or to a silk web spun over the
foodplant. Many species live communally in such webs, and usually
moult in synchrony with their fellows.
A few
days before a larva is due to moult, a loose baggy elastic second skin
forms under the outer skin. At the same time, the soft tissues within the head
retract, forming a new head which is temporarily housed within the
first thoracic segment. The old head shell slides forward until it
eventually drops off, and the "old" skin splits just behind the head, allowing
the caterpillar to walk forward out of its former costume.
At
first the new skin is loose and soft, leaving the larva highly vulnerable to attack by parasitoid
wasps and flies. The larva slowly inflates
it's body by drawing in air through the spiracles, and then waits
for the skin to
harden. After a couple of hours it has usually hardened enough to
allow the larva to walk about without
injuring itself, and feeding recommences.
Larval webs and shelters
The caterpillars of
Peacock, Small Tortoiseshell and Marsh Fritillary butterflies, and
those of many moths, spend most of their lives feeding communally
within substantial silk webs where they are protected from
predators, parasitoids and severe weather. These species tend to
moult synchronously, after which they move as a group to another
clump of foliage where they spin another web.
The communal web of
the Small Eggar caterpillar ( Lasiocampidae )
Many
other species live solitarily and construct individual shelters or
tents in which they hide while resting. These include skipper
butterflies ( Hesperiidae ), Pyralid moths ( Pyralidae ) and
Charaxine butterflies in the genera Memphis
and Consul. The latter hide in rolled
leaves, and plug the entrance with their head capsule. This is
sclerotised and too hard to be pierced by the ovipositor of
parasitoid wasps. Other parasitoids such as Tachinid flies cannot
gain access to lay eggs on the larva's body, and the secrecy
provided by hiding in the shelter also undoubtedly confers a degree
of protection from birds and other foraging predators.
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