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The butterfly
lifecycle
1 - Egg
- anatomy, oviposition
2 - Caterpillar
- anatomy
3 - Caterpillar
- hatching, feeding and development
4 - Caterpillar
- cannibals, carnivores and myrmecophiles
5 - Caterpillar
- survival mechanisms,
armature, camouflage / disguise
6
- Caterpillar
-
co-evolution with plants
7 - Chrysalis
- pupation, metamorphosis
8 - Adult
- emergence, feeding
9 - Adult
- mate location and courtship
10 - Adult
- daily routine, roosting, hibernation, lifespan
Larva
:
hatching, feeding & development
Hatching
Most butterfly eggs undergo colour changes as the
young larvae 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 larvae however
stay long enough to partly devour their eggshell, which contains
vital nutrients. If deprived of the opportunity to eat the
eggshell caterpillars usually die.
Feeding
For the remainder of the larval stage most species feed on the leaves,
stems, flowers or seeds of particular plants. Some species are
polyphagous, i.e. they are adapted to feed on a wide variety of
plants from different botanical families. Most however are
monophagous, i.e.
limited to feeding on just one or two closely related plant species, and
unable to survive on anything else. Many species in the family
Lycaenidae are carnivorous, feeding on ant grubs or aphids. This
group is dealt with in detail on the
cannibals, carnivores and myrmecophiles
page.
1st instar Buff Ermine
Spilosoma luteum larva nibbling tiny
holes in the lower cuticle of leaf.
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
diurnal grazing animals such as rabbits, sheep or cattle. To avoid this they
evolved to become nocturnal feeders, and in the daytime hide away
deep down in grass tussocks where grazing animals can't
reach them.
The duration of the larval stage varies according to 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 up to 2 months. This is 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.
Leaf-miners
The newly hatched
larvae 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 larva 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.
Moulting
Depending on the species, caterpillars
increase their body weight between about 60-200x in the period between
hatching and pupating. Accordingly as
it feeds and grows, a caterpillar's elasticated skin periodically
becomes too tight and has to be moulted and replaced with a looser
baggy elastic second skin that forms under the outer skin.
Moulting is
triggered by nerve cells called scolopidia, which detect stretch in
the skin between the caterpillar's segments.
Two or three days before
moulting caterpillars anchor themselves either to a small button of silk
which
they have spun on a leaf or twig; or to a silk web spun over the
foodplant.
A day or two prior to moulting the soft tissues within the
caterpillar's head retract, forming a new head capsule which is
temporarily housed within the first thoracic segment. When moulting
takes place the
old head shell slides forward and drops off. The
old skin then 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. After a couple of hours
the larva's mandibles ( jaws ) have hardened, at which point a larva
will often eat it's old skin. Another 2 hours or so later the skin
has toughened sufficiently to allow the larva to walk about without
injuring itself, and to resume normal feeding.
The stages between moults
are known as instars. Caterpillars
of the family Lycaenidae usually have 4 instars. Those of the Hesperiidae,
Nymphalidae, Papilionidae and Pieridae usually have 5 instars. The
Riodinidae have between 6-8 larval instars according to species.
Larval webs and shelters
The caterpillars of
many butterflies including Peacock, Small Tortoiseshell and Marsh Fritillary
live gregariously. They spend most of their lives within substantial silk webs.
They emerge periodically to feed, but then retire to the silken
shelter 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.
Sexual development
As
larvae grow
and mature they begin to develop sexual organs internally, but it
is not possible to determine the sex of a larva from its
external appearance. There are however some species of Lepidoptera
in which
the larvae that will ultimately become males have 4 instars, while those that will become females
have 5 instars.
An example is the Vapourer moth
Orgyia antiqua in
which the female larvae grow to a much larger size than the males.
The male moth looks much like any other moth, but the female is
wingless and has an enormous abdomen swollen with hundreds of
eggs.
Duration
In tropical regions
larvae can develop very rapidly, and in some species the entire
larval period may be as short as 2 weeks. In temperate regions
the larval stage usually lasts about 6 weeks, but is often much
longer. Many species particularly amongst the Satyrinae overwinter
as larvae, taking several months to mature. There are even a few
sub-Arctic species in which the larval stage can last for 2 years.
4 - Cannibals,
carnivores and myrmecophiles >>>
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