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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
: survival mechanisms
Caterpillars have soft
bodies, making them extremely vulnerable to predation and
parasitism. They are unable to escape if attacked, so have evolved
numerous survival mechanisms to deal with their enemies which
include
birds, reptiles, amphibians, dragonflies,
predatory /
parasitoid wasps, robber flies,
crickets and spiders. In tropical regions they also fall victim to
ants, mantises, monkeys and numerous other threats.
Armature
Larvae in some subfamilies e.g.
Satyrinae, Hesperiinae, Notodontinae, Noctuinae, Geometrinae and
Sphinginae are normally devoid of hairs. Those in many other
subfamilies including Lasiocampinae, Arctiinae, Lymantridae,
Acronictinae and Hemileucinae bear hair-like setae. Sometimes these
are sparse but in some species such as the 'woolly bear' Garden
Tiger moth Arctia caja they are very
long and dense, giving the larva a furry appearance. The thick coat
of hair makes it more difficult for a bird or reptile to swallow a
larva. It also has the added bonus of cushioning it in the event of
a fall. Additionally hair traps pockets of air around the
caterpillar's body, allowing it to survive if it has the misfortune
to fall into a puddle. Caterpillars of Arctia
caja for example can survive periods of several days
submerged in water.
larva of Drinker moth
Euthrix potatoria
( Lasiocampidae ), England ©
Adrian Hoskins
The
hairs of moths in the families Lasiocampidae and Lymantridae often
have irritating properties. In the case of the Drinker moth
Euthrix potatoria
they do little more than cause a mild itch, but
the hairs shed by the larva of the Brown-tail moth
Euproctis chrysorrhoea
are rather more troublesome and can cause a
severe rash on human skin. It is generally the case however that
larvae from the temperate regions of the world are fairly safe to
handle. In the tropics it is a very different story, and all hairy
or spiky larvae should be treated with caution.
Caterpillar of Flannel moth ( Megalopygidae ), Peru ©
Adrian Hoskins
The
larvae of moths in the family Megalopygidae look fluffy and almost
invite you to handle them, but hidden beneath the soft hairs are a
series of sharp spines. If the larvae are handled the spines break,
releasing a chemical which causes excruciating pain.
Unidentified
moth larva, Peruvian Andes ©
Adrian Hoskins
Many caterpillars,
such as the unidentified species illustrated above, use bright
patterns and colours to warn enemies that they are distasteful or
poisonous. Others including those of the Nymphalinae, Heliconiinae,
Limacodidae and Saturniidae are armed with rows of extraordinary
multi-branched spikes and horns. These are enough to deter many
birds from attacking, and no doubt also offer a degree of protection
against wasps, ants and other insect predators.
Lexias pardalis,
West
Malaysia
©
Gan
Cheong Weei
Brenthis daphne,
Hungary
© Peter Bruce-Jones
Spikes, hairs
and other armature are most pronounced in young larvae which feed
communally, so it seems likely that one of their functions may be to
protect them against cannibalism.
In most species the
spines are harmless or cause only mild irritation in humans, but in
at least one Saturniidae species they can inflict a potentially
lethal sting.
The well camouflaged spiked larvae of Lonomia
obliqua are often found clustered on
tree trunks in Amazonia. There have been thousands of cases where
people have unwittingly
touching or rubbed an arm against them.
The effects can be extreme, including
massive intercranial
haemorrhaging and kidney failure.
Lonomia larvae are a frequent cause of death in southern
Brazil - 354 people died between 1989-2005.
The fatality rate is about 1.7% - roughly
equivalent to that of rattlesnake bites.
The Bullseye moth larva
Automeris liberia
( Saturniidae ) can inflict a painful sting ©
Adrian Hoskins
unidentified Limacodidae
species,
West
Malaysia
©
Gan
Cheong Weei
Larva of
Sphinx ligustri
( Sphingidae ), showing the spiracles and tail horn ©
Adrian Hoskins
Many people believe you can be stung by
the 'hornworm' larvae of
moths in the family Sphingidae but this is entirely untrue. The
caterpillar is completely harmless, and edible to birds.
Mimicry
Diematic mimicry is quite
a common form of defence in caterpillars as well as in adult
butterflies and moths. The larvae of many Swallowtail species
including Papilio polymnester and
Papilio troilus have a pair of false
eye-spots on the thoracic segments. Many hawkmoth larvae such as
Deilephila elpenor and
Hippotion celerio
employ the same strategy. When alarmed the larvae of these species
puff up the thoracic segments causing the eye-spots to expand. This is
considered to be a form of diematic defence in which the larvae are
mimicking the heads of snakes.
Camouflage and disguise
Many species use camouflage to escape detection, and are thus often
coloured green to match the leaves on which they rest.
The Lasiocampidae species illustrated
below is very well camouflaged at rest among lichens and mosses.
Others are disguised as flowers,
feathers, twigs or bird droppings.
unidentified Lasiocampidae larva, Peruvian Andes, 2800m ©
Adrian Hoskins
Heraclides thoas, disguised as a bird
dropping. Rio Alto Madre de Dios, Peru ©
Adrian Hoskins
Gregarious behaviour and larval webs
Generally
speaking larvae which feed as solitary individuals tend to be
palatable to predators, and rely primarily on camouflage - colours,
patterns and textures which help them to avoid detection. Larvae
which feed gregariously tend to be unpalatable or toxic to
predators. They often advertise their toxicity with bold aposematic
colours - a seething mass of brightly coloured wriggling larvae is
much more likely to deter a potential predator than a single larva
could. Gregarious behaviour also serves other purposes, e.g. a group
of larva can quickly construct a communal silk shelter in which they
can hide from predators and parasitoids. These larval shelters or
nests also protect them from the ravages of extreme weather such as
heavy rain, flooding and high winds.
Reflex bleeding
If molested, some
species such as the caterpillar of the Peacock butterfly
Inachis io react by 'reflex bleeding',
i.e. they spurt a foul smelling and noxious fluid from glands behind
the head. This acts as a warning to wasps, spiders and predatory
birds that they are distasteful and should be left alone.
Molestation also causes the caterpillars to wriggle violently and
drop from their leaf into the herbage below, presumably as a defence
against parasitoid wasps or flies. Nevertheless at least 90% of
Peacock larvae fall victim to attack by the Tachinid fly
Zenilla vulgaris. Other Tachinids
including Pelatachina tibialis,
Sturmia bella and
Phryxe vulgaris are also recorded as parasitizing
Inachis io.
Inachis io, 5th instar larva,
Hungerford, Berkshire ©
Adrian Hoskins
Multiple defence strategies
Birds rely primarily on sight to locate prey, so the evolution of
visually directed defences such as camouflage, disguise and aposematic
colouration reduces the likelihood of larvae being eaten.
These strategies work quite effectively against vertebrates but they
provide no protection against invertebrates such as spiders, wasps,
bugs and ants, which rely primarily on smell to locate their prey. The
larvae of many species have consequently evolved a twin pronged
defence strategy:
The larva of the Puss moth
Cerura vinula uses disguise as it's
first line of defence. When at rest its disruptive pattern of green
and dark purplish-brown gives it the appearance of a curled up leaf
with darkened edges. This illusion is reinforced by the presence of
a pair of tail prongs which are held together, simulating a leaf
stalk. The disguise helps it avoid being spotted by birds, but
offers it no protection against parasitoid wasps that track victims
down by scent. To deal with wasp attacks it switches to active
defence mode. When molested it rears up and retracts its head. This
causes the prothorax to expand, exposing a bright crimson 'false
head', complete with prominent false eyes.
If this is insufficient to deter an
attacking wasp the larva then spreads its tail prongs and everts a
pair of whip-like threads which are waved angrily. Even this may not
be enough to deter a wasp however so a last resort defence the larva
can eject formic acid - the same chemical used by bees, wasps and
ants in their stings.
Most Swallowtail larvae are palatable to birds and employ cryptic
colours and patterns as their first line of defence. If discovered
however they activate additional defences. Many for example have a
pair of 'false eye' markings on the thoracic segments, and can inhale
air through the spiracles to puff up the thorax, emphasising their
threatening appearance. This is often enough to deter birds from
attacking. Molestation by insect predators and parasitoids however
elicits a different response from the larvae. In this instance they
evert the 'osmaterium' behind their heads. This discharges airborne
isobutyric and 2-methylbutyric acids which has been shown to repel
ants and Homopteran predators. It also deters oviposition by
parasitoid wasps and flies.
Peruvian Cattleheart larva
Parides anchises with osmaterium extended ©
Adrian Hoskins
Geometrid moth larvae use disguise as their primary defence - they
look just like tiny twigs, and reinforce this similarity by stretching
out their bodies in a straight line so that they project twig-like
from a sprig of their foodplant. If molested they release grip on the
sprig, dropping instantly from a bungee-cord of silk. They dangle at
the end of this thread until the attacker has moved on. After a while
they haul themselves back up, consuming the silk thread as they do so.
Caterpillars of
species such as the Small Tortoiseshell Aglais
urticae, the Tiger moth Arctia caja
and the Fox moth Macrothylacia rubi try
to escape when they perceive a threat. If alarmed they simply roll
into a ball and drop to the ground. Aglais
larvae are covered in spines. Arctia and
Macrothylacia larvae are hairy. In both
cases birds remember from previous experiences that such larvae are
difficult or uncomfortable to swallow, and sight-reject them.
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