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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, anatomy, 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 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 are able to survive periods of several days submerged in water.
larva of
Drinker moth
Euthrix potatoria (
Lasiocampidae ), England
The hairs 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. With these and a few other
exceptions, larvae from temperate regions of the world are fairly
safe to handle, but in the tropics there are many species which can
inflict painful stings.
Caterpillar of Flannel moth ( Megalopygidae ), Peru.
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.
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
( image
© Gan
Cheong Weei
)
Brenthis daphne, Hungary
( image
© 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 Saturniid 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
larva of the Bullseye moth
Automeris
liberia
( Saturniidae ) can inflict a painful sting.
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unidentified Limacodidae
species,
West Malaysia
( image
© Gan
Cheong Weei ) |
Larva of
Sphinx ligustri
( Sphingidae ), showing the spiracles and tail horn
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.
Snake mimicry is also found in adult Atlas moths in the genera
Attacus and
Rothschildia. In these species the forewing apex is lobed,
and bears markings corresponding to the eyes and mouth of a snake.
The illusion is enhanced by rhythmic movements of the wings which
draw attention to the snake-head markings.
Camouflage and disguise
unidentified moth
larva, at rest amongst lichens and moss, Peruvian Andes, 2800m.
Many species use camouflage to escape
detection, and are thus often coloured green to match the leaves
on which they rest. Others are
disguised as flowers, feathers, twigs or bird droppings.
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.
Heraclides thoas, disguised as
a bird dropping. Rio Alto
Madre de Dios, Peru
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.
Larva of Peruvian Cattleheart Parides
anchises with osmaterium extended to deter attack.
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 does
so.
Caterpillars of species such as the Peacock
butterfly Inachis io, 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. Inachis 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.
On the next page you can read
about the evolutionary battle for survival between plants and the
caterpillars that feed on them :
6 -
Co-evolution with plants >>>
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