Butterflies of
Australia & New Zealand
The Monarch
Danaus plexippus
LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily -
DANAINAE
Tribe - DANAINI
subtribe - DANAINA
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Danaus plexippus, New Zealand,
© Rob Herd |
Introduction
The
subfamily Danainae, which includes the Monarchs & Tigers, Nymphs and Crows,
comprises of about 190 species worldwide.
All
butterflies in this subfamily are thought to be toxic or distasteful to avian
predators. Their bodies contain toxins derived from the larval foodplants,
often supplemented by further toxins derived from pyrrolizidine alkaloids in adult food sources.
This form of defence is called Batesian mimicry. It is only effective because
the toxic species far outnumber the non-toxic species. If the situation was
reversed, and most of the butterflies attacked were palatable, the mimicry would
serve no purpose.
Monarchs and Tigers belong to the genus
Danaus.
They are large butterflies, characterised by their orange wings, which have a black
apex, and white subapical spots. On the males there is a patch of raised
androconial scales on the hindwings.
The
bright colours advertise their poisonous qualities
to birds in the same way that the bands of yellow and black of wasps
advertise the fact that they can sting. Any bird that attempts to eat the
butterfly will immediately vomit and suffer nausea and other side
effects. Birds have excellent memories and an ability to learn
from unpleasant experiences, so consequently avoid eating
similarly coloured butterflies in the future.
The Monarch is the most famous migrant in the butterfly world.
Its
powers of migration are so great that it has been able to
spread to across the Americas from Canada to Peru. It has also crossed the
Pacific ocean, reaching Australia, New Zealand and Papua New
Guinea; and across the Atlantic to north Africa, southern Europe
and even to India.
In North America
millions of Monarchs
migrate annually over a distance of 2000 miles ( 3200km ) between their breeding territories in Canada and
their
southern over-wintering grounds in Mexico. Tagging of individual butterflies has
shown that they regularly cover distances
of up to 1100 miles in just a few days. Each
autumn as the climate cools in North America, vast numbers fly
south and channel into a few forested areas in the Mexican Highlands. During
the winter months, fir trees in the tiny El Rosario sanctuary become festooned
with millions of Monarchs. They totally cover the leaves, branches and trunks,
sometimes even causing trees to fall under their weight.
In February
& March they awaken from hibernation and the air becomes a swirling mass of orange and black as tens of thousands of Monarchs take to the air. As the days get
warmer they start to filter out of the sanctuaries and begin their
return journey northwards. The females pause to lay eggs as they travel,
creating temporary colonies along the route. The progeny also migrate
north, laying their own eggs. Most of the original butterflies probably
perish during the return journey, but there is evidence to show that
some manage to return to the original breeding areas in the north.
It is little
known that these amazing butterflies also regularly overwinter in small numbers
in arid desert locations such as Saline valley in California - a moon-crater
shaped valley about midway between Mount Whitney and Death Valley in the Mojave
Desert.
Monarchs
regularly survive the winter there despite the very low daytime humidity ( 5-25%
), scant winter rainfall and the fact the only evergreen vegetation in mid and
late winter are bushes such as tamarisk, mulefat and creosote. One problem with
the Saline Valley habitats however is that once every five years or so,
overnight temperatures drop several degrees below zero, and freeze most or all
of the Monarchs.
The
Monarch's annual migration is
controlled by a "time-compensated sun compass" that depends on light receptors
and a circadian clock, both of which are built into their antennae. The circadian
clock employs rhythms of biochemical, physiological or behavioural processes
which control daily, seasonal and annual activities - including migration. When
scientists removed the antennae from one group of Monarchs they flew strongly
but in random directions, but a control group with their antennae intact all
flew in the same direction - their south-westerly migration route. In another
experiment the antennae of some were painted with black enamel, and these
butterflies when placed in a flight simulator all flew together, but in the
"wrong" direction compared to their normal migration route. Another group had
their antennae painted with transparent paint, and these all migrated together
in the right direction.
Habitats
Because of its
migratory nature the Monarch can be found in virtually any
habitat
, including grasslands & prairies, deciduous temperate forest,
montane pine forest, sub-tropical rainforest, coastal habitats,
deserts, parks, gardens, cities etc. It can be found at
elevations between sea-level and at least 3400 metres. The
butterflies can also be seen out at sea, and often settle on
ships.
Monarchs will
breed almost anywhere where the larval foodplants are available,
typically in fields, meadows, forest glades and roadside
habitats at elevations between sea level and about 1000m.
Lifecycle
The egg is shaped like a tall dome, straw coloured, and covered
in vertical keels, each linked by numerous small horizontal
ridges. It is laid singly on the underside of leaves of the
foodplants which are almost invariably
Asclepias but also include
Calotropis ( Apocynaceae ).
The caterpillar
when fully grown is white, with each segment marked with narrow
black and yellow vertical bands. The 2nd thoracic segment and
8th abdominal segment each bear a pair of black whip-like
protuberances. Caterpillars are often parasitised by a tachinid
fly Lesperia archippivora.
The larval
foodplants Asclepias contain
cardenolides - toxins which can induce cardiac arrest in small
vertebrates. The toxins are sequestered by the caterpillars, and
passed on to the adult butterflies which utilize them for
defence against insectivorous birds, reptiles and rodents.
The pale green
chrysalis is plump and barrel-shaped, with the abdominal
segments compressed, forming a dome. At its widest point there
is a narrow abdominal band studded with yellow and black dots.
The chrysalis is suspended by a stout cremaster from stems on or
near the foodplants.
Adult behaviour
The butterflies
have a powerful but fluttering flight, interspersed with periods
of soaring and gliding in
wide circles as they fly from one clump of flowers to another.
They settle frequently to nectar at
Asclepias, Aster,
Cirsium,
Dispacus, Solidago,
Syringa and
Vernonia.
Courtship takes
place in late morning, at which time the male pursues the female
in flight, nudging and cajoling her until she settles,
typically on a bush, where copulation takes place.
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