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Butterflies of Australia & New Zealand
The Monarch
Danaus plexippus  LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily - DANAINAE
Tribe - DANAINI
subtribe - DANAINA

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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