Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Superfamily: Papilionoidea
Family: Nymphalidae
Subfamily: Danainae
Tribe: Danaini
Genus: Danaus
Species: D. plexippus
Scientific name: Danaus plexippus
Common name: Monarch Butterfly

The monarch arrived in New Zealand about 120 years` ago crossing the Pacific from North America. It is New Zealand's largest resident butterfly. 
Its caterpillar's food is the Swan plant (Asclepias physocarpa). Their live cycle is two weeks as a caterpillar, two weeks as a chrysalis and about two months as a butterfly. 
Female Monarchs have darker veins on their wings, and the males have a spot in the centre of each hindwing from which pheromones are released. Males are also slightly larger.

For information on pheromones visit http://en.wikipedia.org/wiki/Pheromones

A butterfly at rest. 
[1]

Danaus plexippus female (male photo below)[2]

Danaus plexippus male. Notice the pheromone releasing spot on each hind wing.
[3]


Danaus plexippus (female)[4]

Danaus plexippus (female)
[5]

Sunlight shining through Danaus plexippus partly translucent wings.
[6]

A male butterfly of a swan plant the favourite food of the caterpillar.
[7]

Danaus plexippus head ( female)
[8]  
Butterflies mating.


A monarch butterfly caterpillar[9]

[10]

[11]

The caterpillar has just attached itself to a branch in preparation for forming a chrysalis
[12] 
[13]

Next stage, forming a chrysalis.
[14]

[15]

The monarch butterfly chrysalis
[16] 
The monarch butterfly chrysalis with the developing butterfly's wing patterns just visible.
[17] 

A video: How A Caterpillar Becomes A Butterfly 

 
Thanks to Wikipedia for text and information http://creativecommons.org/licenses/by-sa/3.0/