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. 
Monarch butterfly Danaus plexippus. female 2 .JPG

Danaus plexippus female (male photo below)Female Danaus plexippus Monarch Butterfly-001.JPG

Danaus plexippus male. Notice the pheromone releasing spot on each hind wing.
1-male Monarch butterfly Danaus plexippus .JPG


Danaus plexippus (female)Monarch butterfly Danaus plexippus-3.JPG

Danaus plexippus (female)
Monarch butterfly Danaus plexippus-6.JPG

Sunlight shining through Danaus plexippus partly translucent wings.
Danaus plexippus Monarch butterfly.JPG

A male butterfly of a swan plant the favourite food of the caterpillar.
Danaus plexippus Monarch Butterfly.JPG

Danaus plexippus head ( female)
Female Monarch butterfly Head Danaus plexippus.jpg  
Butterflies mating.
Monarchs mating.JPG

A monarch butterfly caterpillarMonarch Danaus plexippus-13.JPG

Monarch Butterfly Danaus plexippus-003.JPG

1- Danaus plexippus Monarch caterpilla.JPG

The caterpillar has just attached itself to a branch in preparation for forming a chrysalis
Monarch Butterfly Danaus plexippus-005.JPG 
Monarch Butterfly Danaus plexippus-004.JPG

Next stage, forming a chrysalis.
Danaus plexippus Monarch Butterfly-003.JPG

Danaus plexippus Monarch Butterfly-004.JPG

The monarch butterfly chrysalis
Monarch Butterfly Danaus plexippus-006.JPG 
The monarch butterfly chrysalis with the developing butterfly's wing patterns just visible.
Monarch Butterfly Danaus plexippus-007.JPG 

A video: How A Caterpillar Becomes A Butterfly 

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