An Apatura iris, illustrated by Roger Vigurs.
This stamp was issued in 1985.
I admit I wasn't exactly sure where Tuvalu was (as a lifelong geography geek, these lapses annoy me) though I was fairly certain it was somewhere in the South Pacific. Tuvalu is a Polynesian island nation midway between Hawaii and Australia. It is about 180º west of the international dateline. Vaitupu is the one of the 'islands' - actually an atoll comprising 9 isles - with a current population of around 1,700, making it the second most populated of Tuvalu.
The Apatura iris, also known as the Purple Emperor, is part of the Nymphiladae family (that also includes other emperors, the Monarch, fritillaries, and admirals). Despite the name, the females have a dull brown appearance with only the males having the purple colour (as seen on the stamp) though the iridescent purple can only be seen from a certain angle. The adults can have a wingspan of around 3".
They spend most of their time in the tree canopy and unlike other butterflies they don't feed on nectar from flowers, instead enjoying a tasty meal from dung, urine and animal carcasses and a dessert of aphid honeydew and oak tree sap.
Turns out they don't live in Tuvalu at all, but are found in the woodlands of Europe and England (but not Wales or Scotland). It is the second largest butterfly found in England and is apparently quite elusive. I found this interesting tidbit of the lengths people go to to find this beauty.
for an easier butterfly trek, follow the links at this week's Sunday Stamps II