The World Divided
For zoologists including entomologists, the world is divided into six main fauna zones.
The Pal(a)earctic (the 'old northern part') is the Old World, comprising all of Europe, Africa north of the Sahara, Asia Minor, northern Arabia, the Middle East and Asia north of the Himalayas.
The Nearctic (the 'new North') consists of North America and Greenland. The southern border is in the Caribbean, roughly along a line dividing the southern tip of Florida and the northern tip of Yucatán along the 25th parallel. For this reason most of Mexico is part of the Nearctic.
Both Palearctic and Nearctic together form the Holarctic (the 'whole North').
South and tropical Central America and the West Indies form the Neotropical fauna zone.
All of Africa south of the Sahara including southern Arabia and Madagascar make up the Afrotropical fauna zone (also called Ethiopian zone).
The Oriental region is Asia south and southeast of the Himalayas plus the Malay archipelago.
Finally, Australia, New Zealand, Tasmania, New Guinea and the islands of Polynesia form the Australian region.
Most butterflies and moths fly in the tropics. According to John B. Heppner's estimate, there are 90,000 species in the Neotropical and 50,000 in the Oriental region whereas the Palearctic has only 25,000 and the Nearctic a mere 14,000. The best studied fauna zone is the Palearctic where the systematic study of insects began and where some 92 per cent of the fauna are known. In North America, the rate is 82 percent while in the four other regions it is only slightly over 50 per cent.
 John B. Heppner, op.cit., p. 2