Sikkim is one of the smallest Himalayan states, with an area of 7,299 sq. km. the biodiversity has given Sikkim an unique status. For example, within 30 km of Tholung Valley the altitude rises from 600 m to 5,500 m. Due to this steepness of the mountain and the geographical and climatic conditions, the floral and faunal diversity ranges from tropical species to high altitude cold desert species.
Of total of about 1,400 butterflies recorded from the Indian Sub-continent almost 50% of butterflies are recorded from Sikkim. Of the total area of Sikkim 40% (North Sikkim) is almost inhabitable and is covered with snow for about 4-8 months to perpetual snow and unsuitable for any life. About 30% of the total area of Sikkim which occupies the altitudinal zone from about 200-1,800 m is represented by about more than 75% species butterflies of Sikkim. Remaining species are found in the in-between zone and some of them overlap all the zones.
The subfamily Amathusiinae occurs mainly below 900 m. The region between 600-1,800 m is occupied by the typical hilly region butterflies. The Lycaenidae and Hesperiidae are highly diversified. Many of the type specimens of Hesperiids have been collected from Sikkim and are in NHM. The nymphalids and danaids are abundant in this region. They include butterflies like the Barons, Pansies, Sailers, Sergeants, Tigers, Crows etc. These butterflies have affinity to Oriental fauna. The Swallowtails also abound this region. Pieridae is represented by Gulls, Puffms, Jezebels and Orangetips.
The zone between 1,800-3,500 m has butterfly fauna typical of temperate zones and have affinity to the Palaearctic fauna. The nymphalids which are found in this region are the Admirals, Tortoiseshells, Silverstripes and Silverspots. Hardly any danaids except for the Chestnut Tiger, occur in this region. Lycaenids mostly consist of the subfamilies Lyaceninae, Theclinae and Polyommatinae- Hairstreaks, Sapphires, Hedge Blues etc. Lybithinae occurs mostly in this zone. Punches and Judies are also seen. But the most diversified is the subfamily Satyrinae, particularly the tribes Lethini and Satyrini. Foresters, Walls, Golden and Silver Forks are numerous. As many as 36 species of tribe Lethini are found in Sikkim. Of the Swallowtails very few larger butterflies occur in this region and include the Krishna and Blue Peacocks. The Yellow Swallowtail in Sikkim has been recorded only from the altitudes above 3,000 m, although it occurs up to 2,000 m in other parts of Himalaya.
The zone beyond 3,000 m is occupied by a very few specialised species which are adapted to harsh climatical conditions and have affinity to the Palaearctic fauna. They include Apollos and Yellow Swallowtail of the Papilionidae. The Clouded Yellows of Piriedae, the Silverstripes and Silverstreaks and the Admirals of Nymphalidae and a very few species of blues like Chumbi Green Underwing, Azure Mountain Blue etc. also occur. Satyrinae is represented by the tribe Satyrini consisting of the Arctic and Mountain Arguses and the Great Satyr.
A few butterflies on account of being polyphagus are found from sea level to the high mountains up to about 4,500 m. They are the Indian and the Large Cabbage Whites, Tortoiseshells, Indian Red Admiral, Painted Lady, Dark Clouded Yellow etc.
The Geographic position, i.e. Sikkim is bound by high mountains of more than 4,000 m on three sides, has led to isolation of the the' population occurring in Sikkim. Probably due to this reasons many of them have become distinct subspecies and forms.
There are many Oriental species which do not occur west of Sikkim. The reason for this may be that the great North-South ridge of the Khangchenzonga spur and Singalila act as barrier for dispersal of the species. Similarly many Palaearctic species like Lassiomata, Hipparchia and Dallacha have not been recorded east of Nepal.
Many of the subspecies are known only from Bhutan and Sikkim. Infact the following species have been so far recorded only from Sikkim -Lethe trisigmata, Lethe atkinsoni, and that too from high altitudes of Lachen and Lachung Valleys. But this does not necessarily mean they are endemic to Sikkim only as the surrounding regions like Bhutan, Arunachal Pradesh, South Tibet have similar biodiversity. But hardly these areas have been explored in detail for butterfly fauna also a very few records are available from these regions in last hundred years.
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