The area covering the states of Arunachal Pradesh, Assam, Bihar, Manipur, Mizoram, Nagaland, Orissa, Sikkim, Tripura and West Bengal in India (ca 506,174m2) supports 294 species and subspecies of Odonata distributed over 112 genera, 17 families and 3 suborders.
The monograph contains keys for identification of the species examined, their variations from the reference collections present in Zoological Survey of India and descriptions available in the Fauna of British lndia volumes, geographical distribution in detail of each species and subspecies recorded from the area. It also embodies description of new species of Calicnemia and Gomphidia and five indeterminate species, females of Agriocnemis nana, Indothemis limbata limbata.
The study reveals that Odonata of India do not, in general, display marked variations from that of other parts of India. Indeed some variations may be noted but are not of much taxonomic importance. Regional variations are random so that no ecological rule could be considered.
Analysis of the geographical distribution of taxa reveals that two genera and 33 species and subspecies are endemic to eastern India. Others occur in the old world. Three species also occur in the New World. About 64 species and subspecies occur in China, 127 species and subspecies in Indo-China, 57 occur in Malayasia, 30 in the Paleartic region (excluding China), 27 in Australia; nine in Africa, seven in Seychelles, and five in Madagascar.
It is reasonable to think that the present relationship of the fauna has been established through the faunal exchange after the formation of the Himalayas. Very likely this trans-exchange took place primarily through the eastern border line (Assam-Burma frontier).
Zoogeographically Odonata of eastern India bear more affinities with the fauna of the Indo-Chinese subregion than with any other part of the world. Furthermore, the fauna of the Assam region, the Gangetic plain and the fauna of the eastern Himalaya are closer to the Indo-malayan forms than those of the Indian peninsula. Moreover, NE India (North Bengal, the Assam area and the eastern Himalaya) is rich in endemic fauna. It is presumed that the topography and dense vegetation help in breaking the population into small populations and reduce the migration pressure. And in consequence population effects are called into play which boosts speciation.
According to the distribution in ecological zones the fauna can be classified into Montane forms, Non-montane forest forms. Eclectic forms, Paraxerophilic forms, Urban forms and species with wide adaptations.
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