Byasa polyeuctes is a large butterfly that is the most common member in India of the Windmill genus (Byasa).
   They are found in Pakistan, the northern ranges of India, Nepal, Bhutan, Myanmar, northern Thailand, Laos, Vietnam, southern China (including Yunnan) and Taiwan.
    Their preferred habitat is forests and woods, and frequent river valleys.
   They occur at low elevations (1,000 to 5,000 feet (300 to 1,520 m)) in northeast India but are found at significantly higher altitudes in the western extent of their range. (up to 11,000 feet (3,400 m) in Kashmir.)
    They are seen on wing between April and September, flying in their very leisurely fashion.
   They have several broods when they find suitable climates.
     They are black and white, with distinctive swallowtail markings.
   Their bodies are red, with a red head and abdomen.
   Their forewings are entirely black.
   Their hindwings are characterized by a traditional swallowtail shape featuring rectangular or parallelogram shaped white patches near the outer margins, sometimes also present on the central portion of the wing, on both the dorsal and ventral sides.
   Typically, there are five red spots scattered near the wing disc towards the tail, with one spot located at the very tip of the tail.
    Both sexes are alike in appearance.