Chaetodon falcula has a white body which is marked with a number of thin vertical dark grey lines. It has two clear black saddle-like blotches on the dorsal part of the body, a vertical black eye band and a black caudal peduncle. The dorsal, anal and caudal fins are yellow.^[3] The yellow continues onto the body adjacent to the dorsal and anal fins.^[4] There are 12–13 spines in the dorsal fin and 23–25 soft rays while the anal fin contains 3 spines and 20–21 soft rays. This species attains a total length of 20 centimetres (7.9 in).^[2]

Chaetodon falcula is found in the Indian Ocean where it occurs along the East African coast between Kenya and South Africa, Madagascar and the other western and central islands of the Indian Ocean, Sri Lanka, southern India, including the Andaman and Nicobar Islands, and from the waters of western Thailand east as far as Java.^[1] It occurs in the Cocos (Keeling) Islands, which are part of Australia, too.^[4]

Chaetodon falcula is found at depths of 1 to 15 metres (3.3 to 49.2 ft).^[1] They are found on the reef edge and upper slope normally in habitats subjected to strong currents although the juveniles hide among corals. Typically seen in pairs or in small aggregations,^[2] of up to 20 fish.^[1] Their diet is made up feeds of the polyps, worms, sea anemones, crustaceans, sponges, the feet of sea urchin and other invertebrates.^[3] This species forms distinct pairs to breed and the pelagic eggs are scattered in open water.^[5]

Chaetodon falcula was first formally described in 1795 by the German physician and naturalist Marcus Elieser Bloch (1723-1797) with the type locality given as Tharangambadi in India.^[6] The specific name falcula is a diminutive of the Latin falx or falcis meaning a "sickle" or a "scythe" and is a reference to the wedge shaped black saddle blotches which Bloch illustrated as sickle-shaped, which they are not in modern photographs.^[7]

With its sister species, the similar-looking Pacific double-saddle butterflyfish (C. ulietensis),^[4] C. falcula belongs to the large subgenus Rabdophorus which might warrant recognition as a distinct genus. Other fairly close relatives are the quite differently-shaped but similarly coloured lined (C. lineolatus) and spot-naped butterflyfishes (C. oxycephalus), while the blue-cheeked butterflyfish (C. semilarvatus) seems to be a far more basal lineage relative to all of these. The bluish vertical lines are present in all of these, while a white body with yellow behind and black on back and caudal peduncle are only shared among the four less ancient species.^[8][9]

• Photos of Chaetodon falcula on Sealife Collection