21 November 2015

Life History of the Bengal Swift

Life History of the Bengal Swift (Pelopidas agna agna)

Butterfly Biodata:
Genus: Pelopidas Walker, 1870
Species: agna Wallace, 1866
Sub-Species: agna Wallace, 1866
Wingspan of Adult Butterfly: 32-36mm
Caterpillar Local Host Plant: Brachiaria mutica (Poaceae, common names: Para Grass, Buffalo Grass, Dutch Grass, Giant couch, Scotch Grass).

The upperside view of a male Bengal Swift.

A close-up view of the forewing upperside of a male Bengal Swift. The lower cell spot is minuscule in this specimen.

Physical Description of Adult Butterfly:
On the upperside, the wings are brown. There are white post-discal hyaline spots in spaces 2-4, 6-8 and two cell spots in the forewing. The male has a narrow oblique brand in the forewing running from the spot in space 2 towards the dorsum (the brand is angled in such a way that a line drawn from and through the two cell spots would touch the brand at its lower end). The female has additional white spots in spaces 1b, consisting of one minute upper spot and one larger lower spot. The line drawn the cell spots would typically pass far from the spot in space 1b. On the underside, the wings are ochreous without a greyish tinge. The forewing have the same spots as per the upperside, and the hindwing has a cell spot and a series of post-discal spots in spaces 2 to 5.

A male Bengal Swift.

The upperside view of a female Bengal Swift, showing the additional post-discal spots in space 1b.

The underside view of the same female Bengal Swift in the above picture.

Field Observations of Butterfly Behaviour:
The Bengal Swift is moderately common in Singapore. Easily confused with the similar-looking Small Branded Swift, the adults have been sighted at multiple locations including grassy wastelands, urban parks and gardens across the island. The adults fly with a swift, strong and darting flight.

Early Stages:
The Bengal Swift has so far been bred on just one grass species locally, Brachiaria mutica, a moderately common grass species found in grassy wastelands. The caterpillars feed on leaves of the host plant, and live in shelters formed by joining edges of a grass blade together.

Local host plant: Brachiaria mutica.

The eggs are laid singly on the upperside of a grass blade of the host plant. Each dome-shaped egg is whitish with a basal diameter of about 1mm.

Two views of an egg of the Bengal Swift.

Two views of a maturing egg, with the black head capsule visible through the egg shell.

Two views of a mature egg, with the head capsule visible through hole in the egg shell.

It takes about 3.5-4 days for the egg to hatch. The young caterpillar eats just enough of the shell to emerge, and has a length of about 2.5mm. Its cylindrical body is pale yellowish with a tuff of moderately long setae at the posterior end. The head capsule is black and right behind it a black collar mark is present on the prothorax. The newly hatched nibbles away most of the egg shell remnant before proceeding to construct its first leaf shelter.

The newly hatched caterpillar eating its own egg shell.

The newly hatched caterpillar resting near the nearly-eaten egg shell.

The body turns yellowish green after the caterpillar has started feeding on the grass blade. By the time the caterpillar lies dormant for its moult to the 2nd instar, its length has reached 5-5.5mm. The 1st instar takes about 3-3.5 days to complete.

Two views of a 1st instar caterpillar, length:4.2mm.

The 2nd instar caterpillar still has a yellowish green body, and the head capsule is still black. The black collar mark on the prothorax has faded to just to hint of its presence. Faint whitish doro-lateral and lateral bands are observable. This instar lasts about 3 days with the body length reaching about 9mm.

Two views of a 2nd instar caterpillar, early in this stage, length: 5mm.

Two views of a 2nd instar caterpillar, length: 7mm.

The 3rd instar caterpillar has a dark brown to black head capsule with two lateral pale yellowish brown bands. On the body, the dorso-lateral and lateral bands are more prominent with a whitish to yellowish coloration. There is no longer any trace of the black collar mark on the prothorax. This instar lasts about 2-3 days with the body length reaching about 14mm.

Two views of a 3rd instar caterpillar, early in this stage, length: 8.8mm.

A 3rd instar caterpillar, length: 12mm.

The 4th instar caterpillar has a head capsule which is whitish in ground colour but reddish brown to black along the periphery and various sulci. Two reddish brown stripes rise from the adfrontal area. This penultimate instar lasts about 3-4 days with the body length reaching up to 21-23.5mm.

Two views of a 4th instar caterpillar, early in this stage, length: 14mm.

Two views of a 4th instar caterpillar, late in this stage, length: 21mm.

The 5th instar caterpillar features a head capsule which is pale yellowish to pale green in ground colour. The periphery is marked with a broad reddish band on each side of the head capsule. Each peripheral band is flanked with whitish bands on both sides. The anal plate is unmarked as in the all previous instars. This final instar takes about 5-6 days to complete with the body length reaching 40-43mm.

Two views of a newly moulted 5th instar caterpillar.

Two views of a 5th instar caterpillar, length: 34mm.

Towards the end of 5th instar, the body of the caterpillar shortens in length and body colour assumes a uniform shade of pale lime green. It seeks out a spot on a leaf blade where it constructs a shallow but half-open shelter with silk threads at both ends. The body excretes a moderate amount of white waxy material at this stage. Within the shelter, a silk girdle and a silk pad are then spun. Once the caterpillar attaches its claspers to the silk pad, it enters the dormant prepupatory phase which lasts about one day.

Two views of a pre-pupa of the Bengal Swift. with the lower one being later in this phase.

The pupa secures itself with the silk girdle and with its cremaster attached to the silk pad. It has a short thorax, a rather long abdomen and a pointed rostrum. The body is deep lime green in the thorax and wing case but yellowish green in the abdomen. Narrow, whitish, dorso-lateral and lateral bands run lengthwise on the abdomen. Length of pupae: 29-30mm.

Two views of a pupa of the Bengal Swift.

After 7 days, the pupa becomes mostly black in color in the wing pads and in the body segments. Eclosion takes place the next day.

Two views of a mature pupa of the Bengal Swift.

A newly eclosed Bengal Swift.

  • [C&P4] The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, Malayan Nature Society, 1992.
  • Butterflies of Thailand, Pisuth Ek-Amnuay, 2nd Edition, 2012.
  • A Field Guide to the Butterflies of Singapore, Khew S.K., Ink On Paper Communications, 2nd Edition, 2015.

Text by Horace Tan, Photos by Bendecit Tay, PF Loke and Horace Tan

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