Life History of the Formosan Swift

Life History of the Formosan Swift (Borbo cinnara)

Butterfly Biodata:
Genus: Borbo Evans, 1949
Species: cinnara Wallace, 1866
Wingspan of Adult Butterfly: 30-34mm
Caterpillar Local Host Plants: Paspalum conjugatum (Poaceae; common name: Buffalo Grass), Setaria barbata (Poaceae; common name: Bristly Foxtail Grass),
Centotheca lappacea (Poaceae, common name: Sefa).

A female Formosan Swift.

A male Formosan Swift.

Physical Description of Adult Butterfly:

On the upperside, the wings are brown with the basal area yellowish green. The forewing has a yellow non-hyaline spot in space 1b and a decreasing series of hyaline spots from spaces 2 to 4, 6 to 8 and two spots in the cell. In some specimens, one or both cell spots could be absent. The hindwing has a few obscure hyaline spots, and does not have any cell spot. On the underside, the wings yellowish brown with greenish scaling. The hindwing has whitish spots in spaces 2-4 and 6, where the spot in space 4 is typically small or even absent.

The upperside view of a female Formosan Swift.

The upperside view of a male Formosan Swift.

A worn specimen of a male Formosan Swift.

Field Observations of Butterfly Behaviour:

The Formosan Swift is relatively common in Singapore. Sightings are rather frequent and spread over many parts of Singapore, at locations such as nature reserves, neighbourhood parks, offshore islands, wastelands and park connectors. The swift flying adults have been observed to visit flowers and sunbath in sunny weather.

Early Stages:

Formosan Swift is known to utilize a number of grass species as larval hosts across its range of distribution in the region. Locally in Singapore, it has been observed to utilize the buffalo grass and the bristly foxtail grass as larval hosts, though it is almost certain that more can be added to the list. The caterpillars of the Formosan Swift feed on the leaves of the host plant in all instars, and live in leaf shelters constructed with silk threads securing opposite edges or a cut segment of a grass blade.

Local host plant: Setaria barbata.

A Formosan Swift laying an egg on the underside of a grass blade.

The eggs are laid singly on a leaf of the host plant, on either side of the blade. The dome-shaped egg is initially creamy white with a greenish tinge, and appears to be smooth to the naked eyes. Each egg has a basal diameter of about 1mm.

Two views of an egg of the Formosan Swift, diameter: 1mm.

It takes about 3 days for the egg to hatch. One the final day, the dark head capsule is visible through the egg shell. The young caterpillar eats just enough of the shell to emerge, and then immediately proceeds to finish the remaining egg shell (see video clip below). The newly hatched has a length of about 2mm. Its creamy white body is cylindrical in shape with a small number of very short and tiny dorso-lateral and lateral setae. There is a tuff of moderately long setae on the posterior segment. Its head is black with a black collar lying behind it on the prothorax

Left: A mature egg. Right: egg site after the newly hatched has eaten the egg shell.

Two views of a newly hatched 1st instar caterpillar, length: 2mm.

After consuming the egg shell, the newly hatched caterpillar  constructs its first leaf shelter,  typically at the tip of the grass blade bearing the egg. The body takes on a yellowish green coloration after a few feeding sessions on the leaf. The 1st instar takes a total of 3 days to complete with body length reaching about 4.5mm.

The (very 1st) leaf shelter of a 1st instar caterpillar.

Two views of a 1st instar caterpillar, late in this stage, length: 3.2mm.

Two views of a late 1st instar caterpillar, dormant prior to its moult.

The body of the 2nd instar caterpillar resembles that of the first instar, but with the dark collar on the prothorax now absent. The tuff of long setae at the posterior end is replaced by much shorter peripheral setae. This instar lasts a total of 3 days with the body length reaching up to 7.5mm.

Two views of a 2nd instar caterpillar, early in this stage, length: 4.5mm.

Two views of a late 2nd instar caterpillar, dormant prior to its moult, length: 7.2mm.

The leaf shelter of a 2nd instar caterpillar of Formosan Swift.

The 3nd instar caterpillar has a pale yelowish body with a green undertone. The body bears a few narrow, faint, whitish longitudinal bands. The head capsule is still black. This instar lasts about 3 days with the body length reaching about 12.5-13.5mm.

Two views of a newly moulted 3rd instar caterpillar, length: 6.8mm.

Two views of a 3rd instar caterpillar, length: 9.8mm.

Two views of a late 3rd instar caterpillar, dormant prior to its moult, length: 12.8mm.

The 4th instar caterpillar resembles the 3rd instar caterpillar closely with somewhat more prominent whitish bands. Whilst a minority of specimens still sport a totally black head capsule, most have two whitish patches of varying extent on both sides of the head capsule (see pictures below). This instar lasts about 3-4 days with the body length reaching up to 20-22mm.

Two views of a 4th instar caterpillar, length: 11.5mm.

Two views of another 4th instar caterpillar, length: 11.5mm.

Two views of a yet another 4th instar caterpillar, later in this stage, length: 17mm.

Two views of a late 4th instar caterpillar, dormant prior to its moult, length: 21mm.

Again the change from the 4th instar to the 5th takes place on the head capsule with most specimens possessing a head dominated by two large and broad white lateral patches, with back patches lining the epicranial suture varying in thickness. In some specimens, these middle black patches are absent. The 5th instar takes about 5-6 days to complete with the body length reaching up to 36-39mm.

Two views of a 5th instar caterpillar, early in this stage, length: 22.5mm.

Two views of a 5th instar caterpillar, later in this stage, length: 33mm.

Two views of another 5th instar caterpillar, showing a variation in the head appearance.

In the last day of this instar, the body decolorises to take on a jade green coloration. The caterpillar ceases feeding and its body shortens. It finds a spot on the underside of a grass blade to be its pupation site. Here it excretes a whitish powdery substance to coat the surface, and spins short transverse silk bands at two ends to shape the pupation site into a shallow cavity. Aftering securing itself with a silk girdle and a short transverse band at its posterior end, the caterpillar becomes dormant. This prepupatory phase lasts about 1 day.

A pre-pupatory larva of the Formosan Swift secured at its pupation site.

The yellowish green pupa is secured with a cremastral attachment and the silk girdle across the wing pads. It has a short thorax, a rather long abdomen and a pointed rostrum. The body bears a few narrow whitish bands, dorsally and dorso-laterally, on its abdomen. Length of pupae: 28-29mm.

Two views of a pupa of the Formosan Swift, length:28mm

After 8 days, the pupa gradually turns black, first in the thorax and wing pads, and then in the abdomen, in the night before eclosion. The next day, the newly minted adult emerges from the pupa.

Two views of a mature pupa of the Formosan Swift.

A newly eclosed male Formosan Swift.

References:

• [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, 2010.

Text by Horace Tan, Photos by Koh Cher Hern, Federick Ho, Khew SK and Horace Tan