THE BUTTERFLY: The Life History of the Sumatran Sunbeam

Life History of the Sumatran Sunbeam (Curetis saronis sumatrana)

Butterfly Biodata:
Genus: Curetis Hübner, 1819
Species: saronis Moore, 1877
Subspecies: sumatrana Corbet, 1937
Wingspan of Adult Butterfly: 40mm
Caterpillar Local Host Plants: Derris trifoliata (Leguminosae)

A Sumatran Sunbeam taking a typical perch on the underside of a leaf.

A sunbathing female Sumatran Sunbeam on a leaf perch.

Physical Description of Adult Butterfly:

Above, the wings are orange with black borders. The female is in paler orange than the male and has much broader black borders. On the hindiwng the orange portion in the female only occupies a small crescentic area. Underneath, the wings are silvery white. On the hindwing, the post-discal striae are lunulate and irregular, with those in spaces 4 and 5 shifted out distad. The post-discal striae are hardly shaded inwardly. Legs are banded in red.

Another Sumatran Sunbeam adult.

Another sunbathing female Sumatran Sunbeam.

Field Observations of Butterfly Behaviour:

This species is locally common in Singapore in coastal mangrove areas such as the Sungei Buloh Wetland Reserve, Kranji Nature Trail, Pasir Ris Park and Pulau Ubin. Adults can be sighted along walking trails in these areas where they are observed to engage in sunbathing, frolicking or dog-fighting activities. The adults also have the habit of perching on the underside of a leaf.

Early Stages:

The host plant, Derris trifoliata, is a scandent shrub with 3-5 leaflets in a pinna, pink flowers, thin and flat pods. This plant is commonly found in coastal areas and thus account for the rather localized distribution of the Sumatran Sunbeam in these areas. The caterpillars of the Sumatran Sunbeam feed on the young foliage of the host plant.

Host plants : Derris trifoliata

Eggs are laid singly on young shoots of the host plant. It is common for a number of eggs to be found on the same shoot. Each egg resembles a flattened sphere with coarse hexagonal reticulations. It is initially light greenish when newly laid but turns whitish as it matures. Each egg has a basal diameter of about 0.8mm.

Two views of an egg of the Sumatran Sunbeam.

It takes 2-3 days for the egg to hatch. The young caterpillar consumes the upper portion of the egg shell to emerge. With a length of 1.9mm, it has numerous fine setae on its body surface, with those occurring dorso-laterally and laterally particularly long. The caterpillar is initially yellowish green but turns dark yellowish as it develops in this instar. Numerous short lateral stripes, pale in coloration, also appear in the later stage of this instar. The first instar lasts about 2 days and with the body reaching about 3mm.

Two views of a newly hatched caterpillar, length: 1.9mm.

Two views of a 1st instar caterpillar, length: 2.6mm.

The 2nd instar caterpillar does not have the long dorso-lateral and lateral setae as in the first instar. Instead, the body is covered with many minute setae. The body colour is dark yellowish initially turn increasing greenish as it grows in this instar. On each body segment, there are several oblique pale stripes laterally. A prominent change occurs on the 8th abdominal segment where a pair of long cylindrical and erect processes, black to dark brown in colour, is now present. When the caterpillar is disturbed or stressed, tentacles with tuffs of white and black hairs at their apices, are everted from these procesess and whirled around at great speed. This instar lasts for about 2 days and reaches a length of about 4.5-5.0mm.

Two views of a 2nd instar caterpillar, early in this stage, length: 3.2mm..

Two views of a 2nd instar caterpillar, later in this stage, length: 5.5mm

The 3rd instar caterpillar looks similar to the 2nd instar caterpillar but has a pair of white, quadrate lateral patches on the 6th abdominal segment. White lateral stripes, varying in lengths but lesser in prominence than that on the 6th abdominal segment, appear later and becoming more prominent on the body segments. The caterpillar reaches up to 9.5mm in this instar which lasts about 2.5-3.0 days.

Two views of a 3rd instar caterpillar, early in this stage, length: 6.3mm.

Two views of a 3rd instar caterpillar, late in this stage, length:9.5mm

The eversion of the tentacles of a Sumatran Sunbeam caterpillar. The same 1-second eversion and withdrawal event is played back three times, with first and last time being at actual speed, and the second time in slow motion (20 times slower).

The 4th (and final) instar caterpillar resembles the 3rd instar caterpillar but with a more prominent contrast of white stripes/patches against the base colour of yellowish green to velvety green. The dorsum of the body segments of the 2nd and 3rd thoracic segments are pale green to whitish in coloration. The lateral oblique white stripes on the 1st two abdominal segments are joined while those on other segments are well separated.

Two views of a 4th instar caterpillar, early in this stage, length: 10mm.

Two views of a 4th instar caterpillar, later in this stage, length: 16mm

Two views of another 4th instar caterpillar, length: 16mm.

The 4th (and final) instar lasts about 3 days and the body reaches a length of about 16-17mm. In the last day of this stage, the caterpillar ceases feeding, and its body shrinks in length. The entire body colour changes to a dark shade of jade green. Eventually it comes to rest on a spot on the surface of leaf. Here the caterpillar prepares for pupation by spinning a silk pad and a silk girdle to secure itself. It then becomes immobile in this pre-pupatory state for about 1 day.

Two views of a pre-pupa of the Sumatran Sunbeam

The next day the pupation takes place. The pupa is held via its cremaster and a weak silk girdle to the silk pad on the substrate. It is almost hemispherical and has a flat under surface, almost completely jade green in ground colour and carries a whitish marking on the dorsum of the thorax. Length: 9-10mm.

Two views of a pupa of the Sumatran Sunbeam

Six days later, the pupa becomes darkened in color signaling the imminent emergence of the adult. The orange patch on the forewing upperside can now be seen through the pupal skin in the wing case. The next day the adult butterfly emerges from the mature pupa.