THE BUTTERFLY: Life History of the Common Disc Oakblue

Life History of the Common Disc Oakblue (Arhopala epimuta epiala)

Butterfly Biodata:
Genus: Arhopala Biosduval 1832
Species: epimuta Moore 1858
Subspecies: apiala Corbet 1941
Wingspan of Adult Butterfly: 40-42mm
Caterpillar Local Host Plants: Vatica pauciflora (Dipterocarpaceae), Vatica rassak (Dipterocarpaceae), Vatica maingayi (Dipterocarpaceae).

A female Common Disc Oakblue resting on a leaf perch between oviposition runs.

Physical Description of Adult Butterfly:
Above, the male is deep shinning blue with a thread-line dark border on both wings, and on the forewing there is a broad circular discal patch; the female is shinning blue with very broad dark borders on both wings. Below, the wings are brown bearing brown spots outlined with whitish to yellowish brown striae. On the forewing, the post-discal spot in space 4 is shifted towards the termen and not aligned with spots in spaces 5 and 6. On the tailless hindiwng, (a) the post-discal spot in space 6 is placed mid-way between the spot in space 5 and the end-cell bar, with its inner edge aligned with the outer edge of spot in space 7; (b) there is a very short tooth at end of vein 2; (c) bluish scales are present in the margin of space 1b and upper end of space 1a, and (d) a small tuff of white cilia is present just below the end of vein 1b.

The upperside of a male Common Disc Oakblue with the "disc" highlighted.

A sunbathing female Common Disc Oakblue showing us its upperside.

A close-up view of the small tuff of white cilia near the end of vein 1b on the hindwing.

Field Observations of Butterfly Behaviour:

In Singapore, the Common Disc Oakblue is moderately common. The adults dwell in shady habitats within the sanctuary of the nature reserves. They typically rest in the shade among the foliage with upright wings, but would open their wings for sunbathing in sun-lit spots. If disturbed, they would fly short distances to other perches among the low-growing shrubs.

A female Common Disc Oakblue checking out a young shoot of the host plant.

A view of a male Common Disc Oakblue captured in the midst of taking off, giving us a view of its bluish upperside and the "disc".

A male Common Disc Oakblue.

Early Stages:

The Common Disc Oakblue has been found to utilize at least two Vatica spp. (V. pauciflora and V. maingayi) in the nature reserves as larval hosts. It has also been bred successfully on a cultivated species, Vatica rassak. The Vatica spp. belong to the Dipterocarpaceae family, members of which are commonly utilized by butterflies in the Arhopala and Flos genera. The early stages of the Common Disc Oakblue feed on the young leaves of the host plants, and often lives i in shallow leaf sheltersiin the company of attending ants of a few species. At times several caterpillars can be observed sharing a leaf shelter.

Caterpillars with ant in attendance in the field. Left: 4th instar; Right: 5th instar.

Local host plant #1: Vatica pauciflora.

Local host plant #2: Vatica rassak.

Eggs are laid singly on a young shoot, a stem or the underside of a leaf of the host plant. Repeated visits by the mother butterflies could result in several eggs being in close proximity on the same site. Each egg is about 0.8mm in diameter, white with a very light green tinge. It is shaped like a pressed bun with a slightly depressed micropylar area atop. The surface has a finely reticulated pattern of intersecting ridges and there are short sharp spikes at the intersections of these ridges.

A mother Common Disc Oakblue laying an egg on the underside of a leaf.

A short animated sequence showing a mother Common Disc Oakblue laying an egg on the stem of a V. pauciflora plant.

An egg of the Common Disc Oakblue.

It takes about 3 days for the egg to hatch. The newly hatched is pale greyish green in body color and has a length of about 1.3mm. It has a rather flattened woodlouse appearance with a large semicircular prothorax, a round anal plate, a yellowish brown head and long whitish dorso-lateral and lateral setae. There are also a fair number of very short setae on the body surface. The newly hatched does not bother to devour the rest of the egg shell after its emergence.

Two views of a 1st instar caterpillar, early in this stage, length: 1.6mm.

The 1st instar caterpillar feeds by skimming the surface of a young leaf, but later instars are able to consume the lamina in whole. The body colour turns pale yellowish brown as the caterpillar feeds and grows. Larval stages of the Common Disc Oakblue are gregarious and a few caterpillars have been observed in the field sharing a leaf shelter and adjacent feeding site with no animosity towards each other. Towards the end of the 1st instar, small whitish dorsal patches and lateral patches becomes visible, though still faint at this stage. In some specimens, small faint red patches also appear between the white lateral patches. After about 2.5-3 days of growth, and reaching a length of about 2.2-2.4mm, the caterpillar moults to the next instar.

Two views of 1st instar caterpillar in its shallow leaf shelter, late in this instar, length: 2.2mm.

The yellowish brown 2nd instar caterpillar features long lateral hairs and a yellowish brown head. Long dorso-lateral setae are no longer present, but many very short setae appear on the entire body surface. Now the whitish dorsal, dorso-lateral and lateral patches are prominent. The numerous small red patches are present to varying degree of prominence in different specimens. Both the dorsal nectary organ (DNO) on the 7th abdominal segment and the pair of the tentacular organs (TOs) on the 8th abdominal segments are now visible.

2nd instar caterpillars. Top: newly moulted, length: 2.5mm; Bottom: 1 day later, length: 3.4mm.

The 2nd instar caterpillar has a functional DNO as ants living in its proximity are observed to actively attend to the young caterpillars, having been attracted to the nectary fluid excreted via the DNO. The ant-caterpillar association continues for all remaining larval stages of the Common Disc Oakblue. The 2nd instar caterpillar reaches a length of about 4-4.5mm, and after about 2.5 days in this stage, it moults again.

Two views of a 2nd instar caterpillar in its shallow leaf shelter, late in this stage, dormant prior to its next moult, length: 3.2mm.

The 3rd instar caterpillar resembles the 2nd instar caterpillar closely. The body is pale yellowish brown to yellowish green with the whitish patches seen in earlier instars becoming pale yellowish. Some specimens could be void of the dorso-lateral and lateral red patches whilst others could be fully adorned with them. The DNO is now rather prominent and sits atop a red glass-like marking stretching into the 8th abdominal segment. The 3rd instar takes about 2.5 days to complete with the body length reaching about 7-7.5mm.

Two views of a 3rd instar caterpillar, length: 6.5mm.

Two views of a 3rd instar caterpillar, late in this stage, length: 7mm.

Ant-caterpillar association involving a 3rd instar caterpillar of the Common Disc Oakblue.

The 4th instar caterpillar has similar markings as the 3rd instar. Again there could be two colour forms with one almost void of red patches, and another with numerous red patches. The whitish to yellowish patches seen in previous instars typically become more luminescent green in appearance in most specimens. This penultimate istar takes about 3-3.5 days to complete with the body length reaching 12-13mm.

Two views of a 4th instar caterpillar, length: 8.8mm.

Two views of a 4th instar caterpillar, length: 10mm.

Two views of a 4th instar caterpillar, length: 12mm.

A 4th instar caterpillar found resting in its shallow leaf shelter next to the midrib on the leaf underside.

4th instar caterpillars in two colour forms.

The 5th instar caterpillar has similar but more striking markings. Visible changes are 1) the yellowish to luminescent green patches are now more centralized towards the dorsal with smaller gaps between individual gaps; 2) the body ground colour ranges from pale yellow to pale green in the green form and to pinky red in the red form; 3) the red glass-like marking in the 8th abdominal segment disappears after being present initially in this instar.

Two views of a 5th instar caterpillar, early in this stage, length: 13.5mm.

Two views of a 5th instar caterpillar, red form, length: 16mm.

Two views of a 5th instar caterpillar, green form, length: 18mm.

The dorsal nectary organ (DNO) and tentacular organs (TOs) of a 5th instar caterpillar.

The 5th instar lasts for 8-9 days, and after reaching a length of about 21-22mm, the caterpillar has its body gradually shrunken in the last 2-3 days. Most caterpillars has its body colour turns pinkish brown or dull reddish brown at this stage. A minority has its body colour remains uniformly green till the pre-pupal stage. Eventually the caterpillar stops food intake for about 1 day and wanders around for its pupation site. Typically the caterpillar seeks out a shallow recess area on the leaf for for this purpose. Here the caterpillar builds its pupation shelter by spinning copious amount of silk on the substrate surface to form a barrier around it.

Two views of a 5th instar caterpillar, green form, late in this instar, length: 21.5mm.

Two 5th instar caterpillars with body shrunken and colour changed.

Two views of a pre-pupa of the Common Disc Oakblue in its shallow pupation shelter.

The pre-pupa caterpillar also prepares for the pupation event by spinning a silk girdle and a silk pad to which it attaches via graspers at its posterior segments. After about 1.5 day as a immobile pre-pupa, pupation takes place. The pupa, with a length of 13.5-14.5mm, has a shape typical of any Lycaenid species, but has a somewhat flattened appearance. Initially pale green to pale pinkish, the pupa soon darkens to a pale reddish brown coloration, adorned with dark patches, within a day. A small reddish-brown slit, likely the remnant of the DNO, has been found to exude fluid droplets for the first few days of the pupal period. This attracts the ants to attend to the pupa, thus securing some degree of protection from predators or parasitoids.

Two views of a pupa of the Common Disc Oakblue, length: 14mm.

Eight days later, the pupa matures enough to show the shining blue patch on the forewing upperside. The next day, the pupal stage comes to an end with the emergence of the adult butterfly.