Butterfly of the Month - June 2024

Butterfly of the Month - June 2024

The Green Oakblue (Arhopala eumolphus maxwelli)

A male Green Oakblue sunbathing with opened wings showing its coppery green uppersides

All too soon, we are almost at the end of the first half of the year 2024. The nature community in Singapore celebrated Pesta Ubin and Ubin Day. In its ninth edition, the event aims to share the essence of a sustainable and conservation-driven “Live Kampung” in harmony with nature, foster deeper affection and appreciation for Ubin’s heritage and ecosystem and solidify the island’s position as a premier “place-based” learning destination, offering enriching educational experiences for all ages.

The recent oil spill incident on 14 Jun is a grim reminder of how delicate our natural environment is. The spill that was sparked by Netherlands-flagged dredger Vox Maxima hitting Singapore-flagged bunker vessel Marine Honour at Pasir Panjang Terminal released about 400 tonnes of low-sulphur fuel into the sea. The impact of the spill could adversely affect organisms living on or near the water's surface such as sea birds, mangrove forests and corals could be smothered by the oil slick and these organisms end up ingesting toxic chemicals.

This month, we feature one of the Oakblues or Arhopala spp. as our Butterfly of the Month for June 2024. The Arhopala genus comprises close to over a hundred species in Malaysia and Singapore - many of which are cryptic and challenging to identify confidently due to their near identical appearances. Most prefer heavily shaded forest habitats and stay close to the ground where they perch on the uppersides of foliage.

A male Green Oakblue sunbathing showing its coppery green uppersides

The uppersides of the majority of the species are iridescent purple or blue in both sexes, with the exception of a few where the males are a metallic green. The Green Oakblue (Arhopala eumolphus maxwelli), our Butterfly of the Month, is one such species. The male of the Green Oakblue is a brassy green on the upperside, with the forewing border decreasing from 2.0 mm at the tornus to 1.0 mm at the apex. When viewed at different angles, the upperside takes on changing shades of green and can even appear bluish at certain oblique angles.

A female Green Oakblue sunbathing showing her purple-blue uppersides

The female is purple blue, and on the upperside of the forewing, the border usually reaches the apex of the cell. The Green Oakblue features a white tipped tail originating from vein 2 of the hindwing. The species of the eumolphus group have the postdiscal spot in space 4 on the underside of the forewing moved out of line with the adjacent spots above and below it.

The butterfly has a strong flight and takes off to the treetops in a very capable fashion if disturbed. At other times, it flies from perch to perch in the forest undergrowth and stops on the tops of leaves with its wings folded upright. At certain hours of the day, both sexes may be encountered with their wings opened to sunbathe in the warm sunshine.

A Green Oakblue feeding on the ripened fruit of the Singapore Rhododendron

The Green Oakblue is rarely observed to feed on flowering plants or puddling, but is occasionally drawn to the ripened fruits of the Singapore Rhododendron (Melastoma malabathricum) growing along the forest edges. The caterpillar host plant in Singapore is Lithocarpus elegans and the caterpillars are attended to by at least 2 species of ants, of which Polyrachis sp. is the one more often encountered.

Text by Khew SK : Photos by Khew SK, Koh CH, Jonathan Soong and Horace Tan

Random Gallery - Green Oakblue

Random Butterfly Gallery
The Green Oakblue (Arhopala eumolphus maxwelli)

Our random butterfly this week is the moderately rare Green Oakblue (Arhopala eumolphus maxwelli).  The species is so named because of the iridescent green upperside of the males. It is one of several species of Arhopalas with green uppersides in Singapore and Malaysia. This pristine male was shot by ButterflyCircle early stages expert, Horace Tan, in the heart of the nature reserves in Singapore. This individual appeared to be sunbathing to dry its wings. Horace explained that it was perched on the underside of a leaf that was exposed to the sun.

Green Oakblue re-discovered in Singapore

As at end of Oct 2007, the Singapore Butterfly checklist stands at 280 species (with 3 subspecies) - not bad for a tiny little island in the sun of about 700 sq km.

There are species which have not yet been confidently identified, and more information would have to be available before these are added to the checklist.

A recent one re-discovered was the Green Oakblue (Arhopala eumolphus maxwelli) which is a forest denizen. The upperside iridescent green of the male of this species is quite characteristic of a group of Arhopala which is green as opposed to purple or blue on the uppersides. The long white tipped tails also distinguish this species from the other two known 'green' Arhopala found in Singapore - A. aurea and A. trogon.

As luck would have it, our hardworking members of ButterflyCircle also managed to discover the host plant of the Green Oakblue and also found eggs of the species. Dr Horace Tan managed to record the life history of this species from egg to adult.

Several known locations within the nature reserves areas of Singapore are home to this species, and only recently have we managed to record the reliable existence of this species in Singapore, without any wild and dubious claims that the species was first found in Singapore when a specimen "flew up to someone's HDB apartment and died in an NSS butterfly collector's flat!"

Special thanks to Sunny Chir for first photographing this species in the wild.

Photo of Green Oakblue courtesy of Federick Ho

Butterflies Galore! : Green Oakblue

Butterflies Galore!
The Green Oakblue (Arhopala eumolphus maxwelli)

The male Green Oakblue is one of three extant species of the genus Arhopala that features metallic green uppersides instead of the usual purple or dark blue uppersides that are more common amongst the species in the genus. The female of the Green Oakblue, however, features purple uppersides with broad black borders on both wings (shown above). The species was re-discovered in Singapore some time in late 2007 in a patch of forested area near a reservoir park. The full life history has been documented here. Click on the link to see the male of the Green Oakblue.

The underside of the Green Oakblue is typical Arhopala - brown with the usual striations. One of the key distinguishing markings is the post-discal spot in space 4 on the forewing being out of line with the spots above and below it. The species is considered moderately rare, but is very local and at times, several individuals may be spotted together. This pristine female Green Oakblue was shot by young ButterflyCircle member, Jonathan Soong.

Life History of the Green Oakblue

Butterfly Biodata :
Genus : Arhopala Boisduval, 1832
Species : eumolphus Cramer, 1780
Subspecies : maxwelli Distant, 1885
Wingspan of Adult Butterfly : 45mm

Caterpillar Host Plant : to be identified.

Adult male Green Oakblue




Adult female Green Oakblue

Physical Description of Adult Butterfly : On the underside, the adult butterflies have the post-discal spot in space 4 of the forewing moved out of line with the spots above and below it; and on the hindwing the post-discal spot in space 6 is below the spot in space 7. The adults are tailed and moderately large. The male has a brassy green upperside with the forewing border decreasing in thickness from the tornus to the apex. In contrast, the female has a purple blue upperside, and on the forewing the border usually reaches the apex of the cell.

Field Observations of Butterfly Behaviour : This species is a forest denizen and the adult butterflies have been observed to fly in the vicinity of its host plant within the nature reserve areas of Singapore. Both sexes spend much time resting among foliage on the uppersides of leaves with wings closed. When disturbed they usually take off with a swift flight. In sunny weather, they can be spotted sunbathing on uppersides of leaves with both wings opened flat. This sunbathing behaviour has been observed to be a well-timed event taking place in the afternoon.

Early Stages :

The female oviposits on the underside of a mature leaf of the host plant. Each time a single egg is laid, but as the mature leaf is rather large, sometimes several eggs can be found (singly, not in a cluster) on the same leaf. The egg is small (about 0.5mm in diameter) and white in colour, circular with a slightly depressed micropylar area and a reticulated pattern of intersecting ridges.


Female Green Oakblue with abdomen curved to lay egg on the leaf underside.


Left : Fertile egg of the Green Oakblue ; Right : and Empty egg shell --- partially eaten

After about 2 days, the caterpillar hatches from the egg. As it makes its way out of the egg shell, the tiny young larva consumes the eggshell partially for its exit. As the caterpillar of all instars only eats the soft young leaves of the hostplant, the newly hatched has to move away from the mature leaf (where the egg was laid), and make its way to the younger leaves of the host plant. Upon arrival at the young leaves, the caterpillar nibbles away on the leaf surface, making tiny holes on the leaf and leaving a trail of frass behind it.


Left : The large (up to 30cm in length) and hardened mature leaves of the host plant where eggs are laid on the undersides by the female Green Oakblue. ; Right : Young shoot of host plant with soft leaves for caterpillars of Green Oakblue to chew on.

The caterpillars of all instars have a much flattened appearance and have a large, flattened, semicircular first thoracic segment . They construct loose leaf shelters to which they return for rest and for hiding when their feeding activity nearby is disturbed. Typically the caterpillars of all instars would rest next to the main rib of the growing leaf. The caterpillars also have a dorsal nectary organ and have been observed to be attended by ants of two species. On one site, the attending ants are of the Polyrachis species, but on another site, the attending ants are much smaller and species ID of which is yet to be determined. The Green Oakblue larvae have also been observed to be victimized by a parasitoid of Apanteles sp.

A 4th instar caterpillar being attended to by an ant of Polyrachis sp.

The 1st instar caterpillar is initially light yellowish with a tinge of beige. As it grows, it loses the beige tinge and becomes increasingly yellowish. It starts at about 1mm in length, and grows to about 3mm in 2 days before the moult to the 2nd instar.

The 2nd instar caterpillar is distinctly yellow in color and has a prominent whitish dorsal marking.

The 3rd instar caterpillar has a similar appearance to the 2nd instar caterpillar. Because of its larger size, the body markings are now more prominent. An ``hour-glass’’ mark on the anal segment is also visible now.

1st instar caterpillar on the young leaf next to the evidence of its feasting.

2nd instar caterpillar . More yellowish, more distinct dorsal marking.

3rd instar caterpillar showing more prominent body markings and the ``hour-glass’’ mark on the anal segment.

The 4th instar caterpillar is essentially similarly patterned as the 3rd instar, but it now takes on a light greenish and somewhat translucent appearance. The dorsal markings become distinctively fluorescent yellow in contrast..

4th instar caterpillar in its shallow leaf shelter

The 5th and final instar caterpillar is much bigger with increases in both body length and width. The ``hour-glass’’ mark on the anal segment is longer visible. Initially the caterpillar has a greenish colouration but slowly this gives way to a yellowish colouration as it matures and grows to a length of about 20-22mm.

Two 5th instar caterpillars, with the green one being earlier in growth.


An ill-fated 5th instar caterpillar with cocoons of a Apanteles sp. underneath it. The poor caterpillar eventually died hours later.

About 16 days after its hatching from the egg, the caterpillar ceases its feeding activity and its body color becomes reddish brown. At the same time its body length gradually becomes shorter. This lasts for about one day. The caterpillar then goes into a one and half day of pre-pupation pose within a loose leaf shelter. During this time, the caterpillar prepares for pupation by spinning a silk girdle and a silk pad to which it attaches itself via cremastral hooks.


The late 5th instar caterpillar is reddish brown and shrunk in length a day before pre-pupation.


The preparatory caterpillar hours before pupation. The silk girdle is already constructed.

The pupa has a shape typical of any lycaenid species, and a length of 14-16mm. It is light brown with a short dorsal stripe, and has a somewhat produced anal segment.

After about 10 days, the pupal shell turns transparent and the wings of the soon-to-be-emerging butterfly can be seen through the pupal shell. Eclosion takes place within the next 24 hours, typically one or two hours after dawn.


Two views of a pupa of the Green Oakblue.


All ready for eclosion. The upperside wing pattern of the adult butterfly can be seen through the now transparent pupal case.

An adult male Green Oakblue

Text by Horace Tan ; Photos by Horace Tan, Sunny Chir & Khew SK