Genus: Arhopala Biosduval, 1832
Species: silhetensis Hewitson, 1862
Subspecies: adorea de Niceville, 1890
Wingspan of Adult Butterfly: 48mm
Caterpillar Host Plants: To be identified.
Physical Description of Adult Butterfly:
The adult is moderately large and has a pair of white-tipped tails at the end of vein 2 on the hindwing, where tornal lobes are also present. As with other members of the cleander group of Arhopala, the post-discal band on the hindwing underside is dislocated at vein 2 with spots in spaces 6 and 7 placed one below the other. For A. silhetensis, the outer edge of the spot in space 7 is well inside the outer edge of the spot in space 6; the latter wider at top than at bottom, with its outer edge sinuous and inside the inner edge of the spot in space 5, and its inner edge not overlapping the end-cell bar. Further, the gap between the spot in space 5 and the end-cell bar are wider than either of these spots. On the upperside, the adult is deep purple blue with narrow borders in the male, and shining purplish blue with broad borders in the female.
An adult with key features of hindwing underside indicated
Field Observations of Butterfly Behaviour:
This species is currently considered rare in Singapore. It is only recently rediscovered in a localized area within the Central Catchment Area. The adults were spotted flying next to the forest trails and at times perching with its wings closed. In sunny weather, some were also observed to fully opening its wings to sunbath on its perch.
An encounter with an oviposting female in October 2007.
Host plant: to be identified.
Host plant: leaf venation pattern of a rather young leaf.
Eggs are laid singly on the underside of an old leaf. The egg shares a similar appearance with a number of Arhopala species. Each egg is small, about 0.5mm in diameter, and white in color, circular with a slightly depressed micropylar area and a finely reticulated pattern of intersecting ridges.
Egg of the Sylhet Oakblue
Empty egg shell
It takes 3 days for the egg to hatch. The young caterpillar eats only the upper portion of the egg shell to emerge. The newly hatched has a length of about 2mm and has a light brown coloration. It has a rather flattened woodlouse appearance with a large semicircular first thoracic segment. This appearance remains as the caterpillar grows through the instars. A large dark brownish marking can be seen on the anal plate. The body also carries long dorsal and lateral hairs. As it grows, the body color becomes more yellowish and a faint white dorsal line appears.
1st instar caterpillar, newly hatched, length: 2mm
1st instar caterpillar, 2.5mm
The 1st instar caterpillar feeds by skimming the surface of a young leaf. After 4 days of growth, and reaching a length of about 3.5mm, it moults to the next instar. The 2nd instar caterpillar has long fine lateral hairs, and a more distinct marking on the anal plate. A pair of reddisk brown markings also sits on the first thoracic segment. The feeding is still done via eating away the upper layer of the young leaf. Nectary organs, including both the dorsal and tentacular organs are now discernible.
2nd instar caterpillar, early in this stage, length: 3.5mm
The 2nd instar caterpillar reaches a length of about 6mm, and after about 4 days in this stage, it moults again. The 3rd instar caterpillar is prominently marked with 1) a large dark patch on the 1st thoracic segment; 2) a wide reddish brown dorsal band running from the 2nd thoracic segment up to the anal plate, with a thin white band sitting within it; and 3) the dorsal organ now is lined with dark "lips" and sits within an oval marking. The caterpillar now feeds by eating along the leaf edge and devours the whole leaf. The 3rd instar takes about 4 days to complete with the body length reaching about 9mm.
3rd instar caterpillar, length: 8.5mm
The 4th instar caterpillar has similar markings as the 3rd instar. All markings have become darker in coloration, and the dark patches on 1st thoracic and the anal plate have also become larger and hence more prominent. The 4th instar takes about 4 days to complete with the body length reaching 16-17mm.
4th instar caterpillar.
The 5th instar caterpillar has similar but more striking markings. Visible changes are 1) all bands and patches are bolder and darker; 2) the lateral edge of the body is marked with a thin black line; and 3) each tentacular organ is highlighted in a black ring which is also connected to the main dorsal band. This last feature on the tentacular organs is also seen in caterpillars for Central Oakblue and the Flos species.After 8 days of frantic feeding and reaching a length of about 27mm, the caterpillar slows down and actually stops food intake for about 3 days. During this time, its body length gradually shortened. Soon it becomes an immobile pre-pupa in its shallow leaf shelter.
5th instar caterpillar, early in this stage, length: 18mm
5th instar caterpillar, late in this stage, length: 27mm
Two views of a pre-pupa of the Sylhet Oakblue
The pre-pupa caterpillar prepares for pupation by spinning a silk girdle and a silk pad to which it attaches itself via cremastral hooks. Also 2 days as a pre-pupa, pupation takes place. The pupa, with a length of 15-16mm, has a shape typical of any lycaenid species, but with a somewhat produced anal segment. It is initially jade greenish but within hours it becomes deep dark brown.
Two views of a pupa, 15-16mm
Ten days later, the pupal stage comes to an end with the emergence of the adult butterfly.
A newly eclosed female
A newly eclosed male
In the wild, the caterpillars of the Sylhet Oakblue have been found to be attended by the rather large ants of the species Polyrhachis armata, as the pictures below show.
A 5th instar caterpillar being attended by Polyrhachis armata ants
A 3rd instar caterpillar attended by a Polyrhachis armata ant
I would like to express my gratitude to Prof. Konrad Fiedler, Mr. Les Day, Dr Rudy Kohout and Dr. Pfeiffer for their generous assistance in the identification of the Sylhet Oakblue (Arhopala silhetensis adorea) and the ant Polyrhachis armata.
- The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, Malayan Nature Society.
- Butterflies of Thailand, Pisuth Ek-Amnuay, 1st Edition, 2006
Text and Photos by Horace Tan