06 June 2009

Life History of the Scarce Silverstreak

Life History of the Scarce Silverstreak (Iraota rochana boswelliana)

Butterfly Biodata:
Genus: Iraota, Moore,1881
Species: rochana Horsfield, 1829
Sub-species: boswelliana, Distant, 1855
Wingspan of Adult Butterfly: 36mm
Caterpillar Local Host Plants:
Ficus microcarpa `Golden' (Moraceae, common name: Indian Laurel Fig), Ficus benjamini (Morrel, 1957).

Physical Description of Adult Butterfly:
Above, the male is dark brown with greenish blue at base of the forewing below the cell and between veins 1-6 of the hindwing; the female is entirely brown. Underneath, both sexes are buff to reddish brown with areas of white streaks and spots. The forewing has a longitudinal silvery white streak in the cell, and the discal spot in space 4 is elongated and nearly reaching the termen. The hindwing has a broad basal streak running along the costa up to the termen. Both sexes have the hindwing tailed at veins 1b and 2, the latter being longer. There is also an additional stumpy tail (or tooth) at vein 3 for both sexes. All tails are white-tipped.

A female Scarce Silverstreak perching near its host plant.

A maler Scarce Silverstreak enjoying the ripened fruit of the
Singapore Rhododendron (Melastoma malabathricum).

A female Scarce Silverstreak sunbathing on its host plant, giving us a view of its
brown upperside.

A male Scarce Silverstreak proudly displaying its striking upperside.

Field Observations of Butterfly Behaviour:
This species is uncommon in Singapore. Sightings have been reported at a few sites around the main island, with the Sourthern Ridges being the most likely place for an encounter. The adults fly rapidly and may sometimes be missed due to the speed at which they fly around. Between the two sexes, the female is more likely to be sighted, especially when they make ovipoisiton visits to the ficus hedges in hill parks. The males usually stay high at tree-top level, and rarely perch low enough for any good photo captures. Both sexes visit flowers for nectar.

Early Stages:
The local host plant, Ficus microcarpa `Golden' is a strangling fig with numerous, slender aerial roots growing from branches, some of which developing into trunk-like pillar roots. Leaves are blunt, with young leaves on new shoots bright yellowish green. Formerly found on coastal and riverine situations, this species is now very common in all parts of Singapore where they are typically used roadside hedges or as ornamental plants in gardens.

Host plant: Ficus microcarpa 'golden'.
Left: young tree with hanging aerial roots; Right: close-up on young leaves.

A mating pair of the Scarce Silverstreak.

A mother Scarce Silverstreak laying one egg on the leaf underside of the host plant.

Eggs of Scarce Silverstreak are laid singly on the stem or the leaf underside of the host plant. The freshly egg is covered in a dark yellowish viscous coating which dries up in a matter of few hours to give the egg a bright yellowish appearance. The egg is shaped like a burger bun with many irregular polygonal pits, and has its micropylar lying at the polar position encircled with a number of smaller pits. The egg is rather large for a lycaenid species with a diameter of about 1.2mm.

Three views of an egg of the Scarce Silverstreak, diameter is about 1.2mm.
Left: a freshly laid egg. Right and middle: 5-hour old egg.

Pics of a mature egg showing the ``infant'' larva eating egg shell to create a sufficiently large exit.

The egg hatches after 2.5-3 days of development. The young larva spends a couple of hours eating away the top portion of the egg shell in order to make its exit. The newly hatched is orangy brown with darker lateral bands. It has long dorsal setae and sub-spiracular setae. Length: about 2mm. Soon after its emergence, the newly hatched turns around and works meticulously at nibbling away the empty egg shell. It only moves on to eating the leaf when it has consumed almost the entire egg shell.

A newly hatched Scarce Silverstreak eating its egg shell.

The 1st instar grazes on the surface of the young leaves, and grows to a length of about 4mm in about 1.5 to 2 days before the moult to the next instar. Prior to the moult, the caterpillar shortens to about 3mm and stays dormant for up to half a day. This body shortening routine occurs for every subsequent moult.

Two views of a 1st instar caterpillar, late in this stage, length: 3.8mm.

A shortened 1st instar caterpillar lying dormant before the moult to 2nd instar, length: 3mm

The 2nd instar caterpillar is orangy brown and features faint and obscure dorsal markings. It has lost the long dorsal setae seen in the 1st instar, but ts body is covered with numerous short setae. The prothoracic shield has the same color as the base color and thus does not stand out. This instar lasts for 2-3 days, and the body length increases to about 8mm.

Two views of a 2nd instar caterpillar, length: 6mm.

The 3rd instar caterpillar initially resembles the 2nd instar caterpillar with its orangy brown body color and short setae. As the caterpillar grows rapidly in 2 days up to a length of 14-15mm, the caterpillar gradually takes on varying shades of green. While some caterpillars retain a large degree of brown coloration, there are others which turn light green. Towards the end of this instar, prior to the moult, two dorsal-lateral brown markings appear on the 1st abdominal segment, and faint reddish brown saddle markings appear on the 5th abdominal segment.

Two 3rd instar caterpillars, lengths: 10mm (top); 8.5mm (bottom).

3rd instar caterpillars, late in this stage, lengths: 15.5mm (top); 14mm (bottom)

One 3rd instar caterpillar found on the underside of a leaf in Southern Ridges.
Note the presence of an attending ant.

The 4th (and final) instar caterpillar is similar in appearance to the late 3rd instar caterpillar. Color variations from the light brown form to the green form can observed from those seen in the wild and those bred in indoor environment. It is also observed that caterpillars of the light brown form become much greener as the instar progresses. For some caterpillars, the two dorsal-lateral markings on the 1st abdominal segment and the saddle marking on the 5th abdominal segment are rather prominent, but for others, these markings can be rather faint or even absent.

Another 4th instar caterpillar, light brown form, early in this stage, length: 16mm

4th instar caterpillar, late in this stage, length: 25mm

Another 4th instar caterpillar, late in this stage, length: 27mm.
This one developed into a male adult.
Note the different ant species attending it as compared to the field shot of
a 3rd instar caterpillar featured earlier.

After 4-5 days of growth and reaching a maximum length of around 27mm in the 4th (and final) instar, the body of the caterpillar gradually shortens, and the color intensifies to either a dark green or deep purplish red. The caterpillar eventually comes to rest on a spot on the leaf surface for its pupation. At the chosen spot, the caterpillar readies itself for pupation by spinning a silk pad and a few silk strands across its body. The caterpillar secures itself to the silk pad via claspers on its posterior end.

Two views of a pre-pupa of the Scarce Silverstreak,

The pupation event of a Scarce Silverstreak caterpillar.

Pupation takes place after one day of the pre-pupal stage. The pupa has the typical lycaenid shape. It is covered in various violet red patches which eventually turns to various shades of brown which remains for most of the remaining pupal period of 9 days. Each pupa has a length of about 13.5-14.5mm.

Two views of a newly formed pupa of the Scarce Silverstreak,

Two views of a pupa of the Scarce Silverstreak.

Eight days later, the pupa becomes darkened in color signaling the imminent emergence of the adult. The markings on the upperside of the forewing become visible through the pupal skin, and it is now possible to tell the gender of the soon-to-emerge adult. The next day the adult butterfly emerges from the mature pupa

Mature pupae: Female (bottom); Male (top). Note the difference in the wing pads.

A newly eclosed female Scarce Silverstreak

  • The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, The Malayan Nature Society.
  • Butterflies of Thailand, Pisuth Ek-Amnuay, 1st Edition, 2006
Text by Horace Tan, Photos by Benedict Tay, Sunny Chir, Khew SK and Horace Tan