Life History of the Starry Bob

Life History of the Starry Bob (Iambrix stellifer)

Butterfly Biodata:
Genus: Iambrix Watson, 1893
Species: stellifer Butler, 1879
Wingspan of Adult Butterfly: 18-22mm
Caterpillar Local Host Plants: Centotheca lappacea (Poaceae, common name: Sefa), Lophatherum gracile (Poaceae).

Physical Description of Adult Butterfly:

The adults are small and the 3rd segment of the palpi is long and thin. On the upperside, both sexes are dark brown. The male is unmarked but the female has several white post-discal spots on the forewing. On the underside, the wings are brown to dark brown overlaid with ochreous scales. The apical area of the forewing is yellowish to yellowish brown. There are variable number of silvery spots on both wings. In fully-spotted specimens, the forewing features post-discal spots in spaces 2-5 and one spot near cell-end, and the hindwing features post-discal spots in spaces 1b, 2, 5 and 7, and one spot near cell-end. Noteworthy is that the spot in space 5 on the hindwing lies further out and about half way between cell-end and the wing margin. In the look alike species Iambrix salsala (Starry Bob), this spot lies just beyond the cell.

Field Observations of Butterfly Behaviour:

Starry Bob is moderately rare in Singapore and is typically found in forested ares in the nature reserves and western wastelands. Within the nature reserve, the adults are usually sighted when they are feeding on flowers of Leea indica in the company of other skippers such as the Yellow Vein Lancer. The adults tend to perch in shady foliage and are rarely sighted puddling on wet ground or organic waste matter.

Early Stages:

Two host plants, Centotheca lappacea and Lophatherum gracile, which are bamboo-like grass species, have recently been recorded as the local larval hosts for Starry Bob. Probably due to the bamboo-like appearance of these plants, the larval host for Starry Bob was previously recorded as "bamboo spp". Both plants can be found in forested areas in Singapore, with Centotheca lappacea much more common and widespread. The caterpillars of the Starry Bob feed on leaves of the host plant, and live in shelters made by joining edges of a leaf or cut segment together with silk threads. As with other skipper species, the caterpillars have the habit of forcefully ejecting their frass pellets.

Host plant #1:Centotheca lappacea.

Host plant #2:Lophatherum gracile.

The eggs are laid singly on the upperside of a leaf of the host plant. Each dome-shaped egg is pinky red mottled with whitish patches. The base diameter is about 1mm with a height of about 0.7mm.

Two views of an egg of the Starry Bob.

Two views of a maturing egg of the Starry Bob.

Two views of a mature egg with the caterpillar in the midst of nibbling away the egg shell to emerge.

It takes about 4-4.5 days for the egg to hatch. The young caterpillar eats just enough of the shell to emerge, and will devour the rest of the egg shell after emergence. The newly hatched has a length of about 2.3mm. Its yellowish golden body bears rows of very short dorso-lateral and sub-spiracular setae. A tuff of long setae can be found at the posterior end. The head capsule is entirely black and there is a black collar just behind the head on the dorsum of the prothorax.

A newly hatched caterpillar of the Starry Bob, almost done with the egg shell.

Two views of a newly hatched 1st instar caterpillar, after completely devoured its egg shell, length: 2.3mm.

The newly hatched constructs its leaf shelter at the leaf tip or along the leaf edge soon after it is done with the egg shell. The body color changes to pale yellowish green after a few feeding sessions on the leaf. The 1st instar takes about 3 days to complete with body length increases to 4.2-4.5mm.

Two views of a 1st instar caterpillar, length: 3.3mm.

Two views of a late 1st instar caterpillar, dormant prior to its moult, length: 4.5mm.

A leaf of Lophatherum gracile bearing a leaf shelter of a first instar caterpillar. Upperside view. The shelter is created by cutting and folding down the leaf fragment near the tip. The egg shell remnant can be seen next to the leaf shelter.

The 2nd instar caterpillar is yellowish with a green undertone. At the posterior end, the tuff of long setae is replaced with a few relatively long setae. The black prothoracic collar is now absent. This instar lasts about 4 days with the body length reaching about 6.5-6.8mm.

Two views of a 2nd instar caterpillar, early in this stage, length: 4mm.

Two views of a 2nd instar caterpillar, length: 5.3mm.

Two views of a late 2nd instar caterpillar, dormant prior to its moult, length: 6.8mm.

The 3nd instar caterpillar resembles the 2nd instar caterpillar but its body more whitish than yellowish. This instar lasts about 4 days with the body length reaching about 10-11mm.

Two views of 3rd instar caterpillar, newly moulted, eating its exuvia, length: 7mm.

Two views of a 3rd instar caterpillar, length: 10.8mm.

Two views of a late 3nd instar caterpillar, dormant prior to its moult, length: 10.2mm.

The leaf shelter of a 2nd/3rd instar caterpillar at the leaf tip of a leaf of Lophatherum gracile.

The 4th instar caterpillar is strongly whitish translucent in its body colour. The head capsule is still black. This penultimate instar lasts about 4 days with the body length reaching up to 14.5-15.5mm.

Two views of a 4th instar caterpillar, length: 14.5mm.

Two views of a 4th instar caterpillar, late in this stage, dormant prior to its moult, length: 14.5mm.

The leaf shelter of a 3rd/4th instar caterpillar on a leaf of Lophatherum gracile, underside view showing the fold.

The 5th instar ushers in a drastic change in the appearance of the head capsule. The base colour of the head is pale purplish brown with dark brown shading along frontal cleavage lines and the periphery. The lower "cheek" area, on both sides of the mouth part, is yellow. The body is strongly whitish with a green undertone. This final instar takes about 5-6 days to complete with the body length reaching up to about 19-20mm.

Two views of a 5th instar caterpillar, early in this stage, length: 16mm.

Two views of another 5th instar caterpillar, late in this stage, length: 20mm.

Towards the end of the 5th instar, the body of the caterpillar shortens and its coloration changes uniformly to beige brown. Typically it picks the underside of a leaf glade to construct its pupation shelter. Typically the basal part of the leaf is devoured bare of any lamina and the shelter is constructed by folding and sealing the remaining part of the leaf blade. Within the shelter, it spins a silk pad and a girdle to secure itself. Whitish waxy substance is deposited on the inner wall of the shelter in moderately large quantity. It then becomes dormant for the prepupatory phase which lasts for about 1-1.5 day.

Two views of a pre-pupa of the Starry Bob.

Two pupation shelters of the Starry Bob.

Within the tight shelter, the pupa secures itself to the substrate with a silk girdle and via a cremastral attachment to the silk pad. It is pale beige brown mostly with its body surface covered in short pale setae. There are two rather large "eye" markings at the anterior end. Length of pupae: 12-14mm.

Two views of a pupa of the Starry Bob.

On the last day of the pupal period, the pupa becomes mostly dark brown to black. The next day, after about 8 days of the pupal phase, eclosion takes place with the adult emerging from the pupal case.

Two views of a mature pupa of the Starry Bob.

A newly eclosed Starry Bob.

References:

• [C&P4] The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, Malayan Nature Society, 1992.
• Butterflies of Thailand, Pisuth Ek-Amnuay, 2nd Edition, 2012.
• A Field Guide to the Butterflies of Singapore, Khew S.K., Ink On Paper Communications, 2010.

Text by Horace Tan, Photos by Benjamin Yam, Nelson Ong, Anthony Wong, Khew SK  and Horace Tan