16 August 2014

Life History of the Palm Bob

Life History of the  Palm Bob (Suastus gremius gremius)


Butterfly Biodata:
Genus: Suastus Moore, 1881
Species: gremius Fabricius, 1798
Sub-species: gremius Fabricius, 1798
Wingspan of Adult Butterfly: 33-35mm
Caterpillar Local Host Plants: Cocos nucifera (Arecaceae; common name: Coconut), Rhapis excelsa (Arecaceae, common name: Lady Palm).





Physical Description of Adult Butterfly:
Above, the wings are brown with the forewing adorned with pale yellow hyaline spots in the cell-end and spaces 1b, 2, 3, 6, 7 and 8. The hindwing is unmarked. The tornal cilia are whitish. Beneath, the wings are greyish brown with the hindwing overlaid with buff scaling and featuring a number of moderately large and well defined black spots.




Field Observations of Butterfly Behaviour:
The Palm Bob is common in Singapore. Sightings are rather frequent in nature reserves and in urban parks and gardens. The adults are fast flyers and are skittish when disturbed. They have been observed to visit flowers and sunbathe in sunny weather, and to puddle on bird dropping.





Early Stages:
Worldwide, a fair number of palm species have been recorded as larval host plants for the Palm Bob. The same is true in Singapore, and two of them have so far been identified as the Coconut Palm and the Lady Palm. Both palm species are rather commonly cultivated in parks and gardens across the island. The caterpillars of the Palm Bob feed on the leaves of the host plant in all instars, and live in leaf shelters constructed by cutting and folding leaf fragment on a palm frond. As the caterpillar grows in size later through progressing instars, it will abandon the current one and construct a larger one to accommodate its increased body size/length.

Local host plant: Coconut.

A female Palm Bob laying an egg on the upperside of a leaf frond of the Coconut Palm.

The eggs are laid singly on the upper surface of a leaf of the host plant. Each shallow dome-shaped egg is wine red with whitish longitudinal ridges emanating from the perimeter of the micropylar area sitting atop the egg. The eggs are rather large with a diameter of about 1.4mm.

Two views of an egg of the Palm Bob, diameter: 1.4mm.

It takes about 4 days for the egg to hatch. The egg decolorizes to a dull shade of pinky brown when fully mature on the last day of this phase. The young caterpillar nibbles away just enough of the shell to emerge, but it makes only a feeble attempt to eat the remaining egg shell. The newly hatched has a length of about 3mm. Its bright orangy red body is cylindrical in shape and there is a tuff of moderately long setae on the posterior segment. Its head is in a bright shade of reddish brown.

A sequence of three views of a mature egg on the last day of the oval stage.

Two views of a newly hatched 1st instar caterpillar, length: 3mm.

After abandoning the empty egg shell, the newly hatched caterpillar constructs its first leaf shelter typically on the same leaflet of the palm frond. From the shelter, the caterpillar leaves the shelter periodically to feed on the nearby leaf lamina. The 1st instar takes a total of 3.5-4 days to complete with body length reaching about 6mm.

Two views of a 1st instar caterpillar, length: 4.5mm.

Two views of a leaf shelter of a 1st instar caterpillar, showing both sides of the leaf. The lower panel shows the actual shelter along the mid-rib.

The unmarked body of the 2nd instar caterpillar resembles that of the first instar at first but in a subdued tone of orangy red for both body and the head capsule (which bears two obscure whitish stripes afront). The tuff of moderately long setae is still present. The prominent anal plate is made of two semi-circular halves outlined in white. As the instar progresses, its body takes on a strong greenish undertone, and gradually losing its reddish tone. This instar lasts a total of 3 days with the body length reaching up to 9mm.

Two views of an early 2nd instar caterpillar, length: 5.5mm.

Two views of a 2nd instar caterpillar, length: 8.2mm.

Two views of a late 2nd instar caterpillar, dormant prior to its moult, length: 8mm.

Two views of a leaf shelter of a 2nd instar caterpillar, showing both sides of hte leaf. The lower panel shows the actual shelter folding down across the mid-rib.

The 3nd instar caterpillar resembles the 2nd instar caterpillar initially but soon loses all remaining traces of reddish tone in its body. For most of this instar, the body is pale yellowish with greenish undertone. The head capsule takes on two additional whitish stripes which occur laterally, and the pale brown patches have small whitish spots embedded. This instar lasts about 3 days with the body length reaching up to 13mm.

Two views of an early 3rd instar caterpillar, length: 8mm.

Two views of a 3rd instar caterpillar, late in this stage, length: 13mm.

The 4th instar caterpillar resembles the 3rd instar caterpillar closely but with minor differences. In the head capsule, both the white and brown coloration are now more intense, giving the head a more striking appearance. Another observable change is that the spiracles are now black in colour. This penultimate instar lasts about 5 days with the body length reaching up to 20mm.

Two views of a 4th instar caterpillar, length: 16mm.

Two views of a late 4th instar caterpillar, length: 20mm.

The body of the 5th instar caterpillar is pale yellowish to whitish. The most prominent change from the 4th to the 5th takes place on the head capsule which has the brown coloration turned to black and with the whitish stripes expanded.

Two views of a newly moulted 5th instar caterpillar, lengths: 17mm.

Two views of a 5th instar caterpillar, length: 30mm.

Two leaf shelters of the Palm Bob, used by the 4th (top) and 5th (bottom) instar caterpillars.

The 5th instar takes about 8-9 days to complete with the body length reaching up to 35mm. In the last 1-2 days of this instar, the underside of caterpillar starts to excrete some whitish powdery substance. Within the leaf shelter, a large number of silk threads are spun and a great mass of small whitish particles deposited . At this time, the body shrinks in length and its body colour changed to yellowish green. The shelter is rather tightly sealed as the caterpillar enter its prepupatory phase which lasts for about 1.5 days.

A pupation shelter opened to reveal two views of a pre-pupa of the Palm Bob.

Pupation takes place within the leaf shelter. The pupa does not have a cremastral attachment nor a silk girdle and it is mainly secured with tightly woven silk threads in the shelter. It has a short thorax and a rather long abdomen. The body is darker green in the thorax and wing pad areas, but yellowish brown in the abdomen. Length of pupae: 19-20mm.

Two views of a pupa of the Palm Bob.

After 8 days, the pupa becomes mostly dark brown as the adult development stage within the pupal case comes to an end. Eclosion takes place the next day.

Two views of a mature pupa of the Palm Bob.

A newly eclosed Palm Bob.

References:
  • [C&P4] The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, Malayan Nature Society.
  • 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 Ellen Tan, Tan Chung Pheng, Bobby Mun, Federick Ho, Khew SK and Horace Tan.