Life History of the Tawny Coster (Acraea violae)
Butterfly Biodata:
Genus: Acraea Fabricius, 1807
Species: violae Drury, 1773
Wingspan of Adult Butterfly: 53-64mm
Caterpillar Local Host Plants: Passiflora foetida (Passifloraceae, common names: stinking passionflower, love-in-a-mist), Passiflora suberosa (Passifloraceae, common name: corky-stemmed passion flower), Passiflora edulis (Passifloraceae, common name: passion fruit)
Physical Description of Adult Butterfly:
Above, the male is deep orange while the female is pale tawny yellow. The forewing has two transverse black spots in the cell, a broader spot at cell-end bar and a discal series of black spots in spaces 2-6 and 10. Its costal margin and termen are black. The hindwing has a black spot in the cell, a basal series of 4-5 black spots, a sub-costal spot and a discal series of spots. The termen is edged with a black marginal band which has a series of small spots in ground colour embedded. Underneath, the wings are salmon orange in the male and pale tawny yellow in the female. Both wings have black markings as per the upperside. The forewing coloration becomes paler and turns whitish towards the apex and the termen. Unlike the smallish spots on the upperside, a series of large, prominent white spots are embedded in the broad black terminal margin on the hindwing. Antennae are black, head and thorax are black spotted with pale yellow and white.
Field Observations of Butterfly Behaviour:
This migrant species was recently discovered in Singapore in 2006 (refer to this blog article for a report of its discovery in Singapore). Since then, it has established a firm foothold and can be considered a relatively common species in Singapore. Across the island, Tawny Coster can be found flying in wastelands and park lands where its favourite host plant, Passiflora foetida, grows in relative abundance. The adults have the habit of visiting flowers for nectar and are sluggish and gentle on the wings.
Early Stages:
The local host plant adopted by Tawny Coster as it spred quickly across the island is Passiflora foetida, a member of the Passifloraceae family commonly found in wastelands. A lesser used plant in the wild is Passiflora suberosa. In captive setting, the Tawny Coster has also been found to feed on Passiflora edulis (Passion Fruit).
The caterpillars of the Tawny Coster feed on the leaves, young shoots, tendrils and outer surface of young stems of the host plant. They are gregarious throughout all six instars of the larval stage, often eating, resting and moulting together in groups. However, not all caterpillars from the same clutch of eggs develop at the same rate. Of all who survive the larval stage, the leading group could reach the pupal stage earlier than the slowest ones by up to a week or so.
It is interesting to note that Tawny Coster is one of a group of butterflies where the ovipositing females feature a sphragis (copulatory plug) which is formed after a mating session (caused by a waxy substance deposited by the male) for the purpose of preventing the female from further matings.
A mother Tawny Coster lays its eggs in a tight cluster on the underside of a leaf of the host plant. The cluster size varies from just a few eggs (less than ten) or up to 50-60 eggs. Each yellowish egg is olive-shaped with a ribbed surface. It has a height of about 0.8-0.85mm, and a cross-sectional diameter of about 0.7mm.
Each egg takes about 3-4 days to hatch. The infant caterpillar nibbles away an upper portion of the egg shell in the hatching process. The newly hatched has a pale yellowish brown cylindrical body with an initial length of about 1.8mm. The body is covered in a grid of greyish tubercles, each of which has a single black setae exuding from it. The head is black. The young caterpillar feeds by either skimming the lamina on a young leaf or nibbles away at the tip of a young shoot.
As the 1st instar caterpillar feeds and grows, its body segments take on a bright yellowish brown coloration. It grows to a length of 4mm in the 1st instar which lasts about 2-2.5 days.
The 2nd instar caterpillar is orangy brown in body base colour and features a dark brown to black head. The fine and long setae in the 1st instar are replaced by rows of black spines which occur dorso-laterally, laterally and sub-spiracularly. This instar lasts 2-3 days with the body length reaching about 6.5mm before the next moult.
The 3rd instar caterpillar does not feature any drastic change in physical appearance from the 2nd instar. Small and inconspicuous pale whitish patches occur ventrally. The body length reaches up to 11mm in 2-3 days before the moult to the next instar.
The 4th instar caterpillar has a close resemblance to the 3rd instar caterpillar. There are minor changes such as the paler coloration in the first two thoracic segments, the head as well as larger ventral patches which are now more whitish in coloration. The 4th instar lasts about 3-5 days with body length reaching about 14-15mm.
A clip of a Tawny Coster caterpillar moulting from 4th to 5th instar, complete with stages of spine inflation and eating of exuvia.
The next moult brings the caterpillar to its 5th and penultimate instar. The caterpillar is little changed in appearance coming into this instar. Visible changes are the continuing paling in coloration of the head and first 2-3 body segments, and the increase in the size and prominence of the ventral whitish patches. This instar lasts for 5-7 days and the body length reaching up to 21-22mm.
The sixth and final instar caterpillar has its head capsule turning almost entirely orange. Its first 2-3 and the last 2-3 body segments bear greater infusion of orange compared to the earlier instars. The ventral white patches invariably turn yellowish green in the last few days of this instar. Growth rate is rather variable among individuals and this final larval stage could last between 9 to 13 days with the body length reaching up to 35mm.
On the last day of 6th instar, the caterpillar ceases feeding and wanders around in search of a suitable pupation site. This mobile pre-pupatory larva has two large white patches on the side of the 2nd and 3rd thoracic segments. Both patches will morph to the wing cases in the pupal stage. Typically the caterpillar finds a spot on the underside of a stem or a tendril where it spins a silk pad to which it secures itself with graspers at its posterior end. From this anchor, the caterpillar hangs vertically head-down and becomes a pre-pupa.
Pupation takes place a day later. The slender and whitish pupa hangs vertically with a short cremaster securing it to the silk pad on the substrate. It has two slightly-raised angles in the thorax. There are black dorso-lateral, lateral and ventral bands spotted with orange spots. Black stripes can also be found in the middle, and costal and dorsum margins of the wing case. Length of pupae: 19-21mm.
A Tawny Coster caterpillar molts to its pupal stage.
After about 5 days of development, the mature pupa turns dark in the thorax and wing cases, and salmon orange in the abdomen. Eclosion takes place the next day.
A Tawny Coster butterfly emerges from its pupal stage.
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, 1st Edition, 2006.
- A Field Guide to the Butterflies of Singapore, Khew S K, Ink on Paper Comm. Pte. Ltd, 2010.
- Wikipedia: Acraea terpsicore.
