Butterflies of Singapore: Life History of the Studded Sergeant

Life History of the Studded Sergeant (Athyma asura idita)

Butterfly Biodata:
Genus: Athyma Westwood, 1850
Species: asura Moore, 1858
Subspecies: idita Moore, 1858
Wingspan of Adult Butterfly: 60mm
Caterpillar Host Plants: Ilex cymosa (Aquifoliaceae), and another Ilex species in the nature reserve.

Open-wing shot of a Studded Sergeant perching on a branch.

A closed-wing shot of a Studded Sergeant revealing the underside of its wings.

Physical Description of Adult Butterfly:
Forewing cells are open. Above, the wings have white markings set against black ground colour. Each forewing cell has a short and undivided basal streak with a large detached spot lying beyond. A white post-discal spot is present in space 3, in addition to the spot typically present in space 2 for Athyma spp. On the hindwing, a post-discal series of white spots is present with the leading 2 to 3 spots each marked with an interior black spot. Underneath, the wings are orangy brown in ground colour, and are similarly marked as per upperside. In addition, on both wings, there is a marginal series of white spots. In the forewing, the leading edge of the basal cell streak and the white spot lying beyond are outlined in black. Furthermore, the space between the cell streak and this white spot is dusted in white, and lying beyond the white spot is a triangular black spot.

A Studded Sergeant perching on a leaf of its host plant, Ilex cymosa.

The same Studded Sergeant shown in the previoius pic displaying its underside.

Field Observations of Butterfly Behaviour:
This species is rather rare in Singapore with very few sightings recorded in the past 5 years. Its local distribution is however wide-ranging with sightings of the fast flying adults in as far north as the Sungei Buloh Wetland Reserve, in the Central Catchment Nature Reserve and down south in the Southern Ridges. The males have been observed to puddle on damp grounds, whilst the females are more likely observed flying in the vicinity of its host plants.

A puddling Studded Sergeant.

A Studded Sergeant sun bathing in an open-wing pose.

Early Stages:
Thus far, two species of Ilex have been recorded as the local host plants. Caterpillars of the Studded Sergeant feed on the middle-aged to mature leaves of the host plant, and they adopt the same feeding and frass-management routines as described for the Commander and other Athyma species in earlier blog articles.

Host plant: Ilex cymosa. Left: mature leaves; Right: fruits.

Host plant: an Ilex sp. found in the nature reserve. The red arrow points to a final instar caterpillar.

The eggs of the Studded Sergeant are laid singly at the tip of a leaf on the host plant. The greenish eggs are somewhat globular in shape, with its surface marked with hexagonal pits and bearing short spines at pit corners, giving them the appearance of minute sea-urchins. Each egg has a diameter of about 1.1mm.

A female Studded Sergeant ovipositing on the tip of a leaf of Ilex cymosa.

Two views of an egg of the Studded Sergeant.

Two views of a mature egg of the Studded Sergeant.

The egg takes about 3.5-4 days to hatch. The young caterpillar emerges by eating away part of the egg shell. The rest of the egg shell becomes the first meal for the newly hatched, which has a cylindrical pale yellowish green body covered with many small tubercles and short setae. The head capsule is pale to dark brown in color.

A newly hatched caterpillar of the Studded Sergeant, length: 1.8mm.
Left: pausing of its emergence from the egg shell. Right: eating its egg shell.

As with other Athyma spp., the 1st instar caterpillar of the Studded Sergeant feeds from the leaf tip and leaves the midrib intact and protruding. A frass chain is also constructed laboriously by the young caterpillar at the tip of the exposed midrib. Between feeds, the caterpillar rests on either the exposed midrib or the frass chain. In later instars, the caterpillar tends to rest near where the protruding midrib joins the remaining lamina. At this site, it also attempts to camouflage itself with a collection of frass pellets secured with silk threads to the lamina.

Two views of a 1st instar caterpillar, early in this stage, length: 2.5mm.

As the caterpillar grows in this first instar, tubercles occurring dorso-laterally and laterally become larger and prominent. After reaching about 5.5mm in about 3 days, the caterpillar moults to the 2nd instar.

Two views of a 1st instar caterpillar resting on the exposed midrib, late in this stage, length: 4.6mm.

The body color of the 2nd instar caterpillar is dark brown. Tubercles seen in the first instar have now developed into short and branched spines which appear dorso-laterally, laterally and spiracularly. The pairs of branched dorso-lateral spines on the prothorax, 1st abdominal and 5th abdominal segments are pale in coloration. The head capsule is dark brown and dotted with a number of conical tubercles. This instar lasts about 2.5-3 days with the body length reaching 8-8.5mm.

Two views of a 2nd instar caterpillar, at the base of the exposed midrib
and next to its frass barricade. Length: 6mm

Two views of a 2nd instar caterpillar, later in this stage, length: 7.2mm

The 3rd instar caterpillar has longer spines throughout. Dorso-lateral spines are among the longest and most prominent, particularly so for the pairs on the 2nd and 3rd thoracic segments, 2nd and 7th abdominal segments. Its head capsule is dark brown to black, irorated with dark and pale brown tubercles. The pairs of dorso-lateral spines on the prothorax, 1st abdominal and 5th abdominal segments are whitish to pale yellowish in coloration.. This instar takes about 2.5-3 days to complete with body length reaching up to 12mm.

Two views of a 3rd instar caterpillar, newly moulted, length: 7.5mm

Two views of 3rd instar caterpillar, late in this stage. Resting near its frass barricade. Length:12mm.

The 4th instar caterpillar resembles the 3rd instar closely but its dorso-lateral spines are longer and more heavily branched. This instar lasts about 4 days with body length reaching up to 20mm.

Two views of a newly moulted 4th instar caterpillar, length: 12mm.

Two views of a 4th instar caterpillar, later in this stage, length: 17mm.

Two views of a 4th instar caterpillar moments before its moult to the next instar, length: 18.5mm.

The 5th (final) instar sees a dramatic change in colour throughout the body. The branched dorso-lateral spines are now very well developed with those on the 2nd and 3rd thoracic segments being the longest. The base of these dorso-lateral spines are raised and colored in blue, with the exception of those on the 2nd and 7th abdominal segments which are black. These dorsal-lateral spines are yellow to yellowish green except for the pair on the 2nd abdominal segment which are in red. The posterior segment is black. The body base colour is yellowish green. Three thin and dark bands run dorsally, and another one runs laterally. The head capsule is pale to grey/yellowish brown and is decorated with long lateral spines. All in all, the combination of these brightly coloured body features gives the caterpillar a very eye-catching appearance.

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

Two views of a 5th instar caterpillar, midway in this stage, length: 33mm.

When disturbed, the caterpillar adopts a characteristic posture with its anteror bended backward and the head tucked against the mid body segment.

A 5th instar caterpillar seen in the field adopting a defensive posture.

The 5th instar lasts for 6-7days, and the body length reaches up to 40mm. On the last day, the various colored features decolorise rather drastically. The body base colour changes to pale yellow, the blue bases of spines change to purple and then to dull purple, and red spines on the 2nd abdominal segment decolorize to dull salmon red.

Two views of a 5th instar caterpillar, late in this stage.

On the final day of the 5th instar, the caterpillar ceases feeding and wanders around. Eventually it chooses a spot on the underside of a branch/stem/leaf/petiole and spins a brown-coloured silk pad from which it hangs vertically to take on the pre-pupatory pose. Observations in the field indicate that the underside of a petiole is a popular choice as a pupation site.

Pre-pupatory larva of the Studded Sergeant.

Pupation takes place a day later. The pupa suspends itself from the silk pad with no supporting silk girdle. It is almost entirely pale brown in color with segments and parts outlined in a darker shade of brown. The abdominal segments are slender, and the thoracic portion being larger and expanded laterally. Dorsally, there are two prominent processes curved towards and touching each other. The pupa has a pair of large and curved cephalic horns which tips touching the leading edge of the wing case in most specimens. When disturbed, the abdominal segments flex laterally, and could stay in that position for a period of time. Length of pupae: 26-27mm.

The pupation event of a Studded Sergeant caterpillar

Three views of a pupa of the Studded Sergeant.

A pupa of the Studded Sergeant sighted on the underside of a petiole in a hill park.

After about 7.5-8 days of development, the pupal skin turns translucent as the development within the pupal case comes to an end. The spots and streak on the forewing upperside also become discernible. The next day, the adult butterfly emerges from the pupal case, typically during the morning hours.