Life History of the Yellow Flat

Life History of the Yellow Flat (Mooreana trichoneura trichoneura)

Butterfly Biodata:
Genus: Mooreana Evans, 1926
Species: trichoneura C & R Felder, 1860
Sub-Species: trichoneura C & R Felder, 1860
Wingspan of Adult Butterfly: 32-36mm
Caterpillar Local Host Plant: Mallotus paniculatus (Euphorbiaceae, common name: Turn-in-the-wind).

A newly eclosed female Yellow Flat showing its underside.

Physical Description of Adult Butterfly:

Both sexes are alike in coloration and markings. Above, the wings are dark brown with veins strongly dusted in white or pale yellowish brown. The forewing bears a number of round and stroke-shaped hyaline spots in the outer half of the wing. The hindwing has a large yellow tornal area and has yellow coloured cilia extending up to vein 6. Underneath, the forewing is dark brown with the same set of spots as above. The veins are not marked in white or pale brown. The hindwing is predominately white from the dorsum to vein 6, with the white coloration diffusing into spaces 7 and 8. The male has a hair tuft on its mid- and hind tibiae.

Close-up views of the legs of both sexes of Yellow Flat, the hair tuffs on the mid- and hind tibae of the male are visible.

Close-up view of the front part of a Yellow Flat taking nectar from Lantana flowers.

Field Observations of Butterfly Behaviour:

C&P4 describes Yellow Flat as being rare in the forested plain in Malaya. This holds true for this newly discovered species in Singapore (see this blog article for a report of the discovery in November 2012) as there has only been a handful of field sightings in the few months since the first sighting. The adult has a strong preference for dark and shady area, and rarely does it venture to sunny spots for sunbathing. The adults are fast and strong flyers and have the habit of perching with their wings opened flat.

A Yellow Flat taking nectar from Lantana flowers, note the hyalinity of the forewing spots.

Early Stages:

Although Igarashi listed Mallotus paniculatus and Macaranga triloba as host plants for the Yellow Flat, only the former has been found as the local host. The immature stages of the Yellow Flat feed on leaves of the host plant. Between feeding sessions, the caterpillars seek safety and concealment in a leaf shelter. The construction of the leaf shelter comes in two modes (to be described below). The caterpillars have been found to reside on leaves of the host plant at heights ranging from 1 metre (young plant) to several metres (mature tree) high.

Local host plant: Mallotus paniculatus.

A leaf of the host plant with a resident caterpillar of the Yellow Flat hidden in a leaf shelter (salmon-coloured arrow). A smaller (abandoned and opened) leaf shelter is nearby (red arrow).

The eggs of the Yellow Flat are laid on the upper surface of a leaf of the host plant. It is not uncommon to find a few (2-5) eggs on the same leaf from repeated oviposition runs to the same run. The whitish egg is dome-shaped with a base diameter of about 0.75mm. It is well hidden in a mass of dark brown and whitish hair. A number of longitudinal ridges run from the top of the egg to the rim of the base.

Two views of an egg of the Yellow Flat.

Left: a mature egg with the caterpillar already nibbed away part of the egg shel. Right: an empty egg shell after the emergence of the caterpillar.

The collected eggs take about 6-7 days to hatch. The young caterpillar emerges after it has eaten a sufficiently large upper portion of the egg shell. The rest of the egg shell is not eaten by the newly hatched which is about 1.8mm in length. At this stage, the body is orange in base colour. It has rows of short to moderately long white setae on its body surface, and a small tuff of white setae at the posterior end. The head capsule is orangy brown at the upper end and dark brown elsewhere. Several short white setae can be found on the head capsule too. A reddish to reddish brown band line the body side spiracularly.

Two views of a newly hatched caterpillar, length: 1.8mm.

A newly hatched caterpillar constructing its first leaf shelter. Left: making an initial cut through the leaf lamina. Right: the cut leaf segment is folded over.

A sequence of four shots showing the newly hatched caterpillar hard at work at folding the circular leaf segment over.

The newly hatched makes its way to a part of the leaf surface adjacent to a lateral vein to construct its very first leaf shelter. Here it cuts a near circular leaf fragment with a short stalk and folds it over towards and covers the lateral vein. The leaf fragment is held in place with silk threads. The leaf shelter is easily observed as the leaf fragment has the contrasting whitish underside exposed. The caterpillar ventures out of the shelter from time to time to feed on the leaf lamina in the vicinity. As the caterpillar grows, the body base colour assumes a pale yellowish brown coloration with a green undertone. After reaching about 3.5-4mm in about 5-6 days, the caterpillar moults to the 2nd instar.

Two views of a 1st instar caterpillar, length: 3.5mm.

The 2nd instar caterpillar has a yellowish green body covered with numerous tiny body setae. The reddish brown head capsule has a rough surface texture and also covered with many short setae. The body still has a lateral reddish brown band, but it is more diffused and less prominent than that in the 1st instar. This instar lasts about 5-6 days with the body length reaching about 5.5-6mm.

Two views of a 2nd instar caterpillar, early in this stage, length: 4.2mm.

Top: Late 2nd instar caterpillar dormant prior to its moult. Bottom: Newly moulted 3rd instar caterpillar.

The 3rd instar caterpillar differs from the 2nd instar caterpillar in having a body colour which is strongly whitish with a weak yellowish undertone. Typically at this stage, the caterpillar abandons its first shelter and moves to an adjacent spot to construct a larger but similar leaf shelter. This instar takes about 6-8 days to complete with body length reaching about 9-9.5mm.

Two views of a 3rd instar caterpillar, late in this stage, length: 9mm.

The resident and well eaten leaf of a 3rd instar caterpillar with its leaf shelter indicated with an orange arrow. Its abandoned 1st/2nd instar shelter is indicated with a red arrow.

Close-up view of the leaf shelter of a 3rd instar caterpillar.

Top: A late 3rd instar caterpillar dormant prior to its moult. Bottom: newly moulted 4th instar caterpillar.

The 4th instar resembles the 3rd instar caterpillar closely. The body base colour is pale yellow. The yellow tone is more intense early on in this instar, but as growth progresses and the body length increased, this tone lightens and the body appears more whistish. Numerous tiny yellow specks are visible on the body surface.

Two views of a 4th instar caterpillar, late in this stage, length: 14mm.

Early in the 4th instar, the caterpillar abandons its 2nd shelter (the leaf fragment making up the shelter is usually bitten off by the caterpillar as it does so) and goes on to construct a rather different looking shelter. Invariably the caterpillar selects a spot along the mid-rib closer to the leaf tip on the whitish underside. Here the caterpillar cuts one large fragment on one side, flips it over and secures it via silk threads to the other side to form the leaf shelter. Now the colour contrast is reversed compared to that seen in the previous 2 shelters. The caterpillar next cuts another large fragment on the opposite side and flip it over to form a loose roof over the shelter.

Leaf shelter for 4th and 5th instar caterpillars. Left: upperside view; Right: underside view.

Besides feeding on the leaf lamina on the resident leaf and adjacent leaves, the caterpillar also eats small and separated portions of the roof and ceiling of the leaf shelter, giving its a meshed or sieve-like appearance. The growth rates in this instar among the few caterpillars observed are rather variable, and the entire instar lasts about 6-11 days with body length reaching up to about 14-16.5mm.

Top: A late 4th instar caterpillar dormant prior to its moult. Bottom: Newly moulted 5th instar caterpiollar.

The 5th instar caterpillar has similar body markings and colour as in the late 4th instar. It is noteworthy that the dark reddish brown head capsules in the 3rd to 5th instars has 6-8 pale reddish brown spots just outside the frons area, and short white setae are restricted to the peripheral areas of the head. Typically the 5th instar caterpillar in the field continues to reside in the same leaf shelter used during its 4th instar and even goes on to use it as its pupation shelter. The 5th instar is rather long in duration and lasts for about 14-16 days, and the body length reaches up to 22mm.

Two views of a 5th instar caterpillar, early in this stage, length: 15mm.

Head capsules for the 4th instar (left) and 5th instar (right) caterpillars.

Two views of a 5th instar caterpillar,  early in this stage, length: 22mm.

On the last day of 5th instar, the body shortens in length and its colour turns to an intense tone of yellowish green. It ceases feeding and stays in the shelter. After purging its last few frass pellets, the pre-pupatory larva prepares for the pupal phase with a series of silk construction work with the main pieces being a silk girdle across the dorsum of its early abdominal segments, and a short and thickened transverse silk band on the substrate at its posterior end. Soon the pre-pupatory larva becomes dormant in its leaf shelter. Pupation takes place about 1-1.5 day later.

Two views of a 5th instar caterpillar, late in this stage, prior to constructing its pupal girdle, length: 18mm.

A girdled pre-pupa of the Yellow Flat revealed by partially opening the leaf shelter.

The girdled pupa is secured with its cremaster attached to the short transverse band on the substrate. A thin layer of whitish powdery substance envelops its body. It has a short thorax, a rather long abdomen and a short pointed and brown rostrum. The body is pale yellowish green, darker green in the wing case and brownish in the head area. It has a dark brown patch over its eye, a series of small round-shaped lateral spots just below the spiracles in abdominal segments 4-8, and usually has brown patches around the spiracles in these segments. Ventrally, there are dark brown patches, two to each segment, in abdominal segments 5-7. Length of pupae: 16-17.5mm.

Two views of a pupa of the Yellow Flat.

After about 8-9 days in the pupal stage, the development within the pupa comes to an end and the pupa turns dark brown to black in the wing cases and thorax. Overnight, the abdomen area turns dark brown to black and the adult butterfly emerges from the pupal case in the morning.

Two views of a maturing pupa of the Yellow Flat, night before eclosion.

Two views of a mature pupa of the Yellow Flat, minutes before eclosion.

A newly eclosed female Yellow Flat.

The empty pupal case after the emergence of the adult butterlfy.

References:

• [C&P4] The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, 4th Edition, The Malayan Nature Society.
• The Life Histories of Asian Butterflies Vol.2, Igarashi S. & Fukuda H., Tokyo University Press, 2000.
• Butterflies of Thailand, Pisuth EK-Amnuay, 1st Edition, 2006.

Text by Horace Tan, Photos by Nelson Ong, Simon Sng, Federick Ho, Sunny Chir, Khew SK and Horace Tan

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