THE BUTTERFLY: Life History of the Plain Plushblue

Life History of the Plain Plushblue (Flos apidanus saturatus)

Butterfly Biodata:
Genus: Flos Doherty, 1889
Species: apidanus Cramer, 1777
Subspecies: saturatus Snellen, 1890
Wingspan of Adult Butterfly: 34mm
Caterpillar Host Plants: Terminalia catappa (Combretaceae, Common name: Sea Almond), Syzygium glaucum (Myrtaceae), S. grande (Myrtaceae) and other Syzygium species (yet to be identified).

A Plain Plushblue perching on a leaf in the nature reserve.

Physical Description of Adult Butterfly:
Above, the male is deep bluish violet, with a narrow forewing border (about 1.0mm); the female is bluer and more shining, with rather regular black borders on the costal and distal margins of both wings. Below, the wings are pale yellowish to darker brown with whitish markings featuring a purple tinge. The discal and post-discal bands are evenly curved on the forewings, and there is a pale yellowish end-cell bar on the forewing. Hindwings have irregular discal whitish bands from mid-costa to mid-dorsum. There is a small ochrous brown patch at the base of the costa on both wings. The hindwing is tailed at vein 2 and toothed at veins 1b and 3. The compound eyes are dark yellow brown (see the cover picture of this earlier blog article).

A close-up view of the front portion of a Plain Plushblue. Note that the ochrous brown patches at behind the head.
Field Observations of Butterfly Behaviour:
Worldwide, Flos apidanus is the most widely distributed member of the Flos genus. This is also true locally in Singapore as it can be found in multiple locations, in and out of the nature reserves, and it is the most commonly sighted of the four Flos species residing here. The fast flying adults are usually spotted near flowering shrubs and their host plants. Typically they perch with their wings closed, but they can also be seen to open their wings fully to sunbathe in warm weather.

A female Plain Plushblue perching on a leaf in the Southern Ridges.

Early Stages:

It is mentioned in C&P4 that the Plain Plushblue utilizes a number of Syzygium species as larval hosts. So far we have established that Syzygium glaucum and S. grande serve this role in Singapore. In addition, Terminalia catappa (Sea Almond) has been recorded as another host. This latter plant can grow to be a rather large tree with a pagoda shape. Its leaves are obovate, 20-30cm long, often spirally crowded at ends of branches, and turning orange or red before they fall. The 4-5cm long fruits are flattened ovoid and keeled all round. The seeds are edible. Sea Almond is common in Singapore, either growing wild or cultivated as roadside trees.

Host plant: Sea Almond. Leaves and fruits are featured here.

Host plant: a Syzygium species yet to be fully identified.

A female Plain Plushblue laying eggs at the petiole of a Syzygium species.
Eggs are laid in small groups of 2-5 on the petiole or the underside of a relatively young leaf of the host plant. Each egg is about 0.9mm in diameter, white with a light yellowish green tinge. It is shaped like a pressed bun with a slightly depressed micropylar area atop. The surface has a coarsely reticulated pattern of intersecting ridges.

Groups of eggs of the Plain Plushblue on young shoots of the Sea Almond.

Closer looks at groups of eggs of the Plain Plushblue.

It takes 2-3 days for the egg to hatch. The newly hatched is pale yellowish in body color and has a length of about 1.5mm. It has a rather flattened woodlouse appearance with a large semicircular prothorax, a yellowish brown head and long dorso-lateral and lateral setae. There are also a fair number of very short setae on the body surface.

Two views of a newly hatched caterpillar, length: 1.5mm.

The 1st instar caterpillar feeds by skimming the surface of a young leaf, but later instars are able to consume the lamina in whole. Larval stages of the Plain Plushblue are gregarious and a few caterpillars have been observed in the field sharing a feeding site with no animosity towards each other. As the 1st instar caterpillar grows, its body color becomes more yellowish green. Pinky red patches also appear on the front portion of the prothorax and the posterior abdominal segments. After 2-3 days of growth, and reaching a length of about 2.5mm, the caterpillar moults to the next instar.

Two views of a 1st instar caterpillar, length: 2mm.

The 2nd instar caterpillar features long lateral hairs and a black head. Long dorso-lateral setae are no longer present, but many short setae with blobbed endings appear on the entire body surface. A pair of faint dorso-lateral line runs along the length of the body. The prothorax is marked by a large black patch with a leading edge in pinky red. The dorsal nectary organ (DNO) is already visible on the 7th abdominal segment and highlighted with a small dark reddish patch stretching to the 8th segment. A large anal plate, black in color, dominates the remaining posterior segments.

Two views of a 2nd instar caterpillar, early in this stage, length: 2.5mm.

2nd instar caterpillar, late in this stage, length: 3.5mm.
The 2nd instar caterpillar has a functional DNO as ants living in its proximity are observed to actively attend to the young caterpillars, having been attracted to the nectary fluid excreted via the DNO. The ant-caterpillar association continues for all remaining larval stages of the Plain Plushblue.

A 2nd instar caterpillar being tailed by an ant which is eyeing the nectary fluid excreted by the caterpillar.

A sequence of three pictures showing an ant receiving its pay packet of a nectary droplet from the 2nd instar caterpillar.

The 2nd instar caterpillar reaches a length of about 4mm, and after about 3 days in this stage, it moults again. The 3rd instar caterpillar resembles the 2nd instar caterpillar closely. New to this instar is a dorsal band in darker green against the yellowish green base colour. The DNO is now rather prominent with an dark brown oval ring marking its outer boundary. The 3rd instar takes about 3-4 days to complete with the body length reaching about 7mm.

Two views of a 3rd instar caterpillar, length: 5.5mm.

A small group of two 3rd instar caterpillars attended by ants.

The 4th instar caterpillar has similar markings as the 3rd instar. One notable change is in the prothoracic dark patch which now has two small white patches embedded at its posterior. Another change is in the dark red patch stretching from the DNO to the anal plate as it now has two side-branches reaching the tentacular organs (TOs) on the 8th abdominal segment. The 4th instar takes about 4-6 days to complete with the body length reaching 11mm.

4th instar caterpillar, length: 11mm.

Two views of a 4th instar caterpillar being attended to by three ants.
The 5th instar caterpillar has similar but more striking markings. Visible changes are 1) a white intermittent line running down the middle of the dark patch on the prothorax; 2) both the dark patch on the prothorax and the anal plate are now flanked by white borders which are moderate in thickness.

5th intar caterpillars, legnths: 15.5mm (top) and 19mm (bottom).

A group of three 5th intar caterpillars sharing one leaf.

The DNO and the TOs at the posterior segments of a 5th intar caterpillar.
The right picture shows the wet DNO after a recent excretion.

Dorsal views showing the newly excreted droplet atop the DNO.

Lateral views of the same nectary droplets at the DNO.

After 7-8 days of feeding and reaching a length of about 20mm, the caterpillar slows down and stops food intake for about 1 day. During this time, its body length gradually shortened. Soon it becomes an immobile pre-pupa in its leaf shelter.

Two views of a pre-pupa of the Plain Plushblue.

The pre-pupa caterpillar prepares for pupation by spinning a silk girdle and a silk pad to which it attaches via graspers at its posterior segments. After 1 day as a pre-pupa, pupation takes place. The pupa, with a length of 12-14mm, has a shape typical of any Lycaenid species, but with a somewhat produced anal segment. It is yellowish green in coloration.

A time-lapse sequence of the pupation event for a Plain Plusblue caterpillar.

Two views of a pupa of the Plain Plushblue, length:12mm.

Nine days later, the pupa matures enough to show the markings on the forewing upperside. The next day, the pupal stage comes to an end with the emergence of the adult butterfly.

Mature pupae: female (top) and male (bottom).

A newly eclosed Plain Plushblue resting on a leaf.

Unlike most of the Lycaenidae species, the caterpillars of the Plain Plushblue have the habit of constructing leaf shelters in which they rest and seek safety between feeds on the lamina of nearby leaves and part of the shelter. Pupation also takes place within a leaf shelter.