27 August 2011

Life History of the Yellow Streak Darter

Life History of the Yellow Streak Darter (Salanoemia tavoyana)



Butterfly Biodata:
Genus: Salanoemia Eliot, 1978
Species: tavoyana Evans, 1926
Wingspan of Adult Butterfly: 29-32mm
Caterpillar Host Plants: Licuala spinosa (Arecaceae, common name: Mangrove Fan Palm).


A Yellow Streak Darter taking nectar.


A Yellow Streak Darter sighted on a leaf perch.

Physical Description of Adult Butterfly:
Above, both sexes are dark brown with the forewing having a number of hyaline spots: two small sub-apical spots at the base of spaces 6 and 7, two large spots at the base of spaces 2 and 3, and two sub-equal spots in the cell. There is a yellow streak in the basal half or two-thirds on the forewing. Both sexes have a yellow discal patch on the hindwing and a yellow-colored costal margin, both of which are broader and more prominent in the male than in the female. In addition, the male has a sub-costal yellow streak in the basal half of the forewing, and a yellow-colored dorsal margin on the hindwing. Underneath, the wings are yellowish orange in base color with a number of irregularly-shaped post-discal black spots on both fore- and hindwings. Additional black spots are found in the cell and the base of spaces 1b, and 7 in the hindwing. Several of these spots take on the appearance of a yellow oval spots encircled in black. The blacks spots in the female are somewhat larger and more prominent than in the male.


The first sighting of a Yellow Streak Darter.


Another Yellow Streak Darter on a leaf perch.

Field Observations of Butterfly Behaviour:
This species was only recently discovered and added to the Singapore checklist as species #300. According to C&P4, the Malayan species of the genus Salanoemia are all very rare, and the Yellow Streak Darter is no exception. In Singapore, adult sightings have so far been very rare and only confined to the offshore island, Pulau Ubin.


A male Yellow Streak Darter.


A female Yellow Streak Darter.

Early Stages:


Host plant: Licuala spinosa.

The host plant, Licuala spinosa, is a palm with fan-like leaf in a pinwheel layout. The petiole is armed with spines. This plant can be found growing in back mangrove habitats or in gardens/parks as an ornamental and landscaping plant. The caterpillars of the Yellow Streak Darter feed on leaves of this plant, and live in shelters made by joining edges of leaf fragments together with silk threads.


A leaf shelter of a final instar caterpillar of the Yellow Streak Darter.

The eggs are laid singly on the leaf of the host plant. Each hemispherical egg has four concentric colored zones, starting from the centre and moving outwards, the colors are dark pinky red, reddish brown, pinky white and finally dark pink red. The micropylar sits atop. There are 20 disheveled radial ridges in all. The base diameter is about 1.4mm.


Two views of an egg of the Yellow Streak Darter.


A mature egg with the young caterpillar already nibbled away the polar part of the egg shell.

It takes about 4 days for the collected egg to hatch. The young caterpillar eats just enough of the shell to emerge, and has a length of about 3mm. Its has a cylindrical body shape, very short dorso-lateral and sub-spiracular setae and a tuff of long setae at the posterior end. The body is mainly orange in base color with faint reddish bands running dorso-laterally. The head capsule is yellowish brown.


A newly hatched 1st instar caterpillar with its empty egg shell nearby.


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

The body color changes to pale yellowish green after a few feeding sessions on the leaf. The newly hatched constructs its leaf shelter as one of its first tasks after the hatching. Between feedings, the caterpillar retreats to its shelter for rest and security. By the time the caterpillar lies dormant in its shelter for the moult to the 2nd instar, its length has reached about 5mm. The 1st instar takes about 2-3 days to complete.


1st instar caterpillar, late in this stage, dormant prior to its moult, length: 5mm.

The 2nd instar caterpillar is whitish, but appears to be yellowish green with its "stomach" contents showing through the translucent cuticle. The short dorso-lateral and lateral setae are now absent. The anal plate still carries a few setae but these are not as long and prominent as in the 1st instar. The head is pale yellowish brown. This instar lasts about 3-4 days with the body length reaching about 8mm.


Two views of a 2nd instar caterpillar, early in this stage, length: 4.5mm.


A 2nd instar caterpillar hard at work at carving out a leaf fragment. The fragment (with a small portion not cut) will be joined to the uncut portion to form its shelter.


Two views of a 2nd instar caterpillar, late in this stage, length: 7.5mm.

The 3nd instar caterpillar resembles the 2nd instar caterpillar closely in body markings and coloration. This instar lasts about 5-7 days with the body length reaching about 11mm.


Two views of a 3rd instar caterpillar, female, early in this stage, length: 6.8mm.


Two views of a 3nd instar caterpillar, later in this stage. length: 9.5mm.

The 4th instar caterpillar is pale yellowish white in body color with a light green undertone. The head capsule is pale beige brown with two lateral white stripes sitting just above the mouth parts. Dark brown meshed markings adorn the surface above the white stripes. This penultimate instar lasts 5-7 days with the body length reaching up to about 18mm.


Two views of a 4th instar caterpillar, female, length: 17mm.


A 4th instar caterpillar, male, late in this stage, length: 19mm.

The 5th (and final) instar caterpillar closely resembles the 4th instar caterpillar with few variations. The head capsule has a base color ranging from pale beige brown to greyish white, and it appears to have more extensive black meshed markings than in the previous instar. This instar takes about 9-10 days to complete with the body length reaching up to about 26mm. On the last one to two days of this instar, the still feeding caterpillar secretes a thin layer of whitish substance on the lower sides of its body.


A newly moulted 5th instar caterpillar.


Two views of a 5th instar caterpillar, length: 24mm.


A 5th instar caterpillar, male, late in this intar, length: 26mm.


A 5th instar caterpillar in the field. Hard at work in repairing its damaged leaf shelter.

Towards the end of the 5th instar, the the body of the caterpillar shortens and its coloration changes to a translucent pale green. Soon it becomes dormant in its leaf shelter and enters the prepupatory phase which lasts for one day During this time period a copious amount of white waxy substance is deposited within the tight confine of the leaf shelter.


A pre-pupa of the Yellow Streak Darter.

Pupation takes place within the leaf shelter. The pupa does not secure itself with any cremastral attachment nor any silk girdle. It is pale brown in the anterior end, pale greenish in the rear thorax and wing cases, and pale yellowish brown in the abdomen. Length of pupae: 14.5-17.5mm.


Two views of a pupa of the Yellow Streak Darter.

On the last day of the pupal period, the pupa becomes mostly brown to dark brown. Yellow markings against a dark brown background are now visible in the wing cases. Finally after about 8 days of pupal phase, eclosion takes place with the adult emerging from the pupal case.


Two views of a mature pupa of the Yellow Streak Darter.


A newly eclosed male Yellow Streak Darter.


A newly eclosed female Yellow Streak Darter.

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
Text by Horace Tan, Photos by Simon Sng, Khew SK and Horace Tan.

1 comment:

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