Butterfly Biodata:
Genus: Orsotriaena Wallengren, 1858
Species: medus Fabricius, 1775
Subspecies: cinerea Butler, 1867
Wingspan of Adult Butterfly: 21-22mm
Caterpillar Local Host Plants: Axonopus compressus (Poaceae, common names: Wide-leaved Carpet Grass, Cow Grass), Urochloa maxima (Poaceae, common name: Guinea Grass), Ischaemum ciliare (Poaceae, common names: Smut Grass).
Physical Description of Adult Butterfly:
On the upperside, the wings are dark brown and unmarked. On the underside, both wings are dark brown in ground colour and have a clear-whitish stripe extending from the costa on the forewing to the tornus on the hindwing. There are two large submarginal ocelli (eye-spots) on the forewing, and three more (two large, one small) in the hindwing.
Field Observations of Butterfly Behaviour:
The Dark Grass Brown is a common butterfly in Singapore. Adults prefer shaded habitats and can be sighted flying low, in and around grassy patches at multiple locations on the fringe of the nature reserves, nature parks and catchment areas.
Early Stages:
The caterpillars of the Dark Grass Brown feed on various grass species in its distribution region around the globe. In Singapore, three grass species, Ischaemum ciliare, Urochloa maxima and Axonopus compressus, have been recorded as larval hosts. The caterpillars feed on grass blades of the host plant, and have been observed to forcefully ejecting their frass pellets, just like their Mycalesis (Bush Brown) cousins. They tend to rest on the underside of a leaf during pauses between feeds.
Local host plant #1: Axonopus compressus.
Local host plant #2: Urochloa maxima.
Local host plant #3: Ischaemum ciliare.
A mating pair of the Dark Grass Brown.
The eggs are laid singly on the underside of a grass blade. Each oblate ellipsoidal egg (about 1.1mm in diameter and 1mm in height) is pale whitish. The surface appears smooth to the naked eyes, but is actually sculptured with numerous tiny indentations/pits when viewed with a macro lens.
A mother Dark Grass Brown laying an egg on the underside of a grass blade.
Two views of an egg of the Dark Grass Brown.
The egg takes about 3.5 days to hatch. The young caterpillar nibbles away a portion of the egg shell to exit and then proceeds to devour the rest of the egg shell almost entirely. It has a cylindrical body in whitish colour, and an initial body length of about 2.9mm. The body is covered with dorso-lateral and lateral rows of whitish setae. At the posterior end, there is a pair of backward-pointing processes. Its pale-yellowish head has pale brown markings, a number of dark setae, a pair of short and rounded horns and a few lateral protuberances.
Two views of a mature egg of the Dark Grass Brown.
Two views of a newly hatched caterpillar of the Dark Grass Brown .
As a result of its leaf diet, the 1st instar caterpillar soon takes on a greenish undertone and its body lengthens gradually. The first instar lasts about 3-3.5 days with the body length increases to about 6.5mm.
Two views of a 1st instar caterpillar of the Dark Grass Brown after a few meals of grass lamina, length: 3.2mm.
Two views of a late 1st instar caterpillar of the Dark Grass Brown, dormant prior to its moult.
In the 2nd instar, both the cephalic horns and anal processes become proportionately longer and pointed, and reddish brown in colour. The few lateral conical protuberances on the head capsule are now much smaller and whitish in colour. The body is pale yellowish green. The head and body is also adorned with numerous minute tubercles, each with a short seta emanating from it. The 2nd instar lasts about 3-3.5 days with the body length reaches about 11.5mm.
Two views of a 2nd instar caterpillar of the Dark Grass Brown, early in this stage.
Two views of a 2nd instar caterpillar of the Dark Grass Brown, length: 10.8mm.
Two views of a late 2nd instar caterpillar of the Dark Grass Brown , dormant prior to its moult, length: 11.5mm.
The 3rd instar caterpillar mostly resembles the 2nd instar caterpillar. One visible difference is that the cephalic horns are proportionately longer than in those in the 2nd instar. This stage also takes 3-4 days to complete with body length reaching about 19.5mm (not including the length of the cephalic horns and anal processes).
Two views of 3nd instar caterpillar of the Dark Grass Brown, early in this stage, length: 12mm.
Two views of a 3nd instar caterpillar of the Dark Grass Brown, length: 17mm.
Two views of a late 3nd instar caterpillar of the Dark Grass Brown, dormant prior to its moult.
The 4th (and final) instar caterpillar bears a close resemblance to the 3rd instar caterpillar in most body features and markings. The cephalic horns are again proportionately longer than those in the previous instar, and are now dark reddish brown in color. Dorso-laterally, indistinct pale yellowish bands run lengthwise. subspiracularly a whitish band also run lengthwise along the body fringe. In some specimens, a narrow reddish band also appears above the whitish band. In a period of about 5-7 days, the body grows to a maximum length of about 32-34mm (not including the length of the cephalic horns and anal processes).
Two views of a newly moulted 4th instar caterpillar of the Dark Grass Brown, length: 19mm.
Two views of a 4th instar caterpillar of the Dark Grass Brown with reddish lateral band, length: 28mm.
Two views of a 4th instar caterpillar of the Dark Grass Brown, length: 32mm.
Toward the end of the 4th instar, the body base colour turns pinkish, and a reddish dorsal band and two lateral bands become evident. In the last day, the body length shortened, and the caterpillar will seek out a spot on the side of a grass stem to spin a silk pad. It then anchors itself there via its anal end, and assumes its upside-down pre-pupatory pose.
Two views of a late 4th instar caterpillar of the Dark Grass Brown, still feeding, length: 36mm.
Two views of a late 4th instar caterpillar of the Dark Grass Brown, ceased feeding, and resting on its chosen pupation spot on a stem.
Two views of a late 4th caterpillar of the Dark Grass Brown releasing its grip to enter the dormant pre-pupal stage.
Two views of an early prep-pupa of the Dark Grass Brown.
After nearly one day as a pre-pupa, pupation takes place. The smooth pupa is mostly pale yellowish brown throughout with narrow pale brownish streaks and speckles. It is somewhat angular in appearance, with a small dorsal keel on the thorax and ridges defining the dorsal wing pad margins. The pupa has a pair of long cephalic horns which are tightly adjoined. It starts to take on a gold sheen about one day after pupation. Length of pupae: 19-20mm (including the length of the cephalic horns).
The pupation event of a Dark Grass Brown caterpillar.
Two views of a one-day-old pupa of the Dark Grass Brown.
Three views of a two-day-old pupa of the Dark Grass Brown.
Close-up view of a pupa of the Dark Grass Brown, showing the golden sheen.
After about 5.5 days of development, the pupa becomes darkened in color overall, and its wing pads totally blackened. Total pupal duration: 6.5 days. The next day the eclosion event takes place with the adult butterfly emerges to start the next phase of its life cycle.
Two views of a mature pupa of the Dark Grass Brown.
The eclosion event of a Dark Grass Brown.
A newly eclosed Dark Grass Brown resting on its pupal case.
References:
Genus: Orsotriaena Wallengren, 1858
Species: medus Fabricius, 1775
Subspecies: cinerea Butler, 1867
Wingspan of Adult Butterfly: 21-22mm
Caterpillar Local Host Plants: Axonopus compressus (Poaceae, common names: Wide-leaved Carpet Grass, Cow Grass), Urochloa maxima (Poaceae, common name: Guinea Grass), Ischaemum ciliare (Poaceae, common names: Smut Grass).
Physical Description of Adult Butterfly:
On the upperside, the wings are dark brown and unmarked. On the underside, both wings are dark brown in ground colour and have a clear-whitish stripe extending from the costa on the forewing to the tornus on the hindwing. There are two large submarginal ocelli (eye-spots) on the forewing, and three more (two large, one small) in the hindwing.
Field Observations of Butterfly Behaviour:
The Dark Grass Brown is a common butterfly in Singapore. Adults prefer shaded habitats and can be sighted flying low, in and around grassy patches at multiple locations on the fringe of the nature reserves, nature parks and catchment areas.
Early Stages:
The caterpillars of the Dark Grass Brown feed on various grass species in its distribution region around the globe. In Singapore, three grass species, Ischaemum ciliare, Urochloa maxima and Axonopus compressus, have been recorded as larval hosts. The caterpillars feed on grass blades of the host plant, and have been observed to forcefully ejecting their frass pellets, just like their Mycalesis (Bush Brown) cousins. They tend to rest on the underside of a leaf during pauses between feeds.
The eggs are laid singly on the underside of a grass blade. Each oblate ellipsoidal egg (about 1.1mm in diameter and 1mm in height) is pale whitish. The surface appears smooth to the naked eyes, but is actually sculptured with numerous tiny indentations/pits when viewed with a macro lens.
The egg takes about 3.5 days to hatch. The young caterpillar nibbles away a portion of the egg shell to exit and then proceeds to devour the rest of the egg shell almost entirely. It has a cylindrical body in whitish colour, and an initial body length of about 2.9mm. The body is covered with dorso-lateral and lateral rows of whitish setae. At the posterior end, there is a pair of backward-pointing processes. Its pale-yellowish head has pale brown markings, a number of dark setae, a pair of short and rounded horns and a few lateral protuberances.
As a result of its leaf diet, the 1st instar caterpillar soon takes on a greenish undertone and its body lengthens gradually. The first instar lasts about 3-3.5 days with the body length increases to about 6.5mm.
In the 2nd instar, both the cephalic horns and anal processes become proportionately longer and pointed, and reddish brown in colour. The few lateral conical protuberances on the head capsule are now much smaller and whitish in colour. The body is pale yellowish green. The head and body is also adorned with numerous minute tubercles, each with a short seta emanating from it. The 2nd instar lasts about 3-3.5 days with the body length reaches about 11.5mm.
The 3rd instar caterpillar mostly resembles the 2nd instar caterpillar. One visible difference is that the cephalic horns are proportionately longer than in those in the 2nd instar. This stage also takes 3-4 days to complete with body length reaching about 19.5mm (not including the length of the cephalic horns and anal processes).
The 4th (and final) instar caterpillar bears a close resemblance to the 3rd instar caterpillar in most body features and markings. The cephalic horns are again proportionately longer than those in the previous instar, and are now dark reddish brown in color. Dorso-laterally, indistinct pale yellowish bands run lengthwise. subspiracularly a whitish band also run lengthwise along the body fringe. In some specimens, a narrow reddish band also appears above the whitish band. In a period of about 5-7 days, the body grows to a maximum length of about 32-34mm (not including the length of the cephalic horns and anal processes).
Toward the end of the 4th instar, the body base colour turns pinkish, and a reddish dorsal band and two lateral bands become evident. In the last day, the body length shortened, and the caterpillar will seek out a spot on the side of a grass stem to spin a silk pad. It then anchors itself there via its anal end, and assumes its upside-down pre-pupatory pose.
After nearly one day as a pre-pupa, pupation takes place. The smooth pupa is mostly pale yellowish brown throughout with narrow pale brownish streaks and speckles. It is somewhat angular in appearance, with a small dorsal keel on the thorax and ridges defining the dorsal wing pad margins. The pupa has a pair of long cephalic horns which are tightly adjoined. It starts to take on a gold sheen about one day after pupation. Length of pupae: 19-20mm (including the length of the cephalic horns).
The pupation event of a Dark Grass Brown caterpillar.
After about 5.5 days of development, the pupa becomes darkened in color overall, and its wing pads totally blackened. Total pupal duration: 6.5 days. The next day the eclosion event takes place with the adult butterfly emerges to start the next phase of its life cycle.
The eclosion event of a Dark Grass Brown.
References:
- [C&P5] The Butterflies of The Malay Peninsula, A.S. Corbet and H.M. Pendlebury, G. and N. van der Poorten (Eds.), 5th Edition, Malayan Nature Society, 2020.
- Butterflies of Thailand, Pisuth Ek-Amnuay, 2nd Edition, 2012.
- A Field Guide to the Butterflies of Singapore, Khew S.K., Ink On Paper Communications, 2nd Edition, 2015.
