eTawau > Insects of Borneo > Caterpillars > A tussock moth at Ikutmaju Saw Mill

Yellow tussock in a silk tent

A tussock moth caterpillar on a grass blade at Ikutmaju Saw Mill — the characters that name it at subfamily, and what three photographs can settle

It was about ten millimetres long and it was not moving. It sat in a loose web of silk it had spun around itself on a grass blade, and it gave no sign of noticing the camera. That is the complete field note: one small caterpillar, one silk retreat, one grass blade, not moving, in the late afternoon at Ikutmaju Saw Mill.

Ten millimetres is not much to work with. What the photographs give you is a set of characters on the back and the sides, and those characters are enough to name it to subfamily — which, for a caterpillar this size in a single brief encounter, is a reasonable result.

The characters

Five characters, all visible here:

• A single rounded dome of yellow hairs on the back. This is the tussock. It sits near the front of the abdomen, and it is the most conspicuous thing on the animal.
• A long black hair pencil projecting forward from the head end. These pencils are long, thin, and pointed — they look nothing like the dense tussock brushes.
• A long black hair pencil projecting backward from the tail.
• A fringe of cream-beige hairs running along each side. Fine and lying flat, not projecting upward.
• Small orange-red dots along the flank. These are raised warts, one to a segment, each carrying a small spray of hairs.

Taken together — one dorsal tussock, hair pencils fore and aft, lateral warts — these are the characters of Erebidae, subfamily Lymantriinae: the tussock moths.

The yellow tussock

The black markings on the tussock are worth noting because they are easy to overlook at ten millimetres and at arm’s length. In the photograph they are clear, and they matter: a plain yellow tussock and a yellow-with-black-markings tussock are not the same, and that distinction can matter to an identification. The camera settles things the eye misses.

Looking straight at it

The head-on view gives a third angle on the same animal, and it confirms three things. The tussock sits on the centreline, the hair pencils are paired (one on each side, projecting symmetrically), and the cream fringe wraps all the way around the lower body. None of this adds to the identification, but it removes doubt about what was there.

The silk retreat

The caterpillar was sitting inside a loose web of silk it had laid across the grass blade. This is common behaviour in Lymantriinae: the larvae often spin a thin open shelter of silk where they rest during the day, leaving it at night to feed and returning before dawn.

The retreat here is not a cocoon. A cocoon is a tightly woven structure built just before pupation, and a ten-millimetre caterpillar is not near that stage — it is early in its growth, with several moults still ahead of it. What you are looking at is a resting tent, not a departure lounge.

The grass blade it was on has not been identified below family (Poaceae). Tussock moths feed on a very wide range of plants, and a grass-feeding Lymantriinae larva is not unusual. Whether this caterpillar was feeding on the grass it sat on, or had simply chosen a convenient blade to rest against, the photographs do not say.

A different tussock from the Tawau River

The site now holds two tussock moth caterpillars, photographed two years and twelve kilometres apart, and they look nothing alike except in the characters that name the subfamily.

• The Tawau River Triangle animal, December 2011: four dense flat-topped brushes in a row on the back; pale grey-white body; three black pencils (two forward, one back). Not measured, but clearly a large caterpillar.
• This animal, February 2009: one rounded yellow dome on the back, with black markings on its surface; reddish-brown body; one pencil at each end. About ten millimetres.

Same subfamily, different species — and possibly a different genus. The four-brush plan and the one-dome plan both occur in Lymantriinae, and recognising them as the same family does not mean confusing them with each other. The field characters that separate the two are visible without magnification, and the difference would not be missed.

The record

Ikutmaju Saw Mill

Ikutmaju Saw Mill is a timber yard at Merotai, on the coastal flat south of Tawau. The mill runs under lights after dark, which draws flying insects from the surrounding country, and arriving timber brings bark fauna in from forests across Sabah — but neither of those routes puts a caterpillar on a grass blade in the yard. This one was almost certainly feeding on vegetation growing at the site.

The mill has now turned up five records on this site, spread across seven years. The lappet moth caterpillar found on the gate post in March 2012 goes into the arithmetic of how the mill gathers its insects, and that page is the place to read about the site in more detail.

Words you may meet

Lepidoptera

The moths and butterflies. The caterpillar is the larval stage of the same animal that will fly as an adult.

Erebidae

One of the largest moth families, holding the tiger moths, tussock moths, underwings and others. The tussock moths were long placed in a family of their own, Lymantriidae, and older books use that name.

Lymantriinae

The tussock moths: a subfamily of Erebidae whose caterpillars carry dense brushes or tussocks on the back, and whose adults include many species with a flightless female.

Tussock

A dense rounded tuft of hairs on the dorsum of a caterpillar. In this animal, a single yellow dome near the front of the abdomen. In other Lymantriinae, four flat-topped brushes in a row.

Hair pencil

A long thin pointed bundle of hairs projecting well clear of the body, forward from the head end or backward from the tail. Different from the tussock: it is thin and pointed, not dense and dome-shaped.

Wart

A raised knob on a caterpillar’s body, here carrying a small cluster of hairs. The orange-red row on each flank of this animal.

Poaceae

The grass family. The plant this caterpillar was resting on was a grass, but the genus was not identified.

Silk retreat

A temporary shelter of loosely spun silk used by some caterpillars for resting. Not the cocoon, which is a tight structure built only before pupation.

Related pages

• Four brushes and three pencils — a tussock moth at the Tawau River Triangle, December 2011 — the same subfamily, a different plan: four flat-topped brushes in a row, pale body, three pencils
• The caterpillar that came to the gate post — Ikutmaju Saw Mill, March 2012 — a lappet moth (Lasiocampidae) at the same saw mill, with the five-second test that tells a lappet from a tussock
• Caterpillars of the Tawau district — the index this record belongs to
• The dead leaf that was holding on — Bukit Gemok  ·  A nettle caterpillar and the ants that found it — Kampung Sayap
• Moths of Borneo  ·  Insects of Borneo

About the photographs. Three frames by Wong Fook Yee with a Canon PowerShot A470, f/3.0, ISO 100, 6.3 mm, no flash, at 16:55:12, 16:57:33 and 16:58:34 on 22 February 2009 at Ikutmaju Saw Mill, Merotai, Tawau, Sabah. Cropped, resized and labelled for this page; nothing has been sharpened, recoloured, added or removed. The unmarked originals are attached to the iNaturalist record above.

The subfamily characters and the account of silk retreat behaviour follow the general entomological literature. The locality, the date, the times and the field description are the photographer’s own observations. The grass is Poaceae, genus not identified; the plant identification goes no further. All photographs © Wong Fook Yee. Page written 23 September 2026.

Note: the field record says September 2009; the EXIF date in all three image files is 22 February 2009, a Sunday. The page uses the EXIF date.