Fred the Thread is the caterpillar of a native moth (Houdinia flexilissima) and is quite possibly the world’s thinnest caterpillar.

Fred was found following studies conducted by Dr Corinne Watts of Landcare Research investigating the natural ecosystem of peat bogs in the Waikato. Peat bogs are mined for use in horticulture (peat is widely available in most local garden centres) but also constitute a unique ecosystem. Dr Watts carries out this research to ensure that peat mining does not destroy the bogs.

In order to do this, she needed to understand the plants and animals that help maintain the peat. Her investigations started with the cane rush (Sporodanthus ferrugineus), which is one of the plants that helps form peat. While looking for insects that live with the plant, she found a very small, thin, orange caterpillar – nicknamed Fred the Thread.

Hard to see

Finding Fred the Thread was quite difficult. While these caterpillars can grow to around 2 centimetres long, living in the stem of the plant means they remain very thin throughout their caterpillar stage – only 1 millimetre or so wide. This makes them very hard to see, so finding such a thin caterpillar amongst the vast numbers of stems of cane rush took patience and a very good eye. When the caterpillar was found, it was so small that it needed to be studied under an electron microscope.

The detective hunt was on

The caterpillar had never been identified before, so Dr Watts decided to consult with other scientists at Landcare Research to see if they could identify this species. She first showed the caterpillar to Dr Robert Hoare, an expert on native moths. He had never seen anything like it and didn’t think it was likely to be a moth species. An expert on beetles said it wasn’t a beetle, and an expert on flies said it wasn’t a fly.

What was this caterpillar?

Dr Hoare and Dr Watts decided to raise a caterpillar and see what happened. Very carefully, they gathered caterpillars from the stems of the cane rush and transferred them to plastic bags. They were then fed and reared through until an adult moth eventually emerged – a very, very, tiny adult moth. Each adult is less than half a centimetre long with a wingspan of only 12 mm. Partly because of their small size, scientists were unaware of the existence of the moth and its caterpillar until this research was done.

Let’s call it…

Because the scientists had discovered a new species, they were allowed to name the species. The caterpillar was sufficiently novel that a new species and a new genus name were needed. They gave Fred the scientific name Houdinia flexilissima.

The genus name (Houdinia) is a Latin derivation of the name Houdini. The caterpillar reminded the scientists of Harry Houdini, a famous escape artist of the early 20th century, who was very good at getting out of tight spaces, much like Fred the Thread, who was also very good at avoiding detection by scientists for a long time.

The species name (flexilissima) refers to Fred the Thread’s amazing flexibility. Living in the very small tubules within the plant means there is not a lot of room to move. Fred pulls himself through the plant by his head, and a hinge allows him to drag his body through the plant as he eats. Also, when the caterpillars are removed from the plant stems, they are very soft and flexible.

Fred the Thread is special

The plants Fred eats and calls home are currently under threat through the mining process and as bogs are drained for use as pasture. The research conducted by Dr Watts and Dr Hoare has helped identify the ecological importance of these habitats, and Dr Watts is now trying to work with commercial companies to ensure that these habitats are fully restored and conserved for future generations of Freds.

Nature of science

Sometimes we think that scientists follow a set scientific method with an aim, a prediction, an experimental method, data gathering and a conclusion. This account of Fred’s discovery and description shows that anything goes when scientists are working together to develop knowledge. There is no single scientific method.

    Published 30 April 2009