While I was watching the heron catch a fish in what appeared to be the middle of a field, something crawling on a nearby rock caught my eye. It was a very brightly coloured caterpillar – lime green – covered in long, equally bright hairs, and a series ofdorsal ‘humps’ on its back – it looks like it had crawled straight off the page of Alice’s Adventures in Wonderland. It is the caterpillar (larval stage) of a Pale Tussock moth, and it is considered fairly common in Ireland, though I don’t recall ever seeing one before.
Caterpillar hairs are for protection – they can contain an irritant to deter predators (which would be handy for something as easily noticeable as the Pale Tussock). The image above shows the defence posture of the caterpillar – I had just gently stroked it, and it froze, and displayed the black patches along the torso. According to my field guide¹, the caterpillar displays luminescence under ultra-violet light. Given that caterpillar-loving birds like Blue Tits can see in the ultravoilet specturm, maybe it is a defence mechanism.
I watched the caterpillar for quite a while, and at one point, it shuffled along the underside of a rock. Just as I was marvelling at its ability to walk upside down, it fell off! It then climbed onto a stalk which gave me a good view of its underside, which is almost as colourful as its other side. The National Biodiversity Centre only had one other record this year for this species (and it’s not clear whether that was a mothor a caterpillar), so maybe they are not that common after all. At this time of year, the caterpillars are preparing to enter their pupa stage to see them through the winter.
¹ Field guide to the Caterpillars of Great Britain and Ireland (Barry Henwood and Phil Sterling)