While walking in the Skiag bridge area (NC 2324) on 14th May 2019, Don O’Driscoll, John Muir Trust Manager for Quinag, took a photo of a caterpillar which looked as if it was a fallen birch catkin (Photo 1).
I thought it would be easy to identify, but could find nothing similar in Caterpillars of the British Isles by Jim Porter. I then searched the internet for caterpillars which resembled birch catkins and also tried the excellent www.ukleps.org website for moth species which have birch as the food plant.
I found some pictures of caterpillars of the Scalloped Hook-tip Falcaria lacertinaria associated with birch, which were reddish-brown, quite knobbly, but with pointed rear-ends (Photo 2). Since caterpillars can drastically change their appearance between the first and final instars, I concluded that Don’s caterpillar was a young specimen of this species. However, it was rather early for Scalloped Hook-tip larvae in Scotland, the timing being similar to those found further south. Just to be sure, I sent Don’s photo to Roy Leverton, an expert and author of the book, Enjoying Moths.
Roy sent me his photos of Large Emerald Geometra papilionaria moth larvae amongst birch catkins hanging on a tree. They were a good match for Don’s caterpillar (Photos 3 and 4).
The Moths of the British Islesby Richard South is an old book, but it gives detailed descriptions of life histories. For the Large Emerald, it says: ‘The caterpillar hatches in the late summer, and feeds on birch, hazel, and beech, until the leaves begin to fall in the autumn; it then constructs a carpet of silk on a twig, and near a bud, upon which it takes up its position for the winter. When thus seen, its reddish brown colour, variegated more or less with green, assimilates so closely with its surroundings that the creature is not easy to detect (Photo 5). In the spring, when it awakens, the green colour increases in extent as the buds open and the leaves unfold; when they are fully expanded, the caterpillar sits amongst the foliage towards the tip, and is then almost entirely green, the reddish brown only showing on the head, slightly on the warts, and more distinctly on the hinder parts which are in contact with the twig.’
Roy suggests that the final instar is normally green and that the catkin form is relatively infrequent. He suspects it is genetic, only triggered by an abundant crop of catkins on the caterpillar’s tree. His photo (6) shows the two colour forms.
That is not the end of this story of mimicry. The Large Emerald is widespread in the U.K and, in June, the adult emerges from a chrysalis, which is enclosed in a silken web among dead leaves on the ground. It comes readily to light and often several are found in moth traps, including mine in Torbreck. The moth is a striking green and holds its wings like a butterfly at rest (Photo 7). The Field Guide to the Moths of Great Britain and Ireland by P. Waring, M. Townsend and R. Lewington states: ‘Occasionally flies high in the canopy on warm, sunny days.’ So, in this stage of its life-cycle, it is also a mimic, this time of fluttering leaves.
Gwen M. Richards