Last August I was examining the sallow cordon (Salix cinerea) that has been growing in the south west corner of the Square Metre for many years when I spotted a strange brown creature on one of the leaves. To begin with I couldn't work out what it was.
After looking at various books I worked out that it was a full grown larva of a comma butterfly (Polygonia c-album) a species that is quite common around the garden and whose larvae I have occasionally found on stinging nettles. While nettle is the usual foodplant, the caterpillars have been recorded from sallow. In our garden there is plenty of both these plants and it is interesting that sallow was chosen by the egg-laying female.
Birds are known to eat comma larvae and as a defence against predation, especially in their earlier stages, they are said to look like bird droppings. In the photograph above the caterpillar is clearly adopting a threat position with its hinder end raised and its penultimate pair of legs imitating short arms. The head, perhaps the most vulnerable part, is kept well down on the right. Also the body is twisted well out of normal caterpillar position to give the impression of a large bird dropping. If, when startled, the caterpillar adopted this position rapidly the sudden movement, combined with the other factors, might be enough be deter a hungry bird.
Despite searching online and in my books, I can find no reference to this kind of distinctive behaviour in comma caterpillars.
