Today I was very happy to see that the 9 eggs that my first female had layed earlier have hatched! I now have my first batch of Papilio canadensis caterpillars - I'm very excited. You won't see any eyespots yet though - Papilio canadensis caterpillars only get their eyespots when they are much larger. As insects grow they progress through a series of "instars", moulting as they pass through each. For now, the caterpillars actually look more like bird-droppings. Almost all of the Papilio genus caterpillars have this bird-dropping colouration in their early instars. In fact, there is a group of Papilio species that retain this colouration throughout their entire caterpillar life stage. In these early instars the caterpillars spend a lot of time in the open on the center of leaves – by resembling a bird-dropping the caterpillars don’t need to hide. You could say that they are hiding in plain sight.
Aside: The adaptive resemblance of an inanimate object or feature of the environment to avoid predation is called masquerade. Check out the recent papers by John Skelhorn and colleagues for some really interesting work exploring the difference between masquerade and crypsis.
Skelhorn J, Rowland HM, Speed MP, Ruxton GD (2010) Masquerade: camouflage without crypsis. Science 327: 51
Masquerading as a bird-dropping enables the caterpillars to use microhabitats that would otherwise risk detection from a predator. If I had to guess I would say the primary advantage of sitting in the open is heat. Insects are ectotherms and their rate of growth depends critically on temperature. Most of their body is dark at this point too, which should help them obtain heat.
From my reading I believe that Papilio canadensis become green with eyespots when they reach the 5th instar. Why the delay? and if masquerading as a bird-dropping works so well why the change? It could be linked to their change in size. As the caterpillars grow eventually they get too big to be a convincing bird-dropping. In addition, the caterpillars wouldn’t make a very convincing or intimidating snake mimic at less than 1 cm long.
Here are some photos of the 1st instar Papilio canadensis caterpillars:
Note: In an earlier post I had originally posted photos of a caterpillar that hatched in my set-up. I thought it was a Papilio canadensis caterpillar becasue I had never seen one for myself, and this one resembled other photos I had seen. That caterpillar has gotten larger now and it clearly is another species. I apologize for the confusion. I have removed the reference in the earlier post and I post 3 pictures here instead for comparison. You can see how it becomes easier to distinguish this species from Papilio canadensis as it grows.