Today a fellow naturalist came across a Black Swallowtail (Papilio polyxenes) caterpillar.
Though still in the Papilio genus, the caterpillars of this species do not have eyespots. Instead they are considered to be aposematic.
Aside: Aposematism is a phenomenon where prey advertize their unpalatability to their potential predators through bright or conspicuous warning colours. Predators learn quickly to avoid consuming prey with colour patterns associated with a toxic or bad tasting meal.
Black Swallowtail caterpillars lay their eggs on a narrow leaved herbaceous dicot called Queen Anne's Lace (Daucus carota), also called Wild Carrot. Most people in Ontario have probably seen this plant:
As you can see, Queen Anne's Lace is a narrow-leaved plant. It has been suggested that prey are likely to evolve aposematism when they live in environments where it is difficult to hide. Prey that can't hide from predators you need something to deter them from attacking, unpalatability is a common solution in nature. Also, there should be a minimal cost to having conspicuous colour patterns for prey living in environments where they can't hide well anyway. There was an amazing paper published in 2007 demonstrating that this seems to explain the evolution of aposematism in Papilio caterpillars very well:
Prudic KL, Oliver JC, Sperling FAH (2007) Signal environment is more important than diet or chemical specialization in the evolution of warning colouration. Proceedings of the National Academy of Sciences 104:19381-19386
The colour pattern of Papilio polyxenes has also been suggested to be cryptic when viewed at a distance, but aposematic when viewed closely. This could give provide an ideal strategy for these caterpillars because some birds may consume these caterpillars despite their unpalatability and some predators may even target aposematic prey when other food resources are low. Also, if predators do need to learn to avoid these prey by eating one, being slightly cryptic could help reduce the number of attacks by naive predators.