A couple of days ago a reader sent us an amazing picture of a yellow caterpillar with black eyes. (These “eyes” are in fact eyespots, which serve no optical function, but more on this in a moment.) She had found a couple of the yellow caterpillars on her back porch and was wondering if we might be able to help with the task of identification. Although caterpillars can be extremely difficult to identify because of the sheer number of different species, we are reasonably confident our reader found a Spicebush Swallowtail caterpillar, one of the hundreds of species of Swallowtail caterpillars.
First, here is the excellent picture our reader submitted:
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In this picture, it looks as though the caterpillar is looking intently into what we think is a hose. Its big black eyes appear to be gazing into the dark tunnel, perhaps evaluating the prudence of venturing into parts unknown. The only problem with this story is that those big black eyes are not eyes at all. Rather, they are, as mentioned, eyespots, which serve as a defense mechanism and are unrelated to the caterpillar’s vision. We explain this in greater depth elsewhere, like in our article about red caterpillars with eyes, but essentially eyespots are an example of biological mimicry. (The eyespots “mimic” the appearance of actual eyes.) The eyespots make the caterpillar look either dangerous or inedible to their natural predators. Caterpillars with eyespots like the one pictured above look like snakes – the caterpillar’s appearance mimics that of a snake, in other words – thereby deterring predators like birds from attacking them.
Eyesports are a very common defense mechanism among caterpillars, and many of the Swallowtail caterpillars have them. However, we think our reader found a Spicebush Swallowtail caterpillar specifically because in addition to the eyespots, everything about the caterpillar in the image above matches this suggestion. All of the body markings, like the circles on top of its head and the grey dots that run down its back, match the markings of a Spicebush Swallowtail caterpillar. And while Spicebush Swallowtail caterpillars are green for much of their maturation period, their colors can vary as they progress through instars (developmental stages). Right before they pupate, Spicebuch Swallowtail caterpillars can be yellow. So, it appears that our reader found a Spicebush Swallowtail shortly before it begins the process of turning into a butterfly.
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As we conclude, it is worth noting that several Swallowtail caterpillars look similar, and they often change colors as they mature, so it is possible we are mistaking one Swallowtail caterpillar for another. There are over 550 species, so there is certainly room for error. Also, the likelihood of our reader finding any given type of Swallowtail depends on her location, which we were not told. Spicebush Swallowtail live primarily in the eastern United States, and if our reader lives far away from this region, so much the worse for our answer. However, we are fairly confident she found some sort of Swallowtail caterpillar, and she can do some research into the types of Swallowtail caterpillars in her part of the world to confirm or revise our answer.