A reader in Algiers, Algeria wrote to us recently to ask about a “worm” he found in his garden. The worm is in fact a caterpillar, which may be one of the species of swallowtail butterfly caterpillar, which make up the family Papilionidae. With these two sentences, we’ve already supplied our reader with everything he wanted to know (and all we can really give him), but we’ll delve a little bit further into the subject to round out our answer, and also explain our rationale a bit more.

Probably the feature you noticed first about this caterpillar is that it has big black eyes. These actually aren’t eyes, but eyespots, which serve as a defense mechanism and aren’t related to the caterpillar’s vision. By making the caterpillar appear to be something it is not, like a snake, the eyespots can deter predators. Eyespots are relatively common among caterpillars, so one can’t reliably identify a caterpillar simply because it has eyespots. However, we still think there is a good chance our reader found a swallowtail caterpillar because these are common caterpillars with eyespots, and the creature above looks like the swallowtail caterpillars we have identified before. (For instance, our reader can compare what he found to what we believe is a Spicebush Swallowtail caterpillar that another reader asked us about. The Spicebush Swallowtail is a bright yellow color [at least in its final developmental stage], but apart from the color, the parallels between this caterpillar and the one above are obvious.)

If our reader did find a swallowtail caterpillar, this narrows the range of possible species from around 180,000 to 550. (The former is the number of species in the Lepidoptera order, which butterflies and moths make up – caterpillars are just the larval form of moths and butterflies – and the latter is the number swallowtail caterpillar species.) This still leaves 550 possible species, but our swallowtail caterpillar suggestion is about specific as we can get in the present circumstances. We did search a bit for Algerian swallowtail species (e.g., Papilio saharae, Iphiclides feisthamelii, and so on), but we weren’t able to identify anything that looked exactly like what our reader found.

However, in the world of caterpillar identification, naming the family of the creature in question is reasonably precise, and perhaps this degree of specificity is all our reader was looking for. (He did start out thinking he found a worm, so we at least pointed him toward the correct basic category of animal.)

We can’t be entirely sure he found a swallowtail caterpillar, but we think this is the most likely possibility, and hopefully that is all our reader cares to know.

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