Black Worm or Caterpillar with Red Dots

We received an interesting and faintly poetic question the other day about a black “worm” that has a red dot on each body segment. Here is her entire question: “Middle of May, Northern California, Black worm about two inches long, seems hairless has what appears to be thin covering of bristles, red dots on each segment, racing across my deck floor. Please identify.” Unfortunately, no picture was submitted along with the question, so this description is all we have to work with. Obviously, this limits our ability to offer a confident suggestion, but we’ll do what we can.

As is so often the case when identifying creatures, the reader has provided us with information that takes us in different directions. We are reasonably confident our reader is describing some sort of caterpillar, but the fact that it was seen “racing across the deck” calls this into question. Caterpillars are fairly slow, with their bodies inching along wavelikely, to use a completely absurd and unnecessary adverb. It would therefore be unusual to describe any caterpillar as “racing.” However, everything else about this creature – from its appearance to the time of year it was found – seems like a caterpillar, and perhaps we are interpreting the word “racing” with too great an emphasis on speed, so we’ll have to stick with this assumption. And if our reader did find a caterpillar, it is of course not a worm.

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As for what kind of caterpillar our reader might have found, it is of course much harder to say, particularly because we don’t have a picture to work with. We can’t simply identify any of the 175,000 species of caterpillar just by looking at a picture, at least not usually, but it gives us something to check possibilities against. We’ve managed to find a couple of caterpillars that kind of look like whatever it is our reader is describing, including Papilio polyxenes (Black Swallowtail) and Battus philenor (Pipevine Swallowtail), but it’s impossible to say if these look anything like what she found without a picture. Our reader should perhaps look into these two options to see if either suggestion holds any promise. If the reader does this, it is worth noting that the appearance of caterpillars can change fairly dramatically over the course of their life, so not every picture of Black Swallowtail and Pipevine Swallowtail caterpillars will be black with red dots. (The former are actually predominately green during certain stages in their development.) Also, both of the possible matches we came up with are swallowtail butterfly caterpillars, which together make up the family Papilionidae, so perhaps this is a promising group for our reader to look into if we’re off the mark.

We wish we could offer more definite information, but unfortunately we’ve said all we can in this instance. Hopefully we’ve helped in some way, and have at least pointed our reader in the right direction.

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Black Worm or Caterpillar with Red Dots
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Black Worm or Caterpillar with Red Dots
We received an interesting and faintly poetic question the other day about a black "worm" that has a red dot on each body segment.

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