Black Caterpillars with Yellow Undersides

black caterpillar with yellow underside

A reader wrote to us via the All About Worms Facebook page recently about some black caterpillars in her backyard. “Some” black caterpillars probably doesn’t quite capture the situation, as the reader reports that they are “all over” her backyard. The black caterpillars are “yellow underneath,” although you can’t really see this in the picture the reader submitted. The reader is wondering what these black caterpillars in her backyard are, if they are even caterpillars.

To begin, let’s take a look at the picture the reader submitted along with her question:


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black caterpillar with yellow underside

Obviously, this shot is taken almost exactly above the caterpillar, so for the most part we can only see the black (or dark grayish) back of the creature, but if you look closely at the right side of the caterpillar, you can partially make out a lighter color, which is perhaps the yellow underside our reader mentioned. If this right, then evidently a significant portion of the caterpillar – almost the bottom half – is yellow, which is helpful for identification purposes; every detail counts.

Technically, the reader never refers to what she found as a caterpillar, so we aren’t even sure of this basic fact, but we don’t know what else it could be. It isn’t a worm, at least not in the way that we use the word “worm,” and it doesn’t appear to be any of the various types of non-caterpillar larvae we write about all the time. (We say “non-caterpillars larvae” because caterpillars are in fact larvae – they are the immature form of moths and butterflies.) It also doesn’t appear to be one of the “worm like” creatures we write about from time to time, like centipedes, millipedes, slugs, or leeches. And finally, it looks like a caterpillar, with its multicolored, segmented body. So, we are reasonably sure our reader found some sort of caterpillar, but which kind?

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Unfortunately, this is an extremely difficult question to answer. There are around 175,000 caterpillar species in the world, and identifying any given one simply by looking at a picture of its back is a tall order. The task is made more difficult by the fact that we have no additional information about this creature. We have no idea where these caterpillars were found – geography is crucial for caterpillar identification – and we also don’t know in what part of the yard they were found. Were they on a particular type of plant or tree, for instance? Caterpillars tend to be picky eaters, so you can often ID a caterpillar by its diet.

This last point is particularly relevant to the present case, as one of our immediate thoughts upon viewing our reader’s picture is that she found Catawba worms, which are also called “Catalpa worm.” Despite their common name, they are not actually worms, but caterpillars, the larval form of the Catalpa Sphinx moth, more precisely. Catawba worms feed on Catawba trees (which are also called “Catalpa trees”), and hundreds of them can infest a single tree. Generally, people with Catawba trees welcome the infestation because Catawba worms makes excellent fish bait, and the tree owners can be become quite distressed when the caterpillars leave the Catalpa trees. Not knowing this (or perhaps not caring about this), the appearance of hundreds of caterpillars can be concerning, and this seems to have been the reaction of our reader. Since Catawba worms are often a dark or black color (at least in their later developmental stages) with green or yellow undersides, their appearance matches what our reader found. However, this possibility entirely depends on if our reader has Catawba trees in her yard. If she doesn’t, she isn’t finding Catawba worms.

So, unfortunately, we don’t have enough information to definitively identify what type of caterpillar our reader found (assuming it is a caterpillar, which it probably is). She might have found Catawba worms, and this is the most plausible suggestion we can offer, but we are far from certain this is correct. We recommend that our reader look into the plants/trees in her yard to see if this yields any clues, and she might also look into common caterpillars in her region of the world, wherever that may be.

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Black Caterpillars with Yellow Undersides
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Black Caterpillars with Yellow Undersides
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A reader wrote to us via the All About Worms Facebook page recently about some black caterpillars in her backyard.
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2 thoughts on “Black Caterpillars with Yellow Undersides

  1. Hi it would appear that I found exactly the same kind of worm. Unfortunately I did not think of taking photographs as I thought it was a common worm/caterpillar, but I was rather taken up by it. It looks like the bark of a tree, lighter on the sides and when provoked pops up yellow like eyes, curls up then the feet underneath are yellow – as if it has socks on and has pinchers on its rear, which sting causing instant swelling like a bee sting. I put it back in a tree, I shouldn’t have as it seems this is a once in a life time experience. This kind of caterpillar does not appear to be common. Has anyone got any other information and a name for this caterpillar – its body is in segments.

  2. I am the one who asked the question. I live on Fort Campbell military post. The Kentucky Tennessee State line runs through post so I technically live on the TN side. My back yard is tiny and we do back up to woods. Back when I wrote you, I took the best picture I could without casting a shadow and as small as they were it was hard to get that one. When I tried to flip them to take a picture of the undersides for you they would curl in a ball. I asked about this bug because it was pretty much an infestation and wanted to know if I should have housing come spay my yard or not. I figured they may be a type of caterpillar from the research I did and they would eventually leave so I left them alone and they eventually did.

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