A couple of days ago we received a question from a reader in Northern California who found a six-inch “worm-like being with black horizontal stripes and a flat, fan-shaped head.” By “fan-shaped head,” we are assuming our reader means the worm’s head is, first, clearly distinct from the rest of its long, striped body and, two, somewhat flat and spread out, perhaps in half-circle shape. If this is true, we have a pretty good idea what our reader found: a hammerhead worm (sometimes spelled “hammer head worm”).

Technically, we could end our response here, as the reader was only interested in what she found. (Or rather who she found. After describing the worm, she asked “who is this?”, which could be related to her use of the phrase “worm-like being.”) However, we’ll at least explain why we think our reader found a hammerhead worm, and we’ll also link to some of the several articles we have written about this creature in case our reader is interested in learning more about what she (likely) found.

Before anything, though, we should note that although our reader didn’t express any worry about the worm she found, she has at least one reason to be concerned: she found the worm in her garden, and hammerhead worms do not make good additions to gardens. Hammerhead worms aren’t dangerous to humans or pets, and they also don’t damage plants, but they feed on the gardener’s best friend, the earthworm, and this can be harmful to your garden because of the vital role earthworms play in aerating soil. One hammerhead worm isn’t going to destroy a garden obviously, especially because their negative impact is indirect, but they should be removed from the gardens they are found in for the sake of the earthworms. (As an interesting side note, the way that hammerhead worms prey on earthworms is fairly disturbing. They release enzymes onto earthworms, causing them to slowly dissolve, and then the hammerhead worm ingests the liquefied earthworm. More information can be found in the article we already linked to, about the dangers (or lack thereof) of hammerhead worms, or in our overview of hammerhead worms.)

We have one very simple reason for thinking our reader found a hammerhead worm: everything she says is consistent with this suggestion. Although hammerhead worms can reach 20 inches (50 centimeters) in length, a six-inch hammerhead worm is perfectly normal. Moreover, several species of hammerhead worms have long, black horizontal stripes on their bodies, just like the worm our reader found. Finally, and most importantly of all, the worm our reader found evidently has a head shaped more or less like a hammer, the telltale sign of a hammerhead worm. The shape of hammerhead worm heads isn’t always exactly the same, and some are more hammerhead-like than others, but it is certainly reasonable and accurate to describe the worms’ heads as “fan-shaped.” So, we have strong reasons to suspect that our reader found a hammerhead worm, and essentially no reason to think she didn’t.

That said, we can’t be absolutely certain of what our reader found, and this is especially true because she was unable to send in a picture. However, as we said, we have good reasons to think she found a hammerhead worm, and she should probably act on this assumption unless she discovers something that compromises this hypothesis.

• 
•