Hellgrammites (Gesundheit!)

Is this not one of the most terrifying things you’ve ever seen?

Image source: Melissa Deluke

It’s not an alien from another planet or an imaginary creature from a science fiction flick, it’s a real live creature…lurking in your local waterways…

These critters are called hellgrammites, and they’re actually juvenile Dobsonflies (Corydalinae).

Female eastern Dobsonfly via Wiki, by SheepNotGoats

Female adult Dobsonfly via Wiki, by SheepNotGoats

This is an adult Eastern Dobsonfly (Corydalus cornutusand even though, at five inches long, they look rather terrifying, I grew up with these insects and never had any bad experiences with them. I’ve read that the females are capable of biting humans, whereas the males’ freakishly large mandibles are essentially useless against hominid flesh.

Male adult Dobsonfly. Credit: T. Duckworth

Dobsonflies spend the vast majority of their lives in their larval form, the hellgrammite, and only up to a couple of weeks as adults. Hellgrammites are ferocious, predacious little creatures, reaching up to four inches. Like other predacious aquatic macroinvertebrates, they’ll eat pretty much anything they can get their little jaws on. They’ve also earned the nickname “Toe-biters” for this reason.

Misidentification of aqautic macroinvert species is pretty easy in some cases, and I’ve uncovered some photos of hellgrammites that don’t quite match the one above. Hellgrammites have only six real legs and eight feathery appendages along the rest of their bodies, where the gills hang out (literally). I’m not sure why the appendages in some photos are so much longer than in other photos – anybody out there know? I’m guessing it’s just individual variation, or has something to do with their long lifespans and molting.

Let’s take a look:

I think here the appendages are folded back. What do you think? Credit: USGS

Gills don’t seem to be visible here. Credit: Fly Fishing Discounters

I tried to find some info on benthos.org but their search engine is not terribly handy at locating images. Credit: http://www.benthos.org

Here we can see shorter appendages and the gill tufts! Credit: cals.ncsu.edu

Hellgrammites live in bodies of water beneath stones and other debris. They prefer well-oxygenated water like that of a quick-moving stream, and are listed by the Maryland Department of Natural Resources as a “sensitive” benthic organism. This means that finding a hellgrammite is an indicator that the stream you’re in is relatively healthy, since hellgrammites are pretty intolerant of pollution. The world of aquatic macroinvertebrates is one of my favorites to explore with kids because most people don’t realize the wonderful world that exists beneath the surface of a stream. A variety of insects that have adult lives above the water start out submerged, attaching themselves to rocks in fast-moving riffles, or even building mobile homes out of tiny pebbles and spit. In the spring, macroinverts are numerous, and the REALLY cool thing is that you can almost instantly gauge the health of a stream based on which species are found there. Clicking on that link for the Maryland DNR will take you to a very cool PDF that breaks down macroinverts into 3 or 4 basic groups categorized around how sensitive to pollution they are.

Hellgrammites are a spectacular find for trout fishermen, despite their reputations. Apparently the trout see a fun-size candybar where we see a startling little predator! Next spring, consider joining a local chapter of stream health surveyors and you can get to experience this tiny, incredible alien world. For a few hours’ worth of volunteering, you can slosh through a stream in waders with a dip net! Okay, maybe that just sounds awesome to me, but I speak from experience when I say it’s a good time. :) Thanks for reading!


Posted on November 4, 2010, in Fauna and tagged , , , , , , , , , , , , , , . Bookmark the permalink. 9 Comments.

  1. I totally agree. A sloshingly good time!

    • I have seen pix of the egg clusters, but when do the eggs hatch? In my research, i have found almost nothing but anecdotal tales of these benthic beauties! Thanks for this post. Oh and by the way… where are the gill tufts on your pictured specimen? Is this Corydalus?

      • Hi Wim! Thanks for stopping by. :) According to most online sources that I could find, the eggs hatch approximately two weeks after being laid, and are laid from May through August. Which means at as early as the first part of spring and as late as the first part of fall. Apparently when the eggs hatch (they’re laid by the female on the undersides of leaves in clusters that look like bird poop – great camo!) the little critters drop into the water and start hunting out a good spot to hide – under rocks, logs, and other debris.

        http://entnemdept.ufl.edu/creatures/misc/eastern_dobsonfly01.htm (egg mass photo!)

        I’ve added a section to the post with more photos of hellgrammites, and in most photos the tufts don’t seem to be visible, including the first one. I imagine when they’re taken out of the water (as many photos feature the critters outside of their natural habitat), the gills probably contract and close up. They’re visible in the last photo I threw in there. Hope that helps! :)

        If you haven’t yet, consider joining a local stream survey chapter – they’re a great deal of fun and you’re almost guaranteed to get to see one in real life!

  2. Thanks for the ammo and encouragement. Info I can use and hope to someday view these larvae in action – at night – with low-light intensifier/macro-video remotes!

  3. I was wondering how I can keep my hellgrammites alive? I have heard water, but then I heard no…just use moss that is cold and wet and maybe burlap…please help

    • I’ve never kept them Darren, but I know that in their natural habitat, they often live in fast-moving freshwater streams, and that’s probably not possible to recreate at home.

  4. I am finding them inside my house! I found 7 last night in my hallway and trapped them between my dustpan and broom before my small indoor pets could get hurt. After each I thought surely there wouldn’t anymore. Then I would check the hallway and there would be another one crawling around. Tonight I went in my bedroom to turn down my bed and there was one on the carpet in the middle of my bedroom floor. How would they be getting in, and how can I stop them from getting in?

    Lyn McCoy
    Buchanan County
    Southwest Virginia

  1. Pingback: Hellgrammite Nymph Fly Fishing Fly Tying | Ultimate Fishing and Hunting Blog

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