We had an insect trading session in the class I’m TAing this semester, so everyone brought in extra insects they had in their collections to trade for things they didn’t have.  I’m going to discuss some of my observations about the trading session in an upcoming post (I was fascinated!) but today I’m g0ing to focus on the specimen I was most excited about: a live hellgrammite.

Hellgrammites are the larvae of the insect known as the dobsonfly and they are fabulous (or at least I think so).  In their adult form, dobsonflies are pretty gnarly looking.  Males tend to have long, intimidating mouthparts:

Dobsonfly male

Dobsonfly male. Awesome photo by Jessica Lawrence, available at http://bugguide.net/node/view/ 419853/bgimage

Though the mouthparts look scary, they’re really pretty wimpy.  The males of most species can only inflict a minor pinch because the mouthparts are so large they can’t get any leverage on them.  But these giant mouthparts do have a purpose – and, as in most cases where insects have supersized body parts, it all comes down to sex.  Female dobsonflies size up potential mates according to the size of his mouthparts, and in the world of the dobsonfly, bigger is definitely better!  The males with the biggest mouthparts are the sexiest, most desirable males, so some dobsonflies have evolved truly massive ones.

So a male with giant mouthparts mates with a female with more reasonably sized mouthparts to produce eggs.  Those eggs then hatch and these crawl out:

Hellgrammite (Corydalus cornutus)


Now I love hellgrammites and find them completely fascinating.  I am always thrilled to find these in the streams I work in and I can spend hours watching them.  Even so, I’ll be the first to admit that these are some truly vile looking larvae.  They’ve got big, strong mandibles they use to rip apart their prey and they are formidable predators.  They’ve got a pair of hooks on each of two fleshy prolegs on the back end (more about these in a moment) that stick to your fingers or clothes like burrs.  They’re big larvae too.  The hellgrammite in the photo is nearly 3 inches long!  And then there are the long, spindly gills sticking off the sides of the abdomen that give them an alien look.  These do nothing to diminish their threatening appearance and I think it makes them look like big, aquatic centipedes.

But those hooks and gills are also part of why I love hellgrammites.  If you’ve kept up with my blog, you know that my research broadly involves respiratory behaviors of aquatic insects.  Judging from the adaptations hellgrammites display and the habitats they live in, they need a lot of oxygen to survive.  That’s where the hooks and the gills come in: they both help the hellgrammite get as much oxygen from the water as possible.

Let’s consider the hooks for a moment.  If you’re an aquatic animal that requires a lot of oxygen, there is a specific type of water that is best suited to your needs: cold, turbulent, fast flowing streams or rivers.  That’s exactly where you’ll find hellgrammites, clinging to rocks right out in the areas of the strongest flow in cool or cold streams.  However, a giant three-inch long larva, even a flat one like a hellgrammite, is going to have a hard time holding onto the rocks when there’s water slamming into it constantly.  So, they’ve got these:

hellgrammite hooks

Prolegs and paired hooks at the posterior end of a hellgrammite.

Those little hooks grab a hold of the rock so that they aren’t ripped off the substrate and washed downstream.  Hellgrammites are also usually found under big rocks in these fast flowing streams, so the currents they experience are weaker than those on the upper surface of the rock.  Those little hooks aren’t always enough to keep a large hellgrammite in place if they venture out onto the top of the rock.

Hellgrammites are highly adapted for collecting oxygen from the water as well.  If you recall from my post on aquatic insect respiration, insects living in turbulent, cold water maximize their opportunities to collect oxygen from the water.  If they expand their exoskeleton into gills, their surface area increases and they can absorb as much of that relatively abundant oxygen as possible.  Hellgrammites have a lot of extra surface area in their gills.  The feathery looking gills sticking off the sides are rather immobile and simply increase the surface area.  The other set of gills, the puffy dandelion fluff looking ones, have muscles attached to them.  When a hellgrammite become oxygen stressed, it can wave those gills around through the water:

Waving the gills around is a form of ventilation and allows the hellgrammite to extract as much oxygen from the water as possible, especially under less than ideal situations.  The gill movements stir the water around the hellgrammite, pushing deoxygenated water away from the body and bringing oxygen-rich water into contact with the gills so that it may be absorbed.  Behavioral ventilation of this sort is common in aquatic insects and gill movements like this have been recorded in several species, especially within the mayflies.  Still, I can’t help but marvel at just how beautiful the hellgrammite gill movements are!  I hadn’t ever seen this behavior before I noticed it in the insect trading session and I was amazed.  I found it shocking that something that ugly could also have such a stunning characteristic.  It was almost hypnotic watching the hellgrammite pulsing its gills and I could have watched it for hours.

But then I was snapped right out of my gill-inspired reverie when the hellgrammite started to swim around the jar:

I don’t know about anyone else, but I find this sort of abdomen flicking, backwards swimming kinda creepy.  Crayfish do it too and it’s just bizarre.  Doesn’t that look like rather inefficient way to maneuver around your environment?  I can’t easily come up with a reason why this sort of swimming would have developed, though I’m sure there’s a good explanation.

Yep.  Hellgrammites are appalling to look at, but they are amazing in so many ways that I have to love them anyway!  I hope I’ve given you at least a little taste of my appreciation for these monsters of streams and rivers.  I’ll probably describe my plan for making a horror movie called “Hellgrammite!” at some point in the future.  I am sure you are all eagerly looking forward to hearing all about it.  It’s going to be fantastic!  :)


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11 thoughts on “Hellgrammites

  1. Hellgrammites have LOTS of entertainment value. I perform volunteer water quality monitoring on several clear, clean Ozark streams. Took my 30-year-old son-in-law out, hoping for some cheap labor. We took a net sample from a riffle, then spread the kicknet out on a gravel bar, and I instructed him to start picking out “anything moving” for later identification. He had his face VERY close to the net, then moved a bit of leaf litter, and a MONSTER hellgrammite scootched out, seemingly going for his nose. Net went one way, son-in-law another, hellgrammite another. The boy screamed like a little girl. I saved the hellgrammite. Most entertaining.

    • Bwah ha ha ha ha! I can picture it perfectly. So funny! I probably wouldn’t have run, but a big hellgrammite coming for my face would likely have been dropped back in the water. :)

      One of my students got it in his head a few weeks ago that he should try to get the hellgrammite in the video I posted to bite his ear so he could walk around with it as a sort of living earring. He looked at the jaws, then grabbed his earlobe, then looked at the mandibles some more. He eventually put the hellgrammite back without trying it. Probably for the best! I haven’t ever been bitten, but I wouldn’t put it past a hellgrammite to bite hard enough to draw blood. They certainly rip insects apart easily!

    • Wow! Thanks for letting me know! I had no idea there was such a character, but it makes me happy that someone thought hellgrammites were interesting enough to name a baddie after.

  2. I have just finished the 50,000-word (plus) manuscript for my first novel, writing as Wim Grundy for NaNoWriMo: National November Writers’ Month. It takes place in the mythic worlds of Bentho and Nekto in the fantasy universe of Silent Stream. The title? “I, Hellgrammite!” Will keep you posted as the editing goes on. But the words are there!

  3. Pingback: Insects and Plants Use the Same Strategy for Breathing Underwater | The Dragonfly Woman

  4. Great article. Two nights in a row a hellgrammite showed up in the bed, freaking us out. Conniption bug, indeed. First night, brushing the hellgrammite off the neck where it was crawling drew a hard pinch. No blood.
    We now have two hellgrammites to release back into the East Fork of the Chippewa River in northern WI, just down the hill from the cabin. I guess they were looking to pupate, but the bed? INSIDE the mosquito net? Anyway, I can see a white gill bud on the side, not deployed. I thought it was debris or a growth, so I appreciate reading your explanation about the extra gills.
    QUESTION: do the hellgrammites go to pupate near where their parents mated, or is it just random where along the fast flowing river they end up crawling up onto land? We are about 70 ft above the river. Snapping turtles come back to the same spots up here to lay eggs where they themselves hatched, so I thought maybe insects could return to the same places to pupate where their ancestors pupated. We’re hoping not to have hellgrammites making a habit of crawling into bed with us!

    • What an odd experience! Wow. I am not aware of any evidence one way or the other about hellgrammite pupation site choices, so I’m not sure whether they do or not. I would lean toward saying no as most aquatic insects end up further downstream than where they started by the time they pupate, but I really don’t know.

      I don’t think I’d want a hellgrammite crawling into bed with me. Was it raining when it happened by any chance? I’ve heard stories about their abandoning rivers before storms set in, so it makes me wonder if you experienced that mass exodus away from the water they occasionally do.

  5. Do giant darners swarm on the same date every year? I experienced a swarm of hundreds of them on the 1st of july or august many years ago and was wondering if it was replicated by date E G barn swallows?

    • I wouldn’t expect them on the exact same date every year, but they do tend to appear when certain conditions are right. If those conditions repeat every year about the same time, you’re like to see them over and over. July and August is swarm prime time, so be on the lookout in another month or two!

      Incidentally, the same insects that draw in the barn swallows will draw in the dragonflies, leading to swarms. You’ll likely see swifts or swallows flying above a dragonfly swarm!

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