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:
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:
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:
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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