Cornelius’ Corner: “Hello, Hellgrammites!”

Hellgrammite found at the Frog Pond

Hi friends!

I have really exciting news to share with you all! A couple of weeks ago, my friends from Thornbury Preschool found a really cool hellgrammite living in the Frog Pond outdoor classroom when they came to visit for a field trip. Why is this bug so exciting? Because none of my coworkers had ever seen that kind of insect at the Frog Pond before! They had been seen in the West Branch of Chester Creek, but never at the Frog Pond.

Some of my friends were a little nervous about the insect, because it does look pretty scary if you’ve never seen one before. Hellgrammites are the aquatic larvae of dobsonflies and have long, somewhat flattened bodies. They have a head with strong mandibles, three pairs of legs with pinchers on each leg, and 6-8 straight filaments that may act as gills or be involved in their sense of touch.

You can typically find hellgrammites in fast-moving, well-oxygenated waters like streams, creeks, and rivers, which is why it was so interesting to see one in the slower-moving Frog Pond. They are nocturnal, and wait for dark to ambush their prey using their strong legs and mandibles. They aren’t picky about what they catch either – pretty much anything moving in the water is fair game, including other aquatic insects and small fish!

Eastern Dobsonfly

Their lifecycle is interesting because of how much time they spend as larvae compared to their adult life stage. Adult dobsonflies lay their eggs on sticks or rocks hanging over the surface of a body of water. The eggs are covered with a chalk-like covering that keeps them from drying out before the larvae hatch. Once hatched, the larvae drop into the creek where they will spend typically 1-3 years, although they can stay in this stage for up to 5 years.

In late spring, the larvae will emerge from the creek to burrow into muddy banks or under rocks and logs. This is often synchronized, with thousands of hellgrammites emerging at the same time! This strategy keeps all the larvae from being eaten by predators – with that many larvae around, the predators get full quickly and leave the rest alone.

After burrowing into the bank, the larvae undergo metamorphosis to become a pupa and stay buried for 3 to 6 weeks. Eventually, the adults emerge in July and August as fully adult dobsonflies that are up to 4 inches long with even longer wings. The males also have long, curved pinchers that scare a lot of humans! They shouldn’t though, because the pinchers aren’t for biting – they are for grappling with other males during mating.

Because they don’t eat in their adult stage, they are not around for very time. The females can live for around 10 days but the males are only seen for 3 days! In that short period of time, however, they are able to fly very long distances to find other adult dobsonflies. Unfortunately, all of that travel puts them at higher risk of being eaten by predators. Trout and bass especially love to eat both the hellgrammite larvae and their adult dobsonfly counterparts.

With all of the predators trying to eat them, it can be hard to spot a hellgrammite or dobsonfly in the wild. If you do see one, be sure to stop by my tank in the Visitor Center to tell me all about it!

Your friendly neighborhood corn snake,

– Cornelius