Dobsonflies are some of the largest and most impressive insects alive today. Out of many species of dobsonfly, the Giant Dobsonfly of China, India, and Northern Vietnam holds the record for the world’s largest insect. Measuring 21 cm, the Giant Dobsonfly’s (right) wingspan is about the size of a large hand stretched thumb to pinky. Yikes! Many will be relieved to learn that we don’t have such monsters lurking in the forests and wetlands here in Minnesota. Here, the Eastern Dobsonfly measures at about 5 ½ inches, and can be found from Canada to Mexico, East of the continental divide. Though similar, dobsonflies differ slightly from alderflies and fishflies. Dobsonflies and fishflies are both of the Corydalidae family, while alderflies are of the Sailidae family.
Larvae are known as hellgrammites, and are a top invertebrate predator of rocky streams. Measuring up to 4 inches, they can be identified by their dark brown/black color, and line of plates from its lower back to its tail. These plates are armor from larger predators such as fish but also other hellgrammites who’re out to swipe new hunting rocks. Hellgrammites are well known by fishermen, as they make excellent bait for bass, and can sometimes be found for sale at bait shops. Some nicknames it’s received over the years include alligator, toe biter, crawler bottom, and hell diver. Their rugged look and fierce pincers surly help build such daunting reputations, and as the name toe-biter implies, hellgrammites can give a hefty bite. While they’re not poisonous, they’ll let you know you stepped in their turf! Despite being a bit testy, hellgrammite’s goal is more set on protection and hunting other insects like mayflies than giving humans the heebie-jeebies.
The series of spikes found along their abdomen helps them anchor to sand and debris at the stream bottom. Having both gills and pores, hellgrammites can breathe both in and out of water. Because of this they’re sensitive to changes in water chemistry, as any contaminants or drastic changes in the water can easily interfere with its breathing. The ability to breathe out of water comes in handy, as they crawl out of the stream and up to 50 feet onto land to pupate (become adults), digging small chambers in moist spots under rocks, logs, or leaves. They live for 1-3 years as larvae, taking longer to grow in northern climates. It is said that the vibrations of thunder act as a sign for mature larvae to begin their land journey. Perhaps they found that this is generally a safer time to trek onto land, as rain would deter birds, turtles, or frogs from making them into a crunchy snack.
Image: Discover Nature
After 1-4 weeks in their cocoon, adult Dobson flies emerge in late Spring or early Summer only to live for a few days. It’s believed that they don’t eat as adults, as their main focus is to mate and lay eggs. Mating is where adult male Dobsonflies’ large mandibles come in handy – they use them to intimidate other males and for courting females. While these huge mandibles are a frightful sight, males don’t even bite! The mandibles are
unable to get enough leverage to use them for harm. Females however, can bite in the way that hellgrammites do. They mean no harm to humans, and bite only in defense.
Adults are nocturnal, and may gather in large numbers around outdoor lights at night during hatching events. Once mated, they lay their eggs on the undersides of leaves and branches, usually over-hanging a stream. Covered in a white protective material, the eggs are laid in up to 5 layers, each layer containing up to 1,000 eggs. These eggs incubate for 2- 3 weeks, then hatch as young hellgrammites drop to the ground to crawl into the stream. If they’re lucky, they’ll avoid the risk of becoming a snack and drop directly into the stream-voila!
While VLAWMO has more slow-moving water than rocky, rushing streams that dobsonflies are known for, they have been spotted in our watershed. If you find hellgrammites or dobsonflies here in VLAWMO, help us assess the health of our water by letting us know or sending a photo!
Send photos to: WAV@VLAWMO.org