Dobsonfly

The hellgrammite

The dobsonfly larva is called a hellgrammite — a name reflecting the reaction most people have on first encounter. The larva is 5–8 cm long, dark brown, flattened, with lateral gills along the abdomen and two pairs of hooked appendages at the tail. When handled, it bites — hard.

Hellgrammites live for 2–3 years under rocks in fast-moving, clean streams, predating other aquatic invertebrates. Anglers prize them as bait for bass and other fish; finding hellgrammites in a stream is also a positive indicator of water quality — they require well-oxygenated, unpolluted water.

The male’s jaws

Male dobsonflies have spectacular curved mandibles that can span 3–4 cm — longer than the rest of the head — giving the insect a terrifying appearance. Despite their size, the jaws are too large to deliver a dangerous bite to humans (the muscles lack leverage at full extension). Females have much shorter, stronger mandibles and can deliver a painful bite.

The male’s jaws are used primarily in mating competition — males grip and wrestle rivals with them.

A brief adult life

Adult dobsonflies do not feed — they have no functional digestive system. Their entire adult existence lasts 3–7 days, spent finding a mate, copulating, and for females, laying eggs. A female may lay 1,000–3,000 eggs on overhanging vegetation above streams, usually in a mass covered with a white chalky substance.

A clean water indicator

Because hellgrammites require high water quality, their presence is used by biologists as a bioindicator of stream health. An abundance of hellgrammites indicates clean, well-oxygenated water with good structural habitat. Their disappearance signals deteriorating water quality before chemical tests may register a change.