Most terrifying insect in Pa. is exhibiting up at our porch lights


Giant, greyish-brown insects with large translucent wings and ominous “gimmicks” terrorize unsuspecting residents across Pennsylvania and appear at night on porch lights, screen doors, and elsewhere near us.

The monstrous insects are Dobson flies, which are the adult form of the well-known Hellgrammite larvae.

They are endowed with powerful mandibles (jaws), those of the male reaching up to more than 1.5 inches. While the male’s long, thin, crescent-shaped mandibles are most threatening, only the female’s much shorter and less imposing mandibles can cause a painful bite that can draw blood.

Female Dobson fly. (Photo by Andy Reago)

From spring to summer, the larva crawls out of the water, up to 15 meters away from the water, and digs a few centimeters into the ground or under leaf litter or into a rotting tree trunk or tree stump. There it pupates for the next 1 to 14 days before emerging as a winged adult.

As terrifying as it looks, a 5 inch long adult Dobsonian fly only lives a few days, hopefully long enough to avoid the many predators that would eat it and find a partner. The male uses these terrifyingly long mandibles to grab the female by her wings to mate on the ground or in low-lying vegetation.

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The female then lays her eggs in several masses on exposed surfaces such as bridge piers or overhanging vegetation over water or rocks protruding from the water.

The larvae hatch from the eggs in a few days to 2 weeks at night and fall into the water.

The young larvae fall to the bottom of the river, stream, lake, or pond where they cling to sticks or rocks, or crawl under rocks, and grow into the 5 inch long hellgrammite (larva) form known to many bait anglers over the next one to three years .

Light grammite

Hellgrammite at the bottom of a stream. (Photo by Della Ray)

The Hellgrammite is equipped with an ugly set of mouthparts that make it a formidable predator for other aquatic insects and other ground-dwelling creatures, even small fish. The hellgrammite, in turn, is prey for a wide variety of fish and other aquatic predators.

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