Dobsonflies and Hellgrammites: Bites, Size and Life Cycle

Dobsonflies are one of those insects that send photos to extension offices every summer. They are big, slow and alarming at a porch light, and almost entirely harmless if you leave them alone. Here is how to tell which ones can nip, what they do, and where in the US you are most likely to meet one.

Which dobsonfly bites? The quick answer

What is a dobsonfly?

Dobsonflies belong to the order Megaloptera, “large wings”, in the family Corydalidae, which also includes the smaller fishflies. The Missouri Department of Conservation (MDC) describes Corydalus cornutus as the only dobsonfly species in eastern North America, and UF/IFAS notes three further Corydalus species in the western US. Mississippi State University Extension calls dobsonflies one of the largest aquatic insects in the country, and UF/IFAS describes the eastern dobsonfly as one of our largest insects outside the moths and butterflies.

What does a dobsonfly look like?

• Colour: drab grey, black or brown, soft-bodied (MDC).
• Wings: long, smoky and densely veined, with tiny white spots, folded along the length of the body at rest (University of Minnesota Extension, MDC, University of Maine).
• Head: large, with long many-segmented antennae and prominent jaws. Males carry the long curved tusks; females have short, stout pincers.
• Behaviour when disturbed: both sexes raise the head and spread the jaws, and University of Maine Extension notes a foul-smelling defensive anal spray used as a last resort.

How big is a dobsonfly?

Big, but the published figures vary with what is measured. University of Minnesota Extension gives 1 1/2 to 3 inches (38 to 76 mm). Mississippi State gives up to 3 inches, with the male’s mandibles adding an inch or more, and a wingspan of up to 5 inches. MDC says adults can exceed 3 1/2 inches, with a wingspan of up to about 5 inches. UF/IFAS gives 100 to 140 mm (4 to 5.5 inches), and University of Maine says both sexes can reach up to five inches. The sources do not always say whether the jaws or wings are included, which probably explains much of the spread.

Compiled by The Pest Control Brief from the sources named. Where they disagree we show both rather than average them.

Are Dobson flies harmless?

To people, very nearly. Can a dobsonfly hurt you? Only if you handle a female or a hellgrammite. Do dobsonflies bite humans? Not unprovoked: MDC describes them as not venomous and says the worst they can do is pinch you if molested. Do eastern dobsonfly males bite? They may try, but UF/IFAS states the male is incapable of inflicting a painful bite, and Maine Extension explains why: the jaws are so long they give weak leverage and cannot break skin. The male uses them in jousting with rival males and in courtship, placing them across the female’s wings.

They are not pests in any sense. Adults are not known to feed in the wild, the larvae live only in streams, so they cannot breed indoors, and none of our sources links them to any disease. UF/IFAS describes dobsonflies as beneficial insects that should be conserved.

What happens if a dobsonfly bites you?

You get a sharp pinch. UF/IFAS says the female is capable of drawing blood, and Maine Extension describes a painful but not venomous bite. Mississippi State calls it a painful, even skin-breaking, nip if the insect is carelessly handled. There is no venom, so the effect is mechanical: a small puncture or scrape. If a bite breaks the skin and you are concerned about it, ask a doctor or pharmacist.

Hellgrammites pinch harder for their size. MDC notes the larva’s sharp pincers can deliver a painful bite and advises anglers to grasp them behind the head. Ozark anglers call them “devil scratchers”, MDC adds.

What does a dobsonfly do?

Almost all of its life is spent underwater as a predator. UF/IFAS calls hellgrammites the top invertebrate predators in rocky streams where they occur, feeding on soft-bodied insects such as caddisfly and blackfly larvae, and MDC says they will snatch nearly anything that swims or ambles by, including small fish. Larvae moult 10 to 12 times over one to three years (UF/IFAS); MDC says two or three years and Mississippi State says one to three years or more.

Then the larva leaves the water, often all within a few days at a given site, and digs a pupal cell under a rock or log, sometimes 15 m (49 ft) or more from the stream. Along Virginia rivers, UF/IFAS records, residents say thunderstorms trigger this “hellgrammite crawling”. The adult that emerges lives briefly: about three days for males and eight to ten days for females, according to UF/IFAS. It is generally believed not to feed in nature, although Parfin reported in 1952 that captive adults took honey water.

Females lay white, chalky egg masses about 2 cm (3/4 in) across on surfaces overhanging water, including leaves, tree trunks and bridge abutments. Each holds about 1,000 eggs, and UF/IFAS notes the masses look like large bird droppings. The eggs hatch at night after one to two weeks, and the young larvae drop or float into the water.

A sign of a healthy stream

Mississippi State calls dobsonflies a key indicator species for stream water quality, because only healthy, unpolluted streams support good populations of them and their prey. UF/IFAS notes hellgrammites tend to be found in relatively unpolluted water and are prized as bait, particularly for smallmouth bass, so their collection for sale is regulated in some states. Mayflies send the same signal, as we explain in our guide to mayflies.

Where do dobsonflies live in the US?

Along flowing streams east of the Continental Divide. UF/IFAS gives the eastern dobsonfly’s range as most of eastern North America east of the Continental Divide from Canada to Mexico, and MDC describes it as statewide in Missouri. Our state analysis matches: in iNaturalist research-grade data, the eastern dobsonfly is recorded most often, relative to all observations, in West Virginia (13.4 records per 10,000 observations), Alabama (8.02) and Virginia (6.84), with no records in California, Arizona or Colorado.

The top of the list is dominated by Appalachian and Ozark states, with the fast, rocky streams MDC and Mississippi State describe as prime habitat. Florida, at the southern edge of the range, sits at just 0.46 records per 10,000 observations, although UF/IFAS photographed hellgrammite habitat on the Santa Fe River.

Is it rare to see a dobsonfly?

Uncommon, but not rare. MDC says the adults are rarely seen because they are short-lived and nocturnal, and UF/IFAS says they spend most of their time in thick vegetation near streams. Yet iNaturalist users have logged 18,869 US research-grade records of the eastern dobsonfly, somewhat more than the American cockroach (15,683). That says more about how photogenic a male dobsonfly is than about true numbers, but it shows they are found every summer near suitable streams.

Timing is the main reason people meet one. University of Minnesota Extension lists them July and August, while Maine Extension says late spring to mid-summer and MDC late spring and early summer. Nationally, iNaturalist records peak sharply: 27.4 per cent of all eastern dobsonfly records fall in June and 39.3 per cent in July, two-thirds of the year’s total in two months.

Dobsonfly look-alikes

The giant water bug is the one most often confused with a cockroach at a porch light; our guide to bugs that look like cockroaches sorts it from the Oriental cockroach. Some people think hellgrammites look like centipedes, MDC notes; our page on house centipedes covers the indoor kind.

Dobsonflies at your porch light: what to do

MDC, Maine and Mississippi State all note that dobsonflies are attracted to lights, and that is where most people find them. No control is needed, and none of the sources recommends any.

1. Leave it alone. An adult will be gone within days. If it must move, slide a jar or box over it and a stiff card underneath, and release it near vegetation.
2. Do not pick up females. Short jaws mean a female, and she can draw blood.
3. Turn off or change outdoor lights near rivers in June and July. Warm white LED and yellow bulbs caught the fewest insects in the porch bulb trial we cover in why moths are attracted to light.
4. Do not spray. Spraying a short-lived adult achieves nothing, and insecticide used near streams risks harming the aquatic life that dobsonflies depend on.

Frequently asked questions

Do dobsonflies bite humans?

Only in self-defence, and only females and larvae can make it hurt. UF/IFAS says the female’s short, powerful jaws can draw blood, while the male’s long tusks cannot inflict a painful bite. Hellgrammites, the larvae, pinch hard if handled. Missouri’s conservation department notes dobsonflies are not venomous and the worst they can do is pinch. Leave them alone and you will not be bitten.

How long do dobsonflies live?

Most of their lives are spent underwater. Hellgrammites take one to three years to develop, according to UF/IFAS, with Missouri giving two or three years. After one to four weeks as a prepupa and pupa in a cell near the stream, the winged adult lives only about three days if male and eight to ten days if female, and is generally believed not to feed.

What are dobsonfly eggs?

White, chalky patches about 2 cm (3/4 in) across, often on leaves, branches, tree trunks or bridge abutments over a stream. UF/IFAS describes each as holding about 1,000 eggs in three layers, coated in a fluid that dries white and makes the mass resemble a large bird dropping. The eggs hatch at night after one to two weeks and the larvae drop into the water.

Why do dobsonflies come to lights?

Adults are nocturnal and strongly attracted to lights, UF/IFAS notes, so a lit porch or car park near a river draws them in during June and July. Dimming or switching off lights near water reduces visits; the reasons insects fly to lights are explained in our guide on moths and light.

Sources

• University of Florida IFAS Extension, Featured Creatures EENY 414: Eastern Dobsonfly (Adult), Hellgrammite (Larva), Corydalus cornutus (Hall; first published 2007, revised 2025).
• Missouri Department of Conservation, Field Guide: Eastern Dobsonfly.
• Mississippi State University Extension, Bug’s Eye View: Eastern Dobsonfly, Vol. 9, No. 24 (Layton, 2023).
• University of Maine Cooperative Extension, Home and Garden IPM: Dobsonflies; Giant Water Bug.
• University of Minnesota Extension, What’s that insect: Dobsonfly; Annual cicadas; Cicada killer; Polyphemus moth.
• Parfin S. Notes on the bionomics of Corydalus cornutus. American Midland Naturalist 47, 1952 (as cited by UF/IFAS).
• Justice MJ and Justice TC, household bulb light-trap comparison, 2021 preprint (as summarised in our moth guide).
• The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.