Introduction

Whether you think of them as humongous, hawkish, and horrifying or hefty, heroic, and hardworking, hellgrammites are anything but humdrum. As members of the aptly named order Megaloptera (i.e., large wings), these impressive aquatic insects ultimately metamorphose during early summer into large, flighted, terrestrial adults. Megaloptera includes alderflies (Family: Sialidae), fishflies and dobsonflies (Family: Corydalidae), but it is the corydalid larvae that are commonly referred to as hellgrammites (Voshell 2002; Bowles and Contreros-Ramos 2019). Ohio is home to at least seven species of hellgrammites (Ohio EPA 2019), but the real titan of the group is Corydalus cornutus (Fig. 1), which is profiled here.

Figure 1. A hellgrammite of the species Corydalus cornutus showing the flattened larval body shape with mandibles held agape and raised for defense.

From hatch to adulthood spanning as many as twelve instars (Bowles and Contreros-Ramos 2019), the life journey of hellgrammites brings remarkable transformations. They undergo a transition from the aquatic to the terrestrial realm, from crawling larvae to flighted adults, from tiny, vulnerable prey to large, fearsome predators, and even a name change to mark their metamorphic progress. The life these creatures live is fascinating from start to finish and worthy of closer inspection.

The Life Aquatic

Hellgrammites of C. cornutus begin life contained within egg masses appropriately deposited above or near water. To an untrained eye, the whitish egg masses appear disguised as bird droppings (Hall 2007) and are affixed to vegetation overhanging a stream or placed on rocks, logs, or walls near a stream (Mangan 1992). The white covering is believed to protect the eggs against overheating and predation (Brown and Fitzpatrick 1978; Mangan 1992). Hatching occurs at night and the tiny, emergent larvae drop into the water. With the assistance of an air bubble, they are conveyed to a desirable stream location (Hall 2007). Upon arrival they will spend from one to five years in this aquatic larval stage, depending on water temperature, latitude, and the time of their hatch (Brown and Fitzpatrick 1978; Bowles 1990; Bowles and Contreros-Ramos 2019).

As they grow larger, the fearsome physical appearance of hellgrammites becomes evident (Figs. 1 and 2). This is likely what spawned the peculiar name of “hellgrammite,” though the origin of the name is a bit uncertain. The first use of the term in print appears to have come in 1866 in an article published in the Spirit of the Times – a popular sportsmen’s newspaper of that era (Univ. Virginia 2014). The name may have originally derived from a combination of Old English words suggesting the creature’s hideous appearance as a goblin, bogey, or haunting spirit (Bugguide.net). Other names have also applied to the larva in the past, including “kill-devil” and “indescribable Barnum-what-is-it-thing” (Univ. Virginia 2014). Modern regional dialects still apply different names, such as the term “devil-scratcher” in the Ozarks (Missouri Dept. Consv.) “toe biter” in Virginia (Univ. Virginia 2014) and “grampus” in West Virginia (Hufford 1999).

Figure 2. Corydalus cornutus showing the impressive hooked mandibles and the characteristic lateral appendages along the abdomen.

In contrast to their colorful array of names, actual hellgrammites are a camouflaged assortment of brown, beige, and black that helps them blend in with the aquatic substrate they inhabit. Although hellgrammites of different species may be found in a variety of aquatic habitats including lakes, ponds, small streams, and larger rivers, C. cornutus (Fig 1.) prefers the fast-flowing, highly oxygenated riffles within streams and rivers (Bowles and Contreros-Ramos 2019). Here they employ their powerful pincers (Fig. 2) to capture prey and defend themselves against intruders. As growth and development continue, hellgrammites may grow up to 90 mm in length (Hall 2007), placing them among the largest invertebrate predators in the stream.

Hellgrammites feed on a variety of other aquatic insects such as free-living caddisfly larvae, black fly larvae, and even other hellgrammites (Hall 2007; Knight and Siegfried 1977). Their voracious appetite contributes to an impressive increase in size from first instar to final instar – up to a 1000x increase in biomass (Barclay et al. 2005). In turn, hellgrammites are preyed upon by larger predators including a variety of fish species. Smallmouth bass seem to have a particular taste for hellgrammites, and the larvae are therefore prized by fishermen as bait (Walsh and Riley 1869; Kirk and Smock 2000; Simonsen and Dombroskie 2008). Consequently, hellgrammites occupy an important position within the freshwater food web.

Sensitivity to pollution also makes hellgrammites valuable as water quality indicators. They are ranked among the most sensitive organisms in the Hilsenhoff Index, a commonly used stream quality assessment method (Hilsenhoff 1988), so their presence in a stream generally indicates clean water. Given that hellgrammites may spend multiple years in the water, they are particularly reliant upon a stable, healthy, aquatic system.

Land of Opportunity

When they are ready to complete their metamorphosis, hellgrammites undertake a risky journey to land, leaving behind the familiar confines of the stream to pupate in the soil under a log or rock. As if in a nod to early childhood memories of hatching at night, older hellgrammites ready for adulthood nocturnally navigate to shore and often do so during a thunderstorm. It is thought that the vibrations from the thunder may trigger the migration (Hall 2007; Univ. Virginia 2014). Their journey conjures images of Andy Dufresne peeling off his old prison garb and emerging from the stream under the cover of a nighttime thunderstorm to enter his new freedom in the Ohio-filmed Shawshank Redemption. Perhaps hellgrammites were subliminally the inspiration for this famous scene in the cinematic classic (Bennett 2019)?

Upon emergence from the pupal stage, hellgrammites become dobsonflies – the terrestrial adult stage equipped with wings and the ability to fly, albeit a weak, awkward form of flight.
Dobsonfly emergence typically occurs in early summer, and the adults tend to remain in the vicinity of water. By day they may be encountered on bridge abutments or damp shower house walls in campgrounds. At night they are attracted to lights, so may be found near lighted parking lots and around campground lanterns and security lights.

Wherever they are encountered, the sight of an adult dobsonfly can be as impressive and fear-inducing as a hellgrammite, since they retain their large size and now possess translucent, membranous wings that enable invasion into human territory on land. The females retain the large mandibles they had as larvae, but the males grow supersized pincers shaped like sickles. Neither use their adult mandibles for acquiring food, however, as adult dobsonflies either do not eat or consume only fruit juices (Brown and Fitzpatrick 1978). Although female dobsonflies can still inflict a painful bite (Hall 2007), the males’ pincers become too large and unwieldy to use as defensive weapons (Fig. 3). Despite the impressive pincers, dobsonflies are not venomous – whether male or female or whether as adults or as larvae. Consequently, they are not the villains their appearance might suggest. Instead, the exaggerated pincers on the male play a role in acquiring a mate, since males will joust and flip each other with them during battle over a female. The elongated pincers also appear to play a role in courtship, as males have been observed resting their pincers perpendicular across a female’s back to inquire about her receptivity (Simonsen et al. 2008).

Figure 3. Preserved adult dobsonfly specimens showing females (left and middle) and male (right). Note the elongated mandibles on the male.

Encountering Hellgrammites

Despite their value as water quality indicators, their use as bait, and their overall curious appearance and life history, hellgrammites and dobsonflies remain understudied (Barclay et al. 2005). They are, consequently, the subject of an ongoing current study by the author and a small team of undergraduate researchers who hope to fill some of the knowledge gaps about these intriguing creatures (Figs. 4, 5).

Figure 4. Measuring hellgrammite head capsule width to assess developmental stage.

Figure 5. Preparing to measure hellgrammite body length to assess developmental stage.

But you don’t have to await the results of our research! The summer months offer prime opportunity to encounter hellgrammites and dobsonflies, so you can conduct informal investigations of your own by having a look under the rocks in a stream or the shower house walls of your favorite streamside campground. Get out and explore and see what you can discover about the fascinating world of hellgrammites and dobsonflies!

References

• • Barclay, A., Portman, R.W., and Hill, P.S.M. 2005. Tracheal gills of the dobsonfly larvae, or hellgrammite Corydalus cornutus L. (Megaloptera: Corydalidae). Journal of the Kansas Entomological Society 78(2): 181-185.
  • Bennett, T. 2019. The Shawshank Redemption at 25: the story behind Andy’s iconic prison escape. IGN. Retrieved June 14, 2023 from https://www.ign.com/articles/2019/12/25/the-shawshank-redemption-andy-prison-escape-break-making-of.
  • Bowles, D.E. 1990. Life history and variability of secondary production estimates for Corydalus cornutus (Megaloptera: Corydalidae) in an Ozark Stream. Journal of Agricultural Entomology. 7(1): 61-70.
  • Bowles, D.E. and Contreras-Ramos, A. 2019. Megaloptera and Aquatic Neuroptera. In R.W. Merritt, K.W. Cummins, and M.B. Berg, An Introduction to the Aquatic Insects of North America (5th ed.). Kendall Hunt Publishing Company.
  • Brown, A.V. and Fitzpatrick, L.C. 1978. Life history and population energetics of the dobson fly, Corydalus cornutus. Ecology. 59(6): 1091-1108. https://doi.org/10.2307/1938225.
  • Bugguide.net. (n.d.) Species Corydalus cornutus – eastern dobsonfly. Retrieved June 13, 2023 from https://bugguide.net/node/view/4873.
  • Hall, D.W. 2007. Featured creatures: common name eastern dobsonfly (adult), hellgrammite (larva). University of Florida Entomology and Nematology. Retrieved June 13, 2023 from https://entnemdept.ufl.edu/creatures/misc/eastern_dobsonfly.htm.
  • Hilsenhoff, William L. 1988. Rapid field assessment of organic pollution with a family-level biotic index. Journal of the North American Benthological Society. 7(1): 65-68. https://doi.org/10.2307/1467832.
  • Hufford, M. 1999. Seining for hellgrammites on Coal River in West Virginia. Folklife Center News. 21(2). https://www.loc.gov/collections/folklife-and-landscape-in-southern-west-virginia/articles-and-essays/seining-for-hellgrammites-on-coal-river-in-west-virginia/.
  • Kirk, D.J., and Smock, L.A. 2000. Interspecific and intraspecific interactions between crayfish (Cambarus longulus) and hellgrammites (Corydalus cornutus) and the influence of a predatory fish (Micropterus dolomieu). The American Midland Naturalist. 144(2): 317-327.
  • Knight, A.W. and Siegfried, C.A. 1977. The distribution of Corydalus cornutus (Linnaeus) and Nigronia serricornis (Say) (Megaloptera: Corydalidae) in Michigan. The Great Lakes Entomologist. 10(2): 39-46.
  • Mangan, B.P. 1992. Oviposition of the dobsonfly (Corydalus cornutus, Megaloptera) on a large river. American Midland Naturalist. 127(2): 348-354. https://doi.org/10.2307/2426541.
  • Missouri Department of Conservation. (n.d.) Eastern dobsonfly. Retrieved June 13, 2023 from https://mdc.mo.gov/discover-nature/field-guide/eastern-dobsonfly.
  • Ohio EPA. 2019. Ohio EPA macroinvertebrate taxa list. Retrieved June 14, 2023 from https://epa.ohio.gov/static/Portals/35/bioassess/MasterTaxaList_123119.pdf?ver=2020-01-07-095148-517.
  • Simonsen, T.J. and Dombroskie, J. 2008. Behavioral observations on the dobsonfly, Corydalus cornutus (Megaloptera: Corydalidae) with photographic evidence of the use of the elongate mandibles in the male. American Entomologist. 54(3): 167-169. https://doi.org/10.1093/ae/54.3.167.
  • University of Virginia. 2014. Eastern dobsonfly larva (hellgrammite). Mountain Lake Biological Station, University of Virginia. Retrieved June 9, 2023 from https://mlbs.virginia.edu/organism/corydalus_cornutus.
  • Voshell, J.R. Jr. 2002. A guide to common freshwater invertebrates of North America. Blacksburg, VA: McDonald and Woodward Publishing Company.
  • Walsh, B.D. and Riley, C.V. 1869. “The Hellgrammite Fly (Corydalus cornutus Linn.)” American Entomologist 1(4): 61-62. Rpt. in American Entomologist by B.D. Walsh and C.V. Riley. 2004. 50(1): 50-51. https://doi.org/10.1093/ae/50.1.50.

About the Author: Dr. Jon Bossley is an Associate Professor of Biology and Coordinator of the Environmental Biology program at Mount Vernon Nazarene University (MVNU), where he teaches several courses including Freshwater Ecology, Principles of Ecology, Field Zoology, two Botany courses, and Tropical Ecology (in Belize). He has also taught Taxonomy and Behavior of Aquatic Invertebrates at OSU’s Stone Lab. He has been a friend, volunteer, and associate at the Triplehorn Insect Collection since 2014.