Eastern Dobsonfly Vol. 9, No. 24
Despite their drab brown color, male eastern dobsonflies are the kind of insects that can stop people in their tracks--just to get a better look. Adults can be up to three inches long and the curved, oversized mandibles of the males add another inch or more. Those sickle-like mandibles may look intimidating, but they are not able to inflict much of a bite. Females, on the other hand, have short, stout mandibles, and can give a painful, even skin-breaking, nip if carelessly handled. Neither males nor females eat as adults. Males use their mandibles for jousting with rivals and possibly for impressing females, while females use theirs for self-defense.
With a wingspan of up to 5 inches, both sexes can fly, but their wings are not designed for quick, maneuverable flight. Dobsonfly flight is more of a slow, fluttering event and happens mostly at night. Where do they fly from? Water. Dobsonflies are one of the largest aquatic insects in the country. Most dobsonflies breed in shallow, rocky bottomed streams, and this is the habitat where they are most common, but eastern dobsonflies sometimes breed in well-oxygenated lakes and ponds. Dobsonflies are attracted to outdoor lights, and this is where adults are most often encountered.
Like moths and butterflies, dobsonflies have a complete life cycle. The larvae, known as hellgrammites, take one to three years or more to develop. Their bodies are elongate, beginning with a large head, a thorax with six legs, and a long, segmented abdomen with a pair of pointed, sensory structures protruding laterally from each section. There is also a pair of strong, hooked prolegs at the end of the abdomen, which they use to anchor themselves to the bottom of the stream, usually hiding under the edge of a rock, where they can snag passing prey. Hellgrammites are strictly predatory. They have big, heavy mandibles of their own, which they use to catch prey such as other aquatic insects, worms, and snails.
Fully-grown hellgrammites can be up to 3.5 inches long. These aquatic larvae breath through feathery gills, which can absorb oxygen from the water, as well as spiracles, which can take in oxygen from the air. When they have completed larval development, hellgrammites leave the water to pupate in a protected place above water. Adults mate soon after emerging, and females deposit white, coin-sized egg masses on leaves or rocks that overhang water, allowing the young hellgrammites to simply drop into the water upon hatching.
In states where they have trout and smallmouth bass, hellgrammites are highly prized as fish bait, and some anglers may devote as much time to catching hellgrammites as they do fishing. Hellgrammites are such good fish bait they are also mimicked by artificial lures and flies.
Dobsonflies are a key indicator species for stream water quality. Only healthy, unpolluted streams support good populations of dobsonflies and the other aquatic invertebrates on which they feed.
Thanks to Beth Hill for sharing this photo of a male dobsonfly resting on a flag.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.
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