- Scientific name: Stylurus amnicola
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
- Endangered (MA Endangered Species Act)
Description
Male riverine clubtail
The riverine clubtail is a large, semi-aquatic insect in the order Odonata, suborder Anisoptera (the dragonflies) and family Gomphidae (the clubtails). The clubtails are a large, diverse group of dragonflies named for the lateral swelling at the tip of the abdomen that produces a club-like appearance which varies greatly among species. Clubtails are further distinguished from other dragonflies by their widely separated eyes, wing venation characteristics, and behavior. Many species are very elusive and are thus poorly known. The riverine clubtail is in the genus Stylurus (hanging clubtails) a group characterized by having moderately flared clubs and relatively short legs. They typically perch on the top surface of leaves high in the canopy, oriented in a vertical position. Riverine clubtails are dark brown to black in coloration with pale yellow to greenish markings on the body and green eyes. The thorax is marked with thin, pale yellow or greenish stripes dorsally and broad pale stripe laterally with incomplete dark stripe. The pale thoracic markings are bright yellow in the young adults, but become a dull, grayish-green as the insect matures. The abdomen is black with complete pale stripes dorsally on segments one through seven, triangular spot(s) on segment 8, and large yellow patches on the sides of segments one, two, eight and nine. The face is dull yellowish-green, and the legs are blackish, but with distinct yellowish tibiae on the rear legs. The sexes are similar in appearance, though the females have thicker abdomens, a less developed club, and more extensive lateral pale markings on the abdomen.
Riverine clubtails range in length from 43-49 mm (1.7-1.9 in), with a wingspan averaging about 62 mm (2.4 in). The nymphs average 27.5-29 mm (just over one inch) in length when fully mature.
The riverine clubtail is one of three species in the genus Stylurus in Massachusetts but differs from the other two species in several features. As in most dragonflies, the shapes of the male’s terminal appendages and hamules (located on the underside of the second abdominal segment) and the female’s vulvar lamina (located on the underside of the eighth and ninth abdominal segments) provide the most reliable means of identification. The riverine clubtail can also be distinguished by its smaller size and yellowish tibiae on the rear legs. The arrow clubtail (Stylurus spiniceps) differs further in having an extremely long ninth abdominal segment. The zebra clubtail (Stylurus scudderi) has distinctive yellow abdominal rings. Other similar species in Massachusetts are black-shouldered spinyleg (Dromogomphus spinosus), cobra clubtail (Gomphus vastus), the midland clubtail (G. fraternus), and the skillet clubtail (G. ventricosus). Black-shouldered spinyleg is most similar but possess dorsal pale markings on segments nine and ten. The remaining species differ in having dark tibiae and thicker pale stripes on the top of the thorax, in addition to features of the male terminal appendages and hamules, and the female vulvar lamina.
The nymphs of the riverine clubtail can be distinguished by subtle features given in the keys of Walker (1958), Soltesz (1996), Needham et al. (2000), and Tennessen (2019).
Life cycle and behavior
Note, nymphs are present year-round.
Riverine clubtails are elusive, and little is known about their life history. The egg and nymph life stages are fully aquatic. The nymphs burrow into muddy and sandy substrate and are predators, feeding on a wide variety of aquatic insects, small fish, and tadpoles. Nymphs undergo several molts (instars) for at least 2 years until they are ready to emerge as winged adults. When ready to emerge, the nymphs crawl out onto exposed rocks, emergent vegetation, partially submerged logs, or the steeper sections of riverbanks, where they emerge from the nymphal exoskeleton as adults (a process known as eclosion). Emergence generally takes place very early in the morning, presumably to reduce exposure to predation. As soon as the freshly emerged (teneral) adults are dry and the wings have hardened sufficiently, they fly off to seek refuge in the vegetation of adjacent uplands, leaving their larval exoskeletons behind. These cast exoskeletons, known as exuviae, are identifiable to species and provide a reliable, useful means of determining species presence and viable nymph habitat.
Immature adults spend several days or more feeding and maturing, before returning to their breeding habitats. Riverine clubtails are seldom encountered during this phase of their life; it may be that they spend most of this time high in the canopy. When mature, the males return to the water where they search for females and drive away competing males. Males patrol over the water or shoreline from mid-morning to dusk. They are very wary and, unlike many clubtails, seem to avoid perching on the shore, or on protruding rocks and logs. When not over the water, they apparently spend their time high in the canopy. Females generally appear at water only for a brief period when they are ready to mate and lay eggs. When a male encounters a female, he attempts to grasp the back of her head with claspers located on the end of his abdomen. If the female is receptive, she allows the male to grasp her, then curls the tip of her abdomen upward to connect with the male’s secondary sexual organs located on the underside of the second abdominal segment, thus forming the familiar heart-shaped “wheel” typical of all Odonata — the male above, the female upside down underneath. In this position, the pair flies off to mate, generally hidden high in nearby trees where they are less vulnerable to predators. The duration of mating in riverine clubtails has not been recorded, but in similar-sized odonates typically lasts from several minutes to an hour or more. Females oviposit by flying low over the water, periodically striking the surface with the tips of the abdomen to wash off the eggs. It is not known how long the eggs of riverine clubtails take to develop.
Although most species of Stylurus fly late in the season, riverine clubtails are on the wing from late June through August.
Distribution and abundance
Riverine clubtails range throughout much of eastern North America from Minnesota, Quebec, and southwestern Maine south to Georgia, Louisiana, and Kansas. In addition to Massachusetts, the species has been recorded in New England from Connecticut and southwestern Maine. In Massachusetts, the species occurs in the Connecticut and Merrimack Rivers. Both adults and exuviae have been observed in low abundances at any given site and may reflect their elusive behavior, potentially clustered distribution, and low abundances within a waterbody.
The riverine clubtail is listed as Endangered in Massachusetts. As with all species listed in Massachusetts, individuals of the species are protected from take (picking, collecting, killing, etc.) and sale under the Massachusetts Endangered Species Act.
Distribution in Massachusetts.
1999-2024
Based on records in the Natural Heritage Database.
Habitat
Riverine clubtails inhabit primarily medium to large-sized streams. Substrates are typically mud, sand, and gravel with slack to rapid streamflow. The nymphs are aquatic and burrow into finer substrate like sand. The adults inhabit riparian areas, forested uplands, fields, and fly along shorelines and over rivers.
Healthy habitats are vital for supporting native wildlife and plants. Explore habitats and learn about conservation and restoration in Massachusetts.
Large stream habitat for midland clubtail.
Threats
Degradation of water quality, alteration of natural streamflows, and upland habitat loss are primary threats to riverine clubtail. Potential threats to the water quality of the Connecticut and Merrimack Rivers include industrial and agricultural pollution, sewage overflow, salt and other road contaminant run-off, and siltation from construction or erosion. The alteration of natural flow regimes by damming and water level regulation on riverine clubtail is likely has considerable impact on riverine clubtail. Extensive use of power boats and jet skis that generate large wave action is a serious concern, particularly during the species summer emergence period (as well as several other clubtail species). Amongst other large river species, riverine clubtail ecloses very close to the water surface on exposed riverbank or other substrate that can be subject to sudden wave action generated from boats or rapid changes in water level from hydropower operations.
Conservation
Survey and monitoring
Surveys should target known sites, new waterbody segments, and new waterbodies to update riverine clubtail occupancy and population status in Massachusetts. Targeting exuviae and adults will likely yield increased species detection and should target potential emergence and flight periods respectively. Occupied waterbodies should be routinely monitored every five years or to the extent practical to document changes in occupancy and habitat conditions. Effort should also target potentially suitable habitat in new waterbodies particularly in eastern Massachusetts.
Management
Upland and waterbody habitat protection is critical for the conservation of riverine clubtail. Protection of forested riparian and uplands are critical in maintaining suitable water quality and are critical for feeding, resting, and maturation. Development of these areas should be discouraged, and the preservation of remaining undeveloped uplands should be a priority. Alternatives to commonly applied road salts should tested to minimize freshwater salinization. Altered streamflows and water levels from hydroelectric operations and water withdrawal should be minimized. Rapid changes in water level increase or decrease may cause mortalities during eclosion or become stranded during the nymph life stage. Hardened bank segments should be restored to promote natural sediment dynamics.
Research needs
Continued effort is needed to define habitat requirements, distribution, relative abundance, and potential breeding sites to update the species status in Massachusetts. Estimation of detection and occupancy rates and how other environmental variables (e.g., sample timing, weather) affect these rates is needed. Additional effort is needed to detect responses to changes in streamflow regimes.
References
Dunkle, S.W. 2000. Dragonflies through Binoculars. Oxford University Press.
Lam, E. Dragonflies of North America. Princeton NJ: Princeton University Press, 2024.
Needham, J.G., M.J. Westfall, Jr., and M.L. May. 2000. Dragonflies of North America. Scientific Publishers.
Nikula, B., J.L. Ryan, and M.R. Burne. 2007. A Field Guide to the Dragonflies and Damselflies of Massachusetts. Massachusetts Natural Heritage and Endangered Species Program.
Paulson, D. Dragonflies and Damselflies of the East. Princeton NJ: Princeton University Press, 2011.
Soltesz, K. 1996. Identification Keys to Northeastern Anisoptera Larvae. Center for Conservation and Biodiversity, University of Connecticut.
Walker, E.M. 1958. The Odonata of Canada and Alaska, Vol. II. University of Toronto Press.
Tennessen, K. Dragonfly Nymphs of North America: An identification guide. Springer, 2019.
Contact
| Date published: | April 9, 2025 |
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