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Identification of Riverine Clubtail (Stylurus amnicola)

HABITAT: Riverine Clubtails (Stylurus amnicola) inhabit primarily medium to large rivers.

DESCRIPTION: The Riverine Clubtail is a large, semi-aquatic insect in the order Odonata, suborder Anisoptera (the dragonflies). Like all dragonflies, the Riverine Clubtail has a long, slender abdomen comprised of 10 segments, four wings (two forewings and two hindwings) with dense venation, and a large head with huge eyes and powerful, chewing mouth parts. It is a member of the family Gomphidae (the clubtails), a large, diverse group comprising nearly one hundred species in North America. Clubtails are named for the lateral swelling at the tip of the abdomen (the seventh through ninth segments) that produces a club-like appearance. The extent of this swelling varies greatly, from extreme to non-existent, depending upon the species. The club is generally more pronounced in males than females. The purpose of the club is uncertain, but it may be used for displays, or it may provide some aerodynamic benefits to the males. Clubtails are further distinguished from other dragonflies by their widely separated eyes, wing venation characteristics, and behavior. Many species are very elusive and thus poorly known.

The Riverine Clubtail is in the genus Stylurus, sometimes referred to as the “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 tree tops, oriented in a more or less vertical position. Riverine Clubtails are dark brown to black in coloration with pale yellow to greenish markings on the body and bright green eyes. The top of the thorax is marked with thin, pale yellow or greenish stripes. The sides of the thorax are mostly pale with narrow dark markings. 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 small, yellow spots on the dorsal surface of segments one through eight, 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 and a less developed club.

Riverine Clubtails range in length from 1.7 to 1.9 inches (43 mm - 49 mm), with a wingspan averaging about 2. 4 inches (62 mm).

SIMILAR SPECIES: The Riverine Clubtail is one of three species in the genus Stylurus in Massachusetts, but differs from the other two species in a number of 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 the Cobra Clubtail (Gomphus vastus), the Midland Clubtail (G. fraternus), and the Skillet Clubtail (G. ventricosus). All 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 and exuvia of the Riverine Clubtail can be distinguished by subtle features given in the keys of Walker (1958), Soltesz (1996), and Needham et al. (2000) .The nymphs average just over one inch in length (27.5 mm - 29 mm) when fully mature.

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