The eggs of Stenacron interpunctatum have attachment structures called polar caps, which are located at both ends of each egg. The presence of these polar caps is an important difference between S. interpunctatum eggs and the eggs of related mayfly species.
In general, mayflies are unique among other aquatic insects because of the presence of wing pads on the thorax, three pairs of segmented legs, gills on the abdomen, and three (sometimes two) thin tail filaments. In S. interpunctatum nymphs, the legs have dark stripes. Early-instar nymphs are identifiable by the presence of gills. Stenacron interpunctatum differs from related species in that its gills are pointed. Later-instar nymphs develop wing pads, which continue developing as the insects molt. Dark wing pads indicate that a nymph will soon emerge as a nonreproductive winged form (subimago). Mature nymphs are 8 to 13 mm long (excluding their tail filaments). The head and body of a mature nymph are dark brown. The head is relatively large, at more than 2.5 mm wide. Like other species in the family Heptageniidae, S. interpunctatum nymphs have flat bodies, which allows them to fit into crevices and under rocks. Spines at the ends of their legs allow the nymphs to dig into small holes and crevices. The sex of a nymph cannot be determined until later instar stages.
The forewings of newly hatched subimagoes are 10 to 12 mm long. Many of the veins in the wings are darkly colored, while some are clear. Their heads are yellowish tan, and the compound eyes of males can be pale green.
Stenacron interpunctatum adults have thin wings that cannot lay flat against their body. Instead, the wings are held together above the body when the insect is at rest. The forewings are larger than the hindwings. Adult bodies often are yellowish but can include the colors brown and white. The brightness of adult coloration can be affected by the length of time that the individual developed as a nymph. Mayflies have paired genital openings.
Adult males have transparent forewings that are 8 to 10 mm long. Their compound eyes often are dull green, though the eyes can be yellowish brown, with a horizontal stripe at the middle of each eye. The eyes are large, well separated, and located toward the top of the head. The length of male forelegs can be shorter or much longer than the body. The males have two penes which have a characteristic shape, with an unusually well-developed cluster of spines on each penis.
Adult females are larger than males. They have glassy forewings that are 10 to 12 mm long. Adult female coloration is similar to that of adult males, but the colors are duller and less defined. The ova of a female are present as she emerges from the nymph stage, creating the brilliant orange color that is visible through her abdomen.
Stenacron interpunctatum can be complicated to identify, because its appearance can vary under different environmental conditions and across its geographic range. Historically, some authors have classified the species within different genera, some have identified multiple subspecies of S. interpunctatum, and some have not separated the subspecies at all. To clear up the confusion, some researchers showed experimentally that physical variation in nymphs is related to variation in temperature. The researchers concluded that previously identified subspecies actually were all S. interpunctatum. ("Mayflies (Ephemeroptera)", 2013; Berner and Pescador, 1988; Edmunds, et al., 1976; Lewis, 1974; McCafferty and Huff, 1978; McCafferty and Pereira, 1984; McShaffrey and McCafferty, 1986; McShaffrey, 1996; Meyer, 2009; Schwiebert, 2007; Wodsedalek, 1912)
Native to North America, Stenacron interpunctatum is a common mayfly species that is found throughout the eastern and central United States and Canada. Populations live as far west as Arkansas and Minnesota, and hatches of S. interpunctatum do not occur west of Denver. The southern limit of its range appears to be Florida, and the species extends north to Manitoba. Some researchers think that S. interpunctatum is the most common species of its genus. ("Mayflies (Ephemeroptera)", 2013; Berner and Pescador, 1988; Edmunds, et al., 1976; Kondratieff, 2013a; Kondratieff, 2013b; McCafferty and Pereira, 1984; Pescador and Rasmussen, 1995; Schwiebert, 2007)
Stenacron interpunctatum nymphs are aquatic, usually living in streams and rivers. Stream organisms are often sensitive to water temperatures, which are related to how fast the stream flows. However, researchers do not agree on the flow rates that S. interpunctatum prefers. Some studies report that S. interpunctatum nymphs prefer slower currents, while other studies claim that the nymphs live in moderately rapid or swift rivers and streams. Stenacron interpunctatum is a common and widely distributed species that can probably live in a relatively broad range of environmental conditions, including water flow rates. The species occupies waters that have a pH between 5.3 and 7.0.
Nymphs spend most of the day under rocks or submerged logs, or in rock crevices, debris, and vegetation. At night, nymphs may crawl to the tops of rocks. Nymphs thrive in mid-sized alkaline creeks. They sometimes can be found on lake shores that have wave action and objects to which the nymphs can cling. Since suitable habitat on lake shores is rare, nymphs gather in large numbers in a single spot on a lake shore.
Stenacron interpunctatum adults live on land. Their terrestrial habitat is close to the water where they lived as nymphs. ("Mayflies (Ephemeroptera)", 2013; Berner and Pescador, 1988; Edmunds, et al., 1976; Flowers and Hilsenhoff, 1978; Giberson and MacKay, 1991; Lamp and Britt, 1981; McCafferty and Huff, 1978; McShaffrey and McCafferty, 1986; McShaffrey, 1996; Meyer, 2009; Snucins, 2003)
Like other mayfly species, Stenacron interpunctatum is hemimetabolous, which means that it undergoes incomplete metamorphosis. Its life stages are egg, nymph, non-mating adult (subimago), and adult. Eggs most often are laid on the surface of rivers and streams and then sink to the bottom. Polar caps at each end of the eggs serve as attachment structures, which prevent the eggs from drifting downstream to potentially inferior habitat. Eggs hatch after about 13 days to 6 weeks.
Mayfly nymphs also are called naiads. The number of nymph instar stages of S. interpunctatum is unknown, but nymphs molt about once every two weeks. The growth of nymphs is continuous, though slower during the colder, winter months. Nymphs emerging in the fall are known to overwinter. When S. interpunctatum nymphs overwinter, they migrate to deeper waters before ice forms. In the spring thaw, nymphs return to shallow waters a few days after the ice disappears. Unlike other species in the family Ephemeridae, S. interpunctatum does not emerge all at once on the same day or within a few days. Nymphs are commonly found throughout the year, though the number of mature nymphs is greatest in late May or early June. When nymphs emerge later in the breeding season, their size at maturity is smaller. Lost appendages in S. interpunctatum nymphs can be regenerated.
Emergence usually occurs in the late afternoon or at sunset. In nature, S. interpunctatum nymphs require a water temperature of 16° C or greater to emerge. When a nymph is ready to emerge as an adult, it floats to the surface of the water in a bubble of air and emerges on the surface. Alternatively, a nymph can crawl up onto a rock, where its cuticle splits across the middle of its back. A mayfly nymph first emerges from the water as a immature winged form called a dun or subimago, and the transformation is complete after about 5 or 6 minutes. The subimago, which is not yet able to mate, flies to a nearby leaf or stem and, after 18 to 22 hours, molts into a reproductive adult called a spinner or imago. Mayflies are the only insects that molt again after they already have developed wings. The adult emergence period extends from April or May through August or September. Adults live for 6 or fewer days.
Development time varies, depending on when the eggs hatch. Nymphal growth to adulthood occurs within a year, sometimes in as little as a few months during the summer. Water temperature and the length of development affect the size, intensity of coloration, and spotting pattern of S. interpunctatum. (Berner and Pescador, 1988; Edmunds, et al., 1976; McCafferty and Huff, 1978; McCafferty and Pereira, 1984; Meyer, 2009; Schwiebert, 2007; Wodsedalek, 1912)
Mating flights occur just after sunset and continue after dark during the summer. Like most mayfly species, Stenacron interpunctatum adult males form swarms at a predictable height above their aquatic habitat. The males move continuously in the swarm by repeatedly flying upward and drifting back down. A female flying into the swarm is grabbed quickly by a male, and copulation occurs in flight. Sternacron interpuncatatum mates once in its lifetime. Males die shortly after mating.
Although S. interpunctatum usually reproduces sexually, unfertilized eggs of this species have developed under experimental conditions. (Berner and Pescador, 1988; Edmunds, et al., 1976; Meyer, 2009; Mingo, 1978; Schwiebert, 2007)
Females lay their eggs within minutes or hours of mating. To lay eggs, a female ascends and descends in flight above the water. She touches her abdomen on the surface of the water, releasing a portion of her eggs each time she descends. Stenacron interpunctatum females usually die shortly after laying eggs. In the laboratory, eggs hatch after about 13 days, but hatching can take up to 6 weeks. In nature, S. interpunctatum eggs usually hatch in the early fall and early spring.
Populations of Stenacron interpunctatum can have complex reproductive cycles. The life cycle can vary significantly among populations, probably depending on water temperatures and other environmental factors. For example, the insect can produce two generations in warm streams and one generation with overlapping broods in cold streams. In one Indiana stream, three broods of S. interpunctatum emerge each year. These broods emerge in the early spring, midsummer, and late summer or early fall. In other areas, such as Florida, adults emerge every month. The complex breeding system in S. interpunctatum ensures that individuals of various growth stages can be found throughout the entire year. Because S. interpunctatum individuals hatch at different times throughout the year, competition for resources among the individuals probably is reduced. The complex life cycle also may maintain genetic diversity in S. interpunctatum populations.
While S. interpunctatum often reproduces sexually, unfertilized eggs of this species have developed in laboratories. (Berner and Pescador, 1988; Edmunds, et al., 1976; Flowers and Hilsenhoff, 1978; Giberson and MacKay, 1991; Lamp and Britt, 1981; McCafferty and Huff, 1978; McCafferty and Pereira, 1984; Meyer, 2009; Rowe and Berrill, 1989; Snucins, 2003; Wodsedalek, 1912)
Stenacron interpunctatum likely provides nutrients in its eggs for the offspring to grow and develop, but it provides no more parental care.
Stenacron interpunctatum can be raised easily in the laboratory. Adult females have been kept alive in the lab for up to 8 days. (Funk, et al., 2010; McCafferty and Huff, 1978; McShaffrey and McCafferty, 1986; Wodsedalek, 1912)
Stenacron interpunctatum is a poor swimmer as a nymph. It usually just drifts or crawls along the bottom of its aquatic habitat. Nymphs are nocturnal and instinctively move away from light. At night, they move upward in the water column, where they can be exposed to the current. They usually rest under stones during the day. Nymphs regulate their oxygen intake by moving toward higher-current waters when dissolved oxygen concentrations are low. Nymphs prefer to cling to rocks and other objects when resting.
Adults fly. Males fly and drift in swarms above the water to attract mates. Dispersal can occur when adult females are blown by winds, possibly when eggs are blown by winds, and when eggs become attached to the bodies of birds.
Nymphs overwinter in colonies, while males gather in large groups during their short life span. (Berner and Pescador, 1988; Edmunds, et al., 1976; Heise, 1992; Meyer, 2009; Schwiebert, 2007; Snucins, 2003; Wiley and Kohler, 1980; Wodsedalek, 1912)
Stenacron interpunctatum nymphs are poor swimmers, and adults live for just a few days. Individuals probably stay close to their original habitat throughout their lives, unless winds or birds disperse the insects. (Edmunds, et al., 1976; Schwiebert, 2007; Snucins, 2003; Wodsedalek, 1912)
Stenacron interpunctatum nymphs are negatively phototactic, which means that they can detect and move away from light. They are not sensitive to infrared light, but they move away from sunlight and green or red lights. Their vision is not very good. Some evidence indicates that nymphs cannot see small objects. Nymphs preferentially cling to rocks or other objects. The labium of a nymph, which is used for feeding, contains hairlike setae that may detect mechanical and chemical cues.
Adults have large compound eyes that occupy most of the space on their head. Vision seems to be their most important sense involved in perception and communication. Vision in S. interpunctatum probably is important for locating patches of food and avoiding predators. (Heise, 1992; McShaffrey, 1996; Meyer, 2009; Wodsedalek, 1912)
Stenacron interpunctatum nymphs are generalist herbivores that eat diatoms, other algae, and scavenged aquatic plants. They are opportunistic collectors/gatherers that exhibit a "behavioral cycle" of feeding, which involves collecting food material, gathering detritus, and filtering detritus from the current. Detritus makes up about 97% of the particles eaten by nymphs and 91.6% of their energy intake (in terms of caloric content). Although their eyes do not play a direct role in finding food objects, vision may be important in locating food patches. A nymph gathers food using its labium, which has simple hairlike setae that probably detect mechanical cues. Other setae on the labium probably detect chemical cues. These setae help S. interpunctatum to filter, trap, retain, and push food particles in its mouth. Its mandibles are asymmetrical, curved, and roughly triangular. Instead of biting or chewing, an S. interpunctatum nymph uses its mandibles to handle food material and pack the food into its mouth. Nymphs can use their forelegs to scrape plant or algae material from surfaces. The forelegs also transport the collected food to the mouth of the insect.
The mouthparts of S. interpunctatum adults do not function in feeding. Stenacron interpunctatum adults do not eat. In fact, the adult digestive system is filled with air, which makes the adults light enough to float. (Lamp and Britt, 1981; McShaffrey and McCafferty, 1986; McShaffrey, 1996; Meyer, 2009; Rowe and Berrill, 1989)
Stenacron interpunctatum nymphs pretend to be dead as a defense response against predators, especially if they are removed from the water. They continue to feign death even if they are handled roughly. This antipredator response lasts an average of 2 to 3 minutes but can last as long as 65 minutes.
Predators of S. interpunctatum include Paragnetina fumosa (stonefly), Myotis lucifugus (little brown bat), fish, and other aquatic animals. (Clare, et al., 2011; Meyer, 2009; Pontasch, 1988; Wodsedalek, 1912)
Stenacron interpunctatum may be an important source of food for Paragnetina fumosa (stonefly), Myotis lucifugus (little brown bat), fish, and other aquatic wildlife. (Clare, et al., 2011; Meyer, 2009; Pontasch, 1988; Wodsedalek, 1912)
The summer emergence of mayflies can occur in extremely high densities. Mayflies can be a nuisance to people if their dead bodies pile up around houses and cars. If mayflies are blown onto roads in high numbers, the roads can become slick and potentially dangerous. (Meyer, 2009)
Along with other mayfly species, Stenacron interpunctatum is valuable to humans as an ecological indicator of water quality. Fly fishers can use S. interpunctatum adults as bait. Fly fishers also tie flies that mimic S. interpunctatum. ("Mayflies (Ephemeroptera)", 2013; McCafferty and Huff, 1978; Meyer, 2009)
In general, mayfly populations have declined dramatically over the past 50 years because of urban development and water pollution. Although mayfly species tend to be sensitive to environmental stress, Stenacron interpunctatum seems able to tolerate some environmental changes and pollutants. However, pollution caused by humans has acidified some aquatic habitats, which has affected S. interpunctatum populations. For example, in a Florida river severely impacted by industrial wastes, S. interpunctatum nymphs were observed to have gills that were significantly larger than in other populations. These abnormally large gills probably developed to help the nymphs get more oxygen out of the water, since oxygen levels would be lower due to pollution.
The taxonomic order Ephemeroptera gets its name from the Greek word "ephemeros", meaning "short-lived" and "ptera", meaning "wing".
Mayflies have the shortest adult lifespan of any living insect. Also, mayflies are ancient - the earliest species evolved before dinosaurs existed. ("Mayflies (Ephemeroptera)", 2013; Funk, et al., 2010)
Elizabeth Wason (author), Animal Diversity Web Staff, Angela Miner (editor), Animal Diversity Web Staff.
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