Stenacron interpunctatum

Geographic Range

Native to North America, Stenacron interpunctatum is a common mayfly species that is widely distributed in the eastern and central United States and Canada. Populations occur as far west as Arkansas and Minnesota, and hatches of S. interpunctatum are not found west of Denver. The southern limit of its range appears to be Florida, and the species extends north to Manitoba. Some researchers suggest 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 thermal conditions in the water, and water temperature correlates with the rate of stream flow. However, accounts vary regarding the flow rates that S. interpunctatum prefers. Some studies report that S. interpunctatum nymphs prefer slower currents, while other studies claim that the nymphs occur in moderately rapid to swift rivers and streams. Stenacron interpunctatum is a common and widely distributed species that probably tolerates a relatively broad range of environmental conditions, including water flow rates. This species inhabits waters that have a pH between 5.3 and 7.0.

Nymphs spend most of the day under rocks or submerged logs, in rock crevices, amid debris, and in vegetation. At night, nymphs may crawl to the tops of rocks. Nymphs thrive in mid-sized alkaline creeks. They sometimes inhabit lake shores that have wave action and objects to which the nymphs can cling. Since suitable habitat on lake shores is rare, nymphs tend to gather at high densities in a single spot on a lake shore.

Stenacron interpunctatum adults are terrestrial. Their terrestrial habitat is close to the water from which they emerged. ("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)

  • Aquatic Biomes
  • benthic
  • lakes and ponds
  • rivers and streams

Physical Description

The eggs of Stenacron interpunctatum have polar caps, which are attachment structures located at the distal ends of each egg. These polar caps distinguish S. interpunctatum eggs from those of related mayfly species.

In general, mayfly nymphs can be distinguished from other aquatic insects by the 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 are striped with distinct, dark bands. Early-instar nymphs are characterized 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 subimago (non-reproductive winged form). 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 enable 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 membranous 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 also be brown and white. The brightness of adult coloration can vary with the length of time that the individual developed as a nymph. Mayflies have paired genital openings.

Adult males have crystalline 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 oriented toward the top of the head. The length of male forelegs varies between shorter to much longer than the body. Males have two penes that have a distinct 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 muted and less defined. The ova of a female are present and visible upon emergence, creating the brilliant orange color that can be observed through the translucent abdomen.

Stenacron interpunctatum presents taxonomic difficulties, due to substantial intraspecific variation under different environmental conditions and across its geographic range. The patterns of phenotypic variation in the species do not fit the normal subspecies concept. Historically, the species has been classified within different genera. Some authors have revised the genus, some have identified multiple subspecies of S. interpunctatum, and some have not attempted to separate the subspecies. To clear up the confusion, some researchers showed experimentally that phenotypic differences in nymphs correlate with 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)

  • Sexual Dimorphism
  • female larger
  • male more colorful
  • Range wingspan
    8 to 12 mm
    0.31 to 0.47 in


Like other mayfly species, Stenacron interpunctatum is hemimetabolous. Eggs most often are laid on the surface of rivers and streams and subsequently 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 unfavorable environments. Eggs hatch after about 13 days to 6 weeks.

Mayfly nymphs also are called naiads. The number of nymph instars in 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. Where 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 observed throughout the year, though mature nymphs peak 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 begin the process. When a nymph is ready to emerge, 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 along the medial line of the dorsal part of its body. A mayfly nymph first emerges from the water as a sexually immature winged form called a dun or subimago, and eclosion is complete after about 5 or 6 minutes. The subimago 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 among broods. Nymphal growth to adulthood occurs within a year, sometimes in as little as a few months during the summer. Water temperature and the duration of development affect the physical characteristics of S. interpunctatum. Increased water temperature or development time positively correlates with 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 discrete height above their aquatic habitat. The males undulate continuously in the swarm. A female flying into the swarm is quickly grabbed by a male, and copulation occurs in flight. The two penes of the male are inserted simultaneously into the paired genital openings of females during copulation. 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 oviposit, 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 ovipositing. 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 exhibit complex reproductive cycles. The reproductive cycle varies markedly among populations, likely depending on water temperatures and other environmental factors. For example, these mayflies can be bivoltine in warm streams and univoltine 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 at various stages of maturity can be found throughout the year, which may reduce intraspecific competition due to resource partitioning in time. The complex life cycle also may facilitate genetic mixing.

While S. interpunctatum usually reproduces sexually, parthenogenesis has been observed in the species under experimental conditions. (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)

  • Breeding interval
    Stenacron interpunctatum individuals breed once in their lifetime.
  • Breeding season
    Courtship, mating, and oviposition occur during the summer, mostly between May and August.

Stenacron interpunctatum likely provides provisioning in its eggs, but provides no further parental care.

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female


Mayflies have the shortest adult lifespan of any living insect. In nature, Stenacron interpunctatum lives for 6 or fewer days after molting into a reproductive adult. 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)

  • Range lifespan
    Status: wild
    6 (high) days
  • Range lifespan
    Status: captivity
    8 (high) days
  • Typical lifespan
    Status: wild
    6 (high) days


Stenacron interpunctatum is a poor swimmer as a nymph. It can swim only by undulating its body and usually just drifts or crawls along the bottom of its aquatic habitat. Nymphs are nocturnal and negatively phototactic. They instinctively move away from light and usually rest under stones during the day. At night, they shift to upper, current-exposed surfaces. Nymphs behaviorally regulate their oxygen consumption, moving toward higher-current waters when dissolved oxygen concentrations are low. Nymphs also are positively thigmotactic; they prefer to cling to rocks and other objects.

Adults are able to fly. Males fly and drift in large undulating swarms above the water to attract mates. Dispersal can occur by means of windblown adult females, possibly windblown eggs, and egg attachment 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)

Home Range

Stenacron interpunctatum nymphs are poor swimmers, and adults live for just a few days. Individuals likely stay close to their original habitat throughout their lives, unless winds or birds facilitate passive dispersal. (Edmunds, et al., 1976; Schwiebert, 2007; Snucins, 2003; Wodsedalek, 1912)

Communication and Perception

Stenacron interpunctatum nymphs are negatively phototactic; 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 are positively thigmotactic, preferentially clinging to rocks or other objects. The labium of a nymph, used for feeding, contains hairlike setae that may detect mechanical and chemical stimuli.

Adults have large compound eyes that occupy most of the space on their head. Vision seems to be their most important means of perception and communication, probably playing an important role in locating patches of food and avoiding predators. (Heise, 1992; McShaffrey, 1996; Meyer, 2009; Wodsedalek, 1912)

  • Communication Channels
  • visual

Food Habits

Stenacron interpunctatum nymphs are generalist herbivores, eating diatoms, other algae, and scavenged aquatic plants. They are opportunistic collectors/gatherers that exhibit a "behavioral cycle" of feeding, which involves brushing food material from the substratum of their aquatic habitat, gathering detritus, and passively filtering detritus from the current. Detritus comprises about 97% of the particles eaten by nymphs and 91.6% of their energy intake (caloric content). While their eyes do not play a direct role in obtaining food, vision may be important in locating food patches. A nymph gathers food using its labium, which contains simple hairlike setae that probably serve as mechanoreceptors. Other setae on the labium resemble the chemoreceptors of other aquatic insect species. 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 remove material from its maxillary palps and pack 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 are vestigial; adults do not feed. 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 feign death as a defense response, 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 be sustained for up to 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)

Ecosystem Roles

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)

Economic Importance for Humans: Positive

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; they also tie flies that resemble S. interpunctatum. ("Mayflies (Ephemeroptera)", 2013; McCafferty and Huff, 1978; Meyer, 2009)

Economic Importance for Humans: Negative

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)

  • Negative Impacts
  • household pest

Conservation Status

In general, mayfly populations have declined dramatically over the past 50 years due to urban development and water pollution. While mayfly species tend to be sensitive to environmental stress, Stenacron interpunctatum is relatively robust to some environmental changes and pollutants. However, the acidification of aquatic habitats caused by anthropogenic pollution has affected S. interpunctatum populations. For example, in a Florida river severely impacted by cellulose mill wastes, S. interpunctatum nymphs were observed to have gills that were significantly larger than in other populations. These abnormal gills probably developed due to reduced concentrations of dissolved oxygen caused by pollution.

The current conservation status of S. interpunctatum has not been evaluated. (Meyer, 2009; Pescador and Rasmussen, 1995; Snucins, 2003)

Other Comments

The 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 predate dinosaurs. ("Mayflies (Ephemeroptera)", 2013; Funk, et al., 2010)


Elizabeth Wason (author), Animal Diversity Web Staff, Angela Miner (editor), Animal Diversity Web Staff.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in landscapes dominated by human agriculture.


reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


mainly lives in water that is not salty.


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


Having one mate at a time.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


development takes place in an unfertilized egg


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season


remains in the same area


offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


living in residential areas on the outskirts of large cities or towns.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


uses sight to communicate


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