Reproduction and Life Cycle

The Shortest-Life Award Goes to...
Twenty-four hours. Add or subtract a few, and you have the entire lifespan of an Isonychia bicolor mayfly. Now we revisit the question that was presented in the introduction to this site, and again we think: just how would we spent such a short time frame if it were all we were given? Read on to find out how this insect uses the hours it has to maximum potential, carrying out what seems should take longer than the mere eye-blink of time that it does. For the mayfly, there are three main stages of life: egg, larvae, and adult (no pupal stage), naming this life cycle hemimetabolous. The organism spends approximately 99 percent of its lifetime working its way to maturity, leaving less than 1 percent -- the equivalent of a few minutes to an hour-- of its life to live as a free-flying, full-grown mayfly (DeWalt et al. 2010, Neuswanger 2012).

Isonychia bicolor: A Rare Bivoltine Species
There are two major reproductive peaks of the year for Isonychia bicolor, making this the only identified bivoltine (semiannual, or twice per year, generation) mayfly species. Although the exact times of year do vary according to the local/regional climate and temperature, as well as the weather trends experienced in any given year, these fecundity (reproduction) patterns are consistent enough to be recognizable:
    ~ The first, or "main" cohort emerges in the late winter to early spring. This generation is called "sadleri" or "christina" by some scientists in the field, and they tend to be the larger cohort, growing to about 15-18 millimeters in length. In addition to their size, the early batch tends to be larger in number, sometimes doubling that of the later, "secondary" ones born in the fall (Echols et al. 2010, Kondratieff and Voshell 1984).
    ~ The second cohort, whose hatching begins in the late summer and continues throughout September, October and sometimes into November, are commonly called the "harperi" and more specifically the "matilda" or "circe," depending on whether they are colored darkly or with skin that is light and pale. Individuals of this variety tend to reach 10-13 mm (Kondratieff and Voshell 1984).

An evolutionary advantage of this virtually year-round existence is the unique filtering habits that come as a beneficial by-product of this species' feeding habits. So long as there is a river rushing with both water and life, it is going to yield more survival and success if it is cleaned periodically by the species who does it best. Since Isonychia bicolor is one of only four North American mayfly genera that does this, it proves itself a good neighbor to have around--throughout the changing seasons of the year (Echols et al. 2010).

The bivoltine life pattern is unique in that it is a slight variation of the more common seasonal-multivoltine-summer life history. This is one of the three prominent patterns (along with seasonal-univoltine-winter and non-seasonal-multivoltine") that are most often seen in mayflies and some other insects (Jacobi and Benke 1991). For more on what distinguishes Isonychia bicolor from others in the animal kingdom, see Classification.

An example of locomotion in Isonychia bicolor nymphs. Video courtesy of Jason Neuswanger
at (Isonychia bicolor species homepage), via
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The Stages Within the Phases
The presence of this aquatic insect cleaning the waters at nearly every time of year is a miraculous thing. Despite their being sensitive to subtle changes in a number of conditions (e.g. pH, temperature, etc.), they can also be considered a 'toughened' species--that is, they may be particular about where they live, but they have 'learned' (evolutionary, as a species on the whole) to survive in these habitats in their many facets of seasonal change which may potentially impact survival--and 'learned' it well (Kondratieff and Voshell 1984). This explains why they are able to carry out their functions dutifully throughout the year, concentrated in the two peaks as described above.

However, we must remember that their time is limited; they only have a day to live. Here is how they generally spend that time:
   ~Once hatched from their eggs (this happens fairly quickly upon oviposition, in which a female lays, or deposits, the eggs on the surface of highwater areas), the young organism crawls onto land to walk. It has officially entered the bulk of its life, the larval stage. At this point they are called "nymphs." The larva form of the insect resides in the same habitat as its adult or 'parent' form but is smaller, lacks genitalia, is more fragile overall, and possesses tiny "wing-pads" as a precursor to fully-developed wings to come (DeWalt et al. 2010).
    *The first winged stage is thelarval subimagostage. In trout fisherman jargon, an individual at this stage is called, simply, a "dun." Their wings are cloudy, delicate, and covered in microtrichia: microscopic hairs which allow them to quickly escape water, though their new wings tend to hold their light bodies back, making them prime targets for hungry fish (DeWalt et al. 2010).

*A matter of hours to a day later, nymphs molt into the second winged form: the sexually mature imago, or "spinner," stage. Most will progress to this stage. Some females, however, do not (DeWalt et al. 2010).

Mating: Completing the Mayfly 'Bucket List'
Toward the end of the day and the end of their short lives, Isonychia bicolor adults "mate in swarms 20-30 feet in the air, and the females drop their eggs from high above the water before they join...males in falling" to the water's surface, "spent" (exhausted). In this condition, they lay flat, outspreading their wings onto the water, and rest in "white-gloved howdy" position until their lives eventually, naturally, and peacefully end (Neuswanger 2012). Soon, the new generation will awaken, and the cycle begins again.

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