A Nymph Against the Norm

In late February to early March, fly anglers are starting to think about Opening Day. Serious attention is paid to organizing and refilling fly boxes with Hendricksons, Blue Wing Olives, Little Mahoganies, Short-horned Sedge, Black Caddis, and a few stoneflies. As stream temperatures warm, development from nymph or larva to emerging subadults or adults gets closer. However, not all growth occurs gradually from the egg stage to the adult. Consider the case of our Siphlonurus species, also known as Gray Drakes. Eggs that were laid in late spring don’t even hatch until January? How and why?

To consider the development of the Gray Drake, let’s first jump ahead to the reproductive adults or what fly fishers refer to as spinners. The Gray Drake is a much-anticipated hatch beginning mid-May on certain rivers, particularly the Muskegon and the Pere Marquette rivers. They can also be found on several other rivers, including sections of the Au Sable and Upper Manistee, to mention a few. Dark-bodied adults are sizable at 13-17 mm (hook size 8-10). They are quickly identified by the presence of two tails and eyes with a dark brown ring around the median, dividing them in half and giving the appearance, especially in males, of having hamburgerlike eyes (see insert in the photo above).

Male spinners will gather over rapidly flowing water in the evening and begin their reproductive flight or dance, flying upwards and fluttering back down repetitively. As a female enters the dancing swarm, she is spied by the male from below. He seizes her around the thorax with his long front legs. Mating will ensue in flight, often taking place within a few seconds. Afterward, the female may rest briefly on surrounding vegetation before laying her mass of green eggs on the water’s surface. Males and females perish soon after, exhausted and spent from the efforts of reproduction.

Like other species of mayflies, the cluster of dense eggs falls to the river bottom and firmly adheres to the substrate. However, unlike other mayfly species, Siphlonurus eggs don’t hatch into nymphs right away. In fact, they will remain in a state of diapause or a resting state for nearly eight months before hatching in late February to early March the following year.

Once the first instars hatch, they feed on detritus, rapidly growing and molting. As the nymphs get bigger, they may become more opportunistic in their feeding, adding chironomid (midges) larvae to their diet. Additionally, the nymphs are on the move, drifting and later swimming well downstream from where eggs

by Ann R. Miller

were initially laid, making their way toward quieter backwaters. Snowmelt and spring rains will fill or even flood some of these riparian zones, providing the perfect habitat for the last few instars of Siphlonurus. These flooded seepages are a veritable nursery for many other aquatic insects, including a few mayfly nymphs. Siphloplecton basale (Great Speckled Olive), Leptophlebia spp. (Great Mahogany), and Baetisca spp. (Bat Fly) all flourish in this environment in late spring.

Taxonomists have nicknamed Siphlonurus nymphs the “primitive minnow mayflies” for a good reason. They are excellent swimmers and closely resemble the nymphs of Isonychia, minus the ‘racing stripes’ found on their dorsal side. Siphlonurus nymphs are long and narrow with three fringed tails, rather large eyes, and platelike gills laced with vein-like streaking (trachea). Both plate-like (or lamellate) gills and long fringed tails are used for swimming, with nymphs undulating through the water like tiny porpoises.

In mid-May to early June, nymphal development will be complete, and duns will begin to emerge from their quiet, watery environs, crawling up aquatic vegetation to emerge. They often go unnoticed by anglers, but under special circumstances, trout may feed on them if their habitat is accessible. Wind can also knock the duns to the water, where they may be ferried to an awaiting trout downstream. Otherwise, most duns escape to surrounding vegetation, resting for a day or so before molting into the spinner stage. Once the spinners have their evening rendezvous, the life cycle is completed, egg to imago in one year.

As you reflect on our rainy Michigan spring weather, realize that it is important not only for spring flowers but also for successful insect hatches. On a perfect day, the astute angler may be able to observe hundreds of Gray Drake nymphs clinging to submerged plants along the riverbank. Meanwhile, as spring fly boxes are readied, imagine these minnow-like nymphs growing and migrating, making their way to backchannels and submerged plants along the river’s edge. Plan and add imitations of each life cycle stage to your late spring to early summer fly box. Enjoy!

Ann Miller is the author of Pocketguide for Upper Midwest Hatches (Stackpole Books, 2023; ISBN - 9780-8117-7232-7; $29.95). You can contact her at MidwestHatches@gmail.com.