In Norman Marsh’s book ‘Trout Stream Insects of New Zealand’ (Millwood press, 1983), he aptly describes the appearance of the spinner in one word…’Brilliant’. Spinners are notably slimmer and more elegant than duns. In their final stages, Deleatidium spinners are a mahogany – red in color with amber, or ginger colored legs and cerci. They have clear, glassy wings, which reflect light when upon the water, giving off a ‘shiny’ appearance.
Again, the differences between the spinners of the two featured species are minimal, and both can be effectively imitated with the one pattern. At times of little wind, male spinners will form ‘mating swarms’ near the water to attract females. After mating the males will expire, while the female returns to the streamside grasses for her eggs to mature. Female spinners require moderately warm, calm conditions in which to lay their eggs. Any wind stronger than a light breeze will bring difficulty to this process, and often precludes spinners from reaching the water. Thus, both dawn and dusk are favored times to experience a fall of returning, egg laden spinners.
Some of the most prolific spinner falls will occur within the final hour of daylight, a time when the wind will often drop along with the setting sun, and when most birds which prey upon mayflies have taken to their roost. However, a spinner fall can occur at any time of the day providing the wind is kind, more so in overcast weather.
In his book, ‘Norman Marsh’s Fly box’ (Halcyon press 1995), Marsh suggests that while aquatic insects enjoy warm conditions, they dislike direct heat. This explains why spinner falls will occur throughout the day in cloudy conditions, and of course, at times when the sun is at its least intense. Early morning spinner falls too are a regular occurrence upon the Mataura, coinciding with the rising sun warming the grasses in which the spinners rest. Female spinners will invariably fly in an upstream direction before laying their eggs. This behavior, in conjunction with the downstream drift of her eggs ensures a stable colonization of mayfly within that section of water. Without these upstream flight patterns, mayfly populations would eventually drift into the sea.
Spinners will land on the ‘ripples’, the sections of increased surface velocity to drop their eggs. The ripply surface minimizes the meniscus, enabling the egg ‘cluster’ to pass through the surface film with minimal difficulty. The meniscus will be thicker upon the pools, and some smaller clusters may not successfully break through this. Each ‘cluster’ comprises of around 500 eggs and is deposited along with a temporary binding agent, to ensure the ‘cluster’ remains intact throughout the descent. This binding agent will soon dissolve, allowing the eggs to disperse amongst the rocks and crevices of the streambed.
After depositing her eggs the spinner will then expire herself, falling ‘spent’ upon the water to provide the trout with a final feast as they drift peacefully down through the pools. Deleatidium vernale are often referred to as the ‘Mataura midge’, and will return to the river in many thousands, inciting great sport at last light. Great swarms may be seen hovering above river, and then just falling like stones, as they meet their watery demise. Trout will ‘lock on’ and feed selectively upon these spent spinners as they drift upon the surface in their thousands.
Spent mayflies drift at the mercy of the currents, and are often concentrated in great numbers amongst the backwaters, eddies and along the slower river margins, providing dry fly opportunities long after the initial rise has ended. These are also productive locations to prospect during the lighter spinner falls, as larger numbers of mayflies here will keep trout more interested than those few drifting down the main river. These will not float forever, and will eventually sink, more so in the more turbulent water of the ripples. Trout will feed upon these sunken spinners well after the rise has ended, and the angler who targets the bases of ripples with a suitable imitation will increase his success as the rise tapers off.