Mayflies & Spinners.
Mayflies (Ephemeroptera)
Ephemeroptera comes from the Greek ephemeros meaning lasting a day and pteron meaning wing. Immediately following emergence, the adults exhibit an erratic up and down flight pattern over water or nearby rocks and bushes, mate in flight, lay eggs on the surface of a water body and die, usually within a day but ranging from a few minutes to several days.
Source Credit - La Trobe University
The adults have no functioning mouthparts with which to feed, their only purpose is reproduction and dispersal. The common name, mayfly, comes from Europe where they emerge in May. In Australia they generally emerge in spring to summer. Mayflies are the only insects to metamorphose a second time after gaining functional wings. Firstly from nymph to a winged, dull coloured sub imago, known to fisherman as the "dun", secondly shedding the entire skin to reveal the shiny imago, known to fishermen as the "spinner". Mayflies are popular with fly fishermen as they are good trout food, even the dead adults.
Mayflies are most diverse in mountain streams of eastern Australia and Tasmania as most species prefer cold, clear flowing streams. Generally mayflies are found in fast flowing mountain streams, but also occur in lakes and reservoirs in either unsheltered stony areas or in weedy sheltered bays. Mayfly nymphs are usually confined to permanent water bodies but a few species do occur in temporary water bodies. Nymphs are benthic macroinvertebrates, living on the bottom of streams and lakes, either in crevices of logs or buried in mud, gravel and leaf litter or clinging to submerged plants.
Mayfly nymphs are either herbivores - grazing on diatoms and algae, or detritivores - scraping detritus off submerged stones and leaves and grow from 4 to 30mm in length.
The abdominal gills appear to "flutter" like feathers, enabling the nymphs to obtain oxygen from the water. Swimming nymphs tend to be flattened and may have overlapping setae on the caudal filaments which are moved together as a swimming aide. Sprawling nymphs, which are found on submerged stones in fast flows, usually have modified abdominal gills such that they also act as an anchoring device. Burrowing mayfly nymphs have frontal processes on the head and modified mandibles. Their abdominal gills are fine, with many trachea, and constantly beat to pump water through the burrow.
Females typically lay 500 - 3000 eggs in flight, "bombing" the water's surface. Small eggs, varied in shape, colour and sculpturing, sink to the substratum or adhere to submerged plants by way of an adhesive outer layer. Early instar nymphs hatch from one week to a year later depending on species and temperature. Temperature effects both egg development and emergence. Species occurring in alpine areas exhibit a winter diapause and so avoid harsh conditions whilst in the egg stage. Species from more arid regions exhibit a summer diapause allowing them to cope with potential drying of the water body whilst in the egg stage. Emergence of alpine and some temperate species is often synchronised; swarms of thousands of individuals can be seen swirling above water bodies at one particular time of year, usually late spring. However tropical species typically do not exhibit any diapause and emergence is unsynchronised; having several generations emerge in a year. Ephemeroptera species undergo ten to fifty nymphal instars (fifteen to thirty most commonly) depending on both temperature and food availability which affect growth rate. The whole life cycle may range from six months to three years.
Source Credit - La Trobe University