F.R.E.S. Unknown to most of us, the streams and rivers of Sri Lanka harbour beneath their surface a hidden fauna of small delicate insects known as mayflies . These interesting and important denizens of the freshwater ecosystem are fascinating objects of study for the amateur and professional biologist alike once one gets to know them. These mayfly nymphs spend their lives in the water, undergoing several molts as the mature. They may live a year or even two before emerging from the water as adults to fly, mate, and reproduce, laying their eggs in the water and starting a new generation. When mayfly nymphs reach the stage of maturity where they are ready to become adults they usually go to the water surface, where they molt from the nymphal stage, emerging directly to a winged form called a subimago.
Mayflies are the only insects which molt once they reach the adult stage.
The subimago stage is unique to this group. The subimagos and imagos lack functional mouthparts and do not feed. The only functions of the imagos are to disperse, mate, and lay eggs. In the winged stages mayflies are very susceptible to predation, especially by birds and this stage has evolved to be extremely short-lived in comparison to the aquatic nymphal stage. Adult mayflies are fragile insects, and can be distinguished from other insects by a long narrow body ending in two or three long tail filaments which are usually longer than the body.