Published in LORIS -- The Journal of the Wildlife and Nature Protection Society of Sri Lanka 19:112-114 (1991)


MAYFLIES OF SRI LANKA

-Michael D. Hubbard, Ph.D., 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. Mayflies spend most of their lives as immature insects (known as nymphs) in the water, feeding on algae, diatoms, litter, detritus, or, sometimes, other insects. 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. After a brief period of time (a few hours to a day or so) usually spent resting on nearby vegetation, the subimagos molt once more and the final stage, the imago, emerges. 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. Ephemeral in their adult winged lives, often living only for a few hours to a few days, they are referred to scientifically as "Ephemeroptera", roughly translated from the Greek as "briefly winged".

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. They have two (sometimes only one) pairs of heavily veined wings which, at rest, are held vertically, meeting over the top of the body. The abdomen of the males usually terminates in a pair of two- to four-segmented claspers, called forceps, which grasp the female during copulation in the mating flights.

The nymphs, which appear quite different from the adults, have chewing mouthparts, and several pairs of lateral gills along the abdomen for extracting oxygen from the water. The abdomen terminates in three or sometimes two moderately long cerci. In more mature nymphs, developing wing pads are quite evident on the top of the thorax.

Mayflies serve as food for many fishes and predaceous insects, such as dragonflies. They, along with other aquatic insects, serve an important function in streams, shredding and breaking down leaves and other litter that otherwise would accumulate and eventually clog Sri Lanka's streams. Nitrates and phosphates are also removed from streams by being incorporated into the bodies of these insects.

In many parts of the world, particularly temperate areas, the emergence of adult mayflies is very seasonal, most species emerging in a short synchronous period. However, in Sri Lanka, I have found no evidence of periodicity, and it appears that most Sri Lankan mayflies emerge throughout the year in an aseasonal pattern.

Mayflies are an ancient group of insects, with fossils known from over 200 million years ago. Modern mayflies have changed little over the past 50 million years. The present distribution of mayflies, especially in the Southern Hemisphere, has been heavily influenced by continental drift. Africa, Australia, South America, and most of India (and Sri Lanka) were once part of a supercontinent, called Gondwanaland, and many of the mayflies of these areas are closely related because they were spread throughout Gondwanaland before it broke apart and carried the separate continental plates to their present locations along with their mayfly fauna.

Mayflies are a small group, as insects go, with only about 370 genera known throughout the world, containing perhaps 4000 species (in contrast, there are over 250,000 described species of beetles).

There are about 38 species of mayflies in some 20 genera which have been described from Sri Lanka and I have specimens of over a dozen more species which are new to science and will be described in the future. Many of these species are endemic to Sri Lanka. Others are known from India and other portions of the Oriental Region as well.

The hill country of Sri Lanka contains myriads of streams and rivers. The species richness of mayflies is much higher in this region than in the lowlands where the water is warmer, the streams are slower, and there is likely to be more silt and pollution. Much of the hill country fauna also extend deep into the lowlands in the streams at the bottom of their steep drop from the hill country highland, especially in the southern and western wet zone lowlands. Most species of mayflies are found throughout the hill country with no obvious signs of vertical zonation. The northern and eastern dry zones appear to have a reduced mayfly fauna in comparison with the upland wet zone fauna.

Sri Lankan mayflies fall into three main biogeographic groups in their relationships. The first group consists of the true Gondwanian derivatives. These mayflies have descended from those widespread throughout ancient Gondwanaland and their closest relatives are found in Africa-Madagascar, South America and Australia as well as southern India. The second group is northern in origin and its closest relatives are found in Europe and the Palearctic Region. The third and largest group probably evolved in the Oriental Region and have spread to Sri Lanka from there. Related species are now found in Africa and other places as well.

Mayflies are dependent upon clean water for their habitat. They are extremely sensitive to the quality of the water in which they live. Polluted or extremely silty streams often have a dearth of mayflies. I found this to be especially common in many of the streams which ran through tea plantations. There is no ground cover among the tea bushes and the red soil readily runs off into nearby streams after heavy rains. Similar nearby streams which did not receive the silty runoff had a healthy mayfly fauna. This same phenomenon was also evident in many lowland streams and others which periodically receive a heavy silt load. I once collected in a clear stream which seemed an ideal mayfly habitat, but was unable to find any mayflies at all. I was at a loss to explain this until I observed a local farmer leading his herd of buffalo across the stream which sent a flood of silt down the water. Learning that he did this twice a day, I realized why this portion of the stream had no mayflies. This periodic heavy silt load made the water quality unsuitable for mayflies. Because of their sensitivity to water conditions, mayflies in some regions of the world, especially North America and Europe, are beginning to be used as indicator organisms for water quality, with the presence or absence of environmentally sensitive species serving as an indication of whether or not the water is pure.

There is room for much work to be done on the study of the biology of these fascinating insects in Sri Lanka. Perhaps this brief discussion will stimulate interest about mayflies among naturalists and many of you may actually notice them on your next trip to the countryside. It is important to remember that the continued survival of many species of mayflies, like many other of Sri Lanka's native organisms, requires your continued efforts to keep streams in as natural a condition as possible, to strive to minimize runoff and other pollution, and to maintain and preserve Sri Lanka's natural resources with all diligence.

My field work on Sri Lankan mayflies was in cooperation with and supported by the Smithsonian Institution's "Biosystematic Studies of the Insects of Ceylon" project.

Some papers of technical interest on mayflies

Brittain, J. E. 1982. Biology of mayflies. Annual Review of Entomology 27:119-147.

Edmunds, G. F., Jr., S. L. Jensen, & L. Berner. 1976. The Mayflies of North and Central America. University of Minnesota Press, Minneapolis. x + 330 p.

Fernando, C. H. 1965. A guide to the freshwater fauna of Ceylon. Supplement 2. Bulletin of the Fisheries Research Station of Ceylon 17:177-211.

Hubbard, M. D., & W. L. Peters. 1984. Ephemeroptera of Sri Lanka: an introduction to their ecology and biogeography. Pages 257-274 in C. H. Fernando (ed.), Ecology and Biogeography in Sri Lanka (Monographiae Biologicae vol. 57), Dr. W. Junk Publishers, The Hague.:257-274.

Figure: Nymph of the mayfly Kimminsula from Sri Lanka.


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