with high levels of dissolved oxygen while pools are relatively slow-flowing environments where the dissolved oxygen levels are not as high as in riffles. The nymphs prefer high dissolved oxygen level environments.
For the purpose of this study it is necessary to introduce the mayflies (Ephemeroptera) because they are such a primitive order of insects that there is much that is unique about their morphology. This uniqueness has to be highlighted to enable better understanding of the experiments conducted to assist this study and the conclusions derived therefrom.
Simply put mayflies belong to Class Insecta Order Ephemeroptera. They belong to the infraclass paleoptera and have primitive wings that cannot be folded over their backs. Fossil records reveal that they may have evolved during the carboniferous period 280-360 mya and their evolutionary history is closely associated with development of wings in Class Insecta as a whole. Modern day mayflies number about 4000 species distributed among 20 families and most are associated with running water (Brooks, Steve, A Natural History of Dragonflies, Mayflies and Stoneflies).
Order Ephemeroptera is well-distributed across the globe except for the two polar regions – the Arctic and the Antarctic – and oceanic islands though it is well-represented in New Zealand (Order Ephemeroptera, “Soil & Water Conservation Society of Metro Halifax” Website, 2004).
The name of the order Ephemeroptera is essentially derived from the fact that adult phase mayflies survive for a very ephemeral period of 2 hr to 3 days. They are unique insects in that they have two adult phases both of which are winged and ephemeral to the tune of 1-2 hr to maximum 14 days. At adult stages the insects do not feed and expend all their time on mating.
The nymphs are ubiquitous and are usually found in shallow streams and littoral areas of lakes. Nevertheless, many species are restricted to specific substrata of macrophytes, sediments of waveswept