primary production. Once burrowing to the bottom of the lake, mayfly nymphs begin to billow their respiratory gills. This motion creates current that carries food particles through the burrow and allows the nymph to filter feed. Other mayfly nymphs possess elaborate filter feeding mechanisms like that of the genus Isonychia. The nymph have forelegs that contain long bristle-like structures that have two rows of hairs. Interlocking hairs form the filter by which the insect traps food particles. The action of filter feeding has a small impact on water purification but an even larger impact on the convergence of small particulate matter into matter of a more complex form that goes on to benefit consumers later in the food chain. Distribution Mayflies are distributed all over the world in clean freshwater habitats, though absent from Antarctica. They tend to be absent from oceanic islands or represented by one or two species that have dispersed from nearby mainland. Female mayflies may be dispersed by wind, and eggs may be transferred by adhesion to the legs of waterbirds. The greatest generic diversity is found in the Neotropical realm, while the Holarctic has a smaller number of genera but a high degree of speciation. Some thirteen families are restricted to a single bioregion. The main families have some general habitat preferences: the Baetidae favour warm water; the Heptageniidae live under stones and prefer fast-flowing water; and the relatively large Ephemeridae make burrows in sandy lake or river beds. Conservation The nymph is the dominant life history stage of the mayfly. Different insect species vary in their tolerance to water pollution, but in general, the larval stages of mayflies, stoneflies (Plecoptera) and caddis flies (Trichoptera) are susceptible to a number of pollutants including sewage, pesticides and industrial effluent. In general, mayflies are particularly sensitive to acidification, but tolerances vary, and certain species are exceptionally tolerant to heavy metal contamination and to low pH levels. Ephemerellidae are among the most tolerant groups and Siphlonuridae and Caenidae the least. The adverse effects on the insects of pollution may be either lethal or sub-lethal, in the latter case resulting in altered enzyme function, poor growth, changed behaviour or lack of reproductive success. As important parts of the food chain, pollution can cause knock-on effects to other organisms; a dearth of herbivorous nymphs can cause overgrowth of algae, and a scarcity of predacious nymphs can result in an over-abundance