FNZ 46 - Nesameletidae (Insecta : Ephemeroptera) - Popular summary
Hitchings, TR; Staniczek, AH 2003. Nesameletidae (Insecta: Ephemeroptera). Fauna of New Zealand 46, 72 pages.
( ISSN 0111-5383 (print), ; no. 46. ISBN 0-478-09349-7 (print) ). Published 14 May 2003
New Zealand mayflies (Ephemeroptera) are average-sized insects, with the adults having two pairs of membranous wings. The forewings are much larger than the hind wings and held vertically at rest. New Zealand mayfly adults have three tail filaments, at least two of which are very long. The juvenile stage is aquatic and has abdominal gills that resemble small leaves, along each side of the first seven abdominal segments. It also has a tail of three filaments.
Like other orders of insects, mayflies are distributed worldwide except for a few oceanic islands and the polar zones. However, the Ephemeroptera is a small order, far from fully identified; with about 2500 named species at present. In New Zealand, this order is represented by 40 described species classified in 8 families, although the list is by no means complete. Three-quarters of the known species are classified in the family Leptophlebiidae. Most New Zealand leptophlebiids are adapted for clinging to the undersides of stones and boulders in streams. However, the species of the family Nesameletidae, represented in New Zealand by a single genus Nesameletus, are free swimming and able to dart about with agility to escape predators. Their torpedo-shaped bodies and three tail filaments, each fringed with lateral hairs, enable them swim rapidly with dorsoventral undulations for short distances. They are sometimes referred to collectively by trout fishermen as "the grey darter". Two other New Zealand families of mayflies also show these body adaptations: the Rallidentidae and the Siphlaenigmatidae. Both of these latter are relatively uncommon. The genus Nesameletus and its five described species are all endemic. Other members of the family inhabit southern South America and southeastern Australia.
The juvenile aquatic stage of mayflies is referred to as a nymph or larva, the latter term being used in this work. New Zealand mayfly species almost invariably develop in unpolluted running water, feeding on algae and plant material and converting it into biomass. This process forms a vital step in the food chain, which is continued by the predators of mayflies such as other insects, crustaceans, fish, and birds.
The Nesameletidae are widely distributed in New Zealand, but three species appear to be confined to the South Island and two of these are known only from restricted areas. All species tend to be found in smaller streams and trickles, often concealed among trailing vegetation where water flow rates are moderate to slow.
Nesameletus larvae develop for a year, increasing in body length to between 10 and 20 mm. This is achieved by a succession of moults. As the last of these is about to take place, the insect leaves the water and climbs a few centimeters up on to a rock surface above the waterline where it sheds its larval shuck and emerges as a subimago, the first of the two successive winged stages. This incompletely developed adult, especially with regard to the reproductive system, flies a short distance to better concealment on streamside vegetation, where it rests for up to two days. A further moult gives rise to the fully mature adult, the imago, which flies, mates, and dies within a day or so. Both subimago and imago have highly atrophied mouthparts, so do not feed. The female imago usually flies in an upstream direction to compensate for the downstream drifting of eggs and larvae. Eggs are laid directly on to a water surface where the current is slow.
The larvae of swimming mayflies are easily recognised by eye, but distinguishing Nesameletus from Rallidens and Siphlaenigma requires a hand lens. Identification of individual species can be done with a low to medium powered stereomicroscope. Confirmation of species may require dissection of the mouthparts and subsequent examination with more powerful magnification.
Subimagos and imagos of the Nesameletidae can be distinguished from other mayfly families by comparison of the fore- and hindwing venation using a hand lens. Identification to species level can also be achieved with low magnification of the forewing pattern of venation. With subimagos, the distinctive patterns of forewing vein clouding can be compared with the photographs of the forewings of reared specimens. In the case of imagos, low magnification of the genitalia and comparison with the diagrams should confirm the identification.