SMALL SWIMMING MAYFLY

Nesameletus ornatus

Mayflies spend almost all of their lives underwater among rocks on a streambed, usually a year. When conditions are right, they ascend to the surface to emerge and mate. In flight they keep their bodies vertical, with the three tails trailing behind, giving an overall impression of dancing in the air. The adult of this species has only two obvious tails and is particularly short-living in its wedding dance – only two days.

Description: Adults are 12–14mm long, with head and body dark brown with pale markings. Legs are yellowish, all with dark-brown bands, and wings have dark veins and yellowish patches marbled with black in males (above), and green in females (centre). The well-camouflaged nymphs (below) have an 11–18mm torpedo-like body shape with short antennae, large wing pads and seven pairs of small oval-shaped gills along the abdomen, all marked with a dark dot. Colour is initially almost transparent, changing to mottled grey and eventually almost black. They have three fringed tail filaments showing conspicuous dark bands, and short, banded legs.

Habitat and distribution: Found widely throughout Aotearoa New Zealand, especially in clear and smaller streams, up to an elevation of 1000m. Nymphs are most common in pools in stony or gravelly bushcovered streams in forests, on the edges among vegetation, where the current is slower.

Biology: Very active swimmers, the nymphs feed on biofilm (algae and other microorganisms) and plant detritus on the streambed. An abundance of this species suggests good habitat and water-quality conditions, especially if other mayfly or stonefly groups are also abundant. In December–January many moulted exoskeletons can be seen on dry stones close to the edge of the streambed.

Status in Aotearoa: Endemic