clings to rocks in the cold current
- Size
- Length: 1–4 cm
- Lifespan
- 1–2 years
- Diet
- Mayfly nymphs feed on algae, detritus and plant material. Stonefly nymphs are predators feeding on small aquatic insects. Adults are short-lived and do not feed.
- Habitat
- High-oxygen lounge of gravel-bed streams. Live in turbulent, cold waters of the bush, clinging to underside of stable rocks where current is strongest.
- Range
- Throughout North and South Islands in clean, cold, well-oxygenated streams and rivers. Most common in native forest catchments with gravel or rocky beds.
- Endemism
- Endemic
- Main Threats
- Siltation from agricultural runoff, forestry operations and urban development. Water pollution. Didymo (rock snot) blooms. Loss of riparian vegetation. Climate change reducing water flow and oxygen levels.
- Population
- The canaries in the coal mine. Require cold, clear, unpolluted water. First to vanish if silt or farm runoff enters stream. Populations declining in regions with degraded water quality.
- Conservation Status
- Not Threatened
- Human Risk
- harmless
- Handling Note
- native mayfly nymph, indicator of clean water leave undisturbed
- Conservation Note
- Endemic mayfly nymph; common in freshwater streams and not subject to conservation assessment.
- Te Ao Māori
- The Mata and Ngunguru are the spirit of the current. They represent the concept of Te Ohonga (the awakening). The sudden, magical transformation from a crawling, mud-dwelling creature into a shimmering winged insect of the air. Māori observers noted the mass hatch of these insects. Where thousands would rise from the water simultaneously. Signalling a time of abundance for fish and birds.
The anchor-man of the rapids. An insect that lives in the fastest water on Earth. Freshwater nymphs have a flattened blueprint that allows them to live in the fastest-moving water without being swept away. The stonefly is the heavyweight of the group. It has a robust, armoured body and two long tails called cerci protruding from its rear. They are powerful crawlers. Using hooked claws to grip the microscopic pits in river stones. A nymph that is a rock-climber.
The mayfly is more delicate. Often possessing three tails and rows of shimmering, leaf-like gills along its abdomen. These beat rhythmically to extract oxygen from the rushing water. These insects are masters of the boundary layer. Because water moves slower right against the surface of a rock, the nymphs flatten their bodies to stay within this sliver of calm. Even while a torrent rages just millimetres above them.
They are the primary processors of the forest's energy. They spend up to a year eating conditioned leaf litter. Leaves that have been softened by fungi and bacteria. By shredding these leaves, they turn the tough carbon of the trees into soft, digestible energy. This fuels the entire aquatic food web. From tiny bullies to apex-predator eels.
The final act of their biology is a suicidal transformation. After months or years in the dark water, the nymph crawls to the surface. Its skin splits. A winged adult emerges. This adult has no mouthparts and cannot eat. Its only mission is to find a mate and lay eggs before it dies. Usually within 24 to 48 hours.
This mass emergence is a critical event for the forest. The cold stream is fast. The stonefly nymph clings to a rock. Flattened body. Hooked claws. Armoured and still. It has been here for a year. It will crawl out. Shed its skin. Fly. Mate. And die. It does not know it is a processor. It does not know it is fuel.
It just wants to eat a leaf. To turn over a rock in a cold stream and see these multi-legged, prehistoric shredders scuttling for cover is to see the foundational pulse of the New Zealand wilderness. The freshwater nymph is proof.