Plecoptera

Plecoptera is an order of insects commonly known as stoneflies.

There are approximately 3,500 described species living across the globe. They inhabit both the Northern and Southern Hemispheres. The only place on Earth where they are not found is Antarctica. These insects are considered one of the most primitive groups of Neoptera. Neoptera are insects that can fold their wings over their backs. Fossil evidence shows their close relatives lived during the Carboniferous and Lower Permian periods. However, the modern diversity of stoneflies likely originated during the Mesozoic era.

The anatomy of a stonefly is relatively generalized. This means they lack many of the highly specialized features found in other insect orders. They possess simple mouthparts equipped with chewing mandibles. Their heads feature large compound eyes and two or three ocelli, which are small simple eyes. They also have long, multiple-segmented antennae to sense their surroundings. Their robust legs end in two claws for gripping surfaces. The name Plecoptera comes from the Ancient Greek words plekein, meaning to braid, and pteryx, meaning wing. This refers to the complex venation, or vein patterns, in their two pairs of membranous wings. These wings fold flat over their backs. While most have wings, some species are entirely wingless.

The life cycle of a stonefly begins in the water. The immature stage is known as a nymph, or sometimes a naiad.

Most nymphs live in the benthic zone, which is the very bottom of lakes and streams. They require well-oxygenated water to survive. Nymphs can take in oxygen through their exoskeleton via diffusion. They also use external gills located behind the head, on the thorax, or around the anus. Most nymphs are herbivorous and eat submerged leaves or benthic algae. However, many species act as hunters that prey on other aquatic arthropods. Some species in New Zealand have terrestrial nymphs that live in very moist environments.

Nymphs undergo a long period of growth before reaching adulthood. This stage can last anywhere from one to four years depending on the species. During this time, they undergo between 12 and 36 molts. A molt is the process of shedding an old exoskeleton to allow for growth. Before they become adults, the nymphs must leave the water. They attach themselves to a fixed surface and undergo one final molt. This transformation allows them to emerge onto land as terrestrial adults.

Adult stoneflies have a much shorter lifespan than the nymph stage. They generally survive for only a few weeks. They usually emerge during specific times of the year when resources are optimal. Most adults are not strong fliers and stay near the water where they hatched. Some adults do not feed at all, while others are herbivorous. The reproductive process is quite efficient for the females. A single female can lay up to one thousand eggs. She may fly over the water to drop them or attach them to a rock or branch. The eggs are protected by a sticky coating that prevents them from being swept away by swift currents.

Some unique species of stoneflies have very unusual life histories. Most stoneflies move from water to land, but a few are different. The Lake Tahoe benthic stonefly, or Capnia lacustra, and the Baikaloperla are known to be exclusively aquatic. They live in the water from birth until death. This is very rare for insects. Most other aquatic insects, such as true water bugs, can still leave the water to travel. These specialized life cycles show how stoneflies have adapted to specific environments over millions of years.

Because of their biology, stoneflies serve as vital biological indicators. All species of Plecoptera are intolerant of water pollution. This intolerance stems from their nymphal requirement for high levels of dissolved oxygen. If the water quality drops, the nymphs cannot survive. Therefore, the presence of nymphs in a stream or still water indicates good or excellent water quality. Scientists use them to monitor the health of aquatic ecosystems. By studying stonefly populations, researchers can understand the impact of pollution on entire environments.