Wetland Insects

by John Walsby

"Wetland Rushes and Sedges and Land Management" discussed the importance of wetland plants, especially rushes and sedges, in creating meadows where water flow was regulated, bank erosion prevented and the land run-off cleansed of silt and polluting chemicals that result from horticultural, agricultural, residential and industrial activities.

Flourishing on the run-off nutrients, the large plants and the microscopic algae that grow over and among them, provide bountiful food supplies for many small insects, worms, snails and crustaceans that also enjoy the shelter given by the rushes. These creatures in turn become a food resource for larger insects such as damsel flies and dragonflies, and for fish and wetland wading birds.

The conditions are also ideal for small semi-terrestrial creatures that include small snails, hoppers and slaters and many insects that would be susceptible to drying out in more open habitats. They can move about easily through the moist but airy layers of living stems and rotting vegetation just above the water line and as the water level gradually rises or falls during wet and dry periods they can remain in the conditions that suit them best by moving up or down just a few centimetres.

Some adult insects - mostly predators and scavengers - live on the water surface and others stay below it but need to rise regularly to breathe air. Meanwhile the larvae of many wetland insects are fully adapted to an aquatic life. They have gills for extracting oxygen underwater and some also have red blood pigments, like the haemoglobin that we have for transporting oxygen in our own blood, to enable them to respire in places where oxygen levels are low.

The tall stems of marshland plants make it easy for the emerging adults of insects with aquatic larvae to climb clear of the water or damp ground below, to dry their wings in preparation for an aerial life. For many, like mayflies and midges, this lasts only a day or two, sometimes less than a few hours; just long enough to find a mate, copulate, fly to a new site and lay eggs for the next generation.

The standing water and sluggish seepages of wetlands are usually well colonised by very small, free swimming crustaceans, such as cyclops (copepods), seed-shrimps (ostracods) and water fleas - that are best observed through a strong magnifying glass or low power microscope. Like the more familiar mosquito larvae, or wrigglers, that are also common in these quiet waters, they feed on microscopic single celled plants and animals (freshwater plankton) and on minute particles of suspended detritus - the decaying remnants of larger animals and plants.

Most aquatic larval insects live on or in the soft mud below. There they feed on settled detritus, filamentous algae, diatoms and single celled protistan organisms (ciliates, flagellates and amoebae) that flourish in the organic broth that covers the bottom.

15K drawing of recycling insects

They include the worm-like larvae of midges and craneflies, burrowing mayfly nymphs, and the larvae of caddis flies that are easily recognised by the homes that they live inside and drag along. These tubular protective cases are made either from hollow plant stems or twigs, or by gluing plant fragments or small stones on to a parchment lining and frequently they are difficult to spot as caddis cases until you notice one moving across the bottom in still water.

All these crustaceans, insect larvae and microscopic life forms play a major part in cleansing run-off water and are important links in the food chain because they help reprocess the finest particles of organic decay. As they grow on this resource, they turn it into animals that are large enough to become food for quite large predators such as fish and wading birds.

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