Friends of Rietvlei

Eco Bites

Wetland Insects

Insects developed on land and the vast majority live in the terrestrial environment, less than 2,000 of the more than 1 million insect species have inhabited the aquatic environment. Most of these live in freshwater, only a small handful of species are found in the marine environment – these live only in the intertidal zone.

Insects are found in every conceivable liquid environment on earth. They occur in running water, lakes, stagnant waters, brine, hot springs, cold glacial streams, and even in tar pits. However, the shallow waters of ponds and lakes offer the most favourable balance of water temperature, sunlight, dissolved oxygen, food and living space for insects. Most species are thus found in shallow waters, none inhabit deep water in lakes or the deep sea.

As air breathers, they have developed a variety of devices to survive under water. Larvae and nymphs extract oxygen from the water through their skin and/or filamentous gills. Larger larvae and nymphs, and all adults living under water, have to breathe surface air – to do this they use a variety of mechanisms which are prototypes of snorkels, aqualungs and diving bells.

The importance of insects in the aquatic environment

Aquatic insects are very important in the food chain. While only a few insect species have adapted to the aquatic environment, those that have done so are present in very large numbers – that means a huge biomass of animal matter available in the food chain in these systems.

They are found in just about every conceivable aquatic habitat, often being the only link between the lowest and highest trophic levels in that ecosystem. Many species are nutrient or energy exporters from one ecosystem to another – the juvenile stages may live in a water body where they grow and accumulate energy, but this energy is transferred to the terrestrial adult where it is now available in a different ecosystem.

Aquatic insects are also good indicators of water quality. Some groups like stoneflies and caddis-flies live only in clear unpolluted water; while others like chironomids prefer polluted waters. The presence or absence of certain groups or species can this be an indication of the health of an ecosystem.

Some aquatic insects

Several of the more common aquatic insects found in our wetlands and rivers are discussed here – the sixteen groups mentioned here do not all occur in Rietvlei, but most are probably represented in the Diep River system or other rivers in the Western Cape.

The insect orders and families described here are:
• Collembola (Springtails)	• Megaloptera (Fishflies)
• Ephemeroptera (May flies)	• Neuroptera - Sisyridae (Sponge flies)
• Odonata (Dragonflies & Damselflies	• Trichoptera (Caddis flies)
• Plecoptera (Stone flies)	• Coleoptera - Gyrinidae (Whirligig beetles)
• Hemiptera - Corixidae (Water boatmen)	• Coleoptera - Dytiscidae (Diving beetles)
• Hemiptera - Notonectidae (Backswimmers)	• Diptera - Chironmidae (Midges & muggies)
• Hemiptera - Nepidae (Water scorpion)	• Diptera - Simuliidae (Black flies)
• Hemiptera - Gerridae (Pond skaters)	• Diptera - Culicidae (Mosquitoes)

Collembola (springtails)

They are primitive wingless hexapods, taxonomists now place them in a separate class, but for the purposes of this discussion we will include them here. They often occur in large numbers on the surface feeding on organic detritus or vegetation, but most of them live in moist, damp conditions like leaf mould, seaweed, etc. They are quite common in tidal pools along our coastline.


Collembola on water	Adult (top) & instar (bottom)	Collembola adult

Ephemeroptera (May flies)

The aquatic nymphs are present in large numbers in quiet waters, mostly feeding on algae and diatoms, only a few are predators. They grow quite rapidly, moulting 20 to 30 times a year. The adults are terrestrial, they do not feed and live only from a few hours to a few days.


Mayfly nymph	Mayfly nymph	Mayfly adult

Odonata (dragonflies & damselflies)

The nymphs inhabit lakes, ponds, streams and marshes, and are ferocious predators, feeding on small fish, mosquito larvae and other small aquatic organisms. The have an extendible lower jaw to catch prey. The adults live near water feeding on a variety of insects, including mosquitoes.

The main visible difference between the adults is that dragonflies hold their wings out at rest, while damselflies fold the wings over their back.


Nymph	Nymph mouthparts	Nymph


Nymph catch small fish	Dragonfly adult	Damselfly adult

Plecoptera (stone flies)

Aquatic nymphs are herbivorous, grazing on algae and plants on the bottom. They prefer substrates with rocks or stones, and they are only found in well-oxygenated water – stonefly larvae are intolerant of pollution. They can live for up to 4 years before emerging as a terrestrial adult.


Stonefly nymph	Stonefly nymph	Stonefly adult

Hemiptera (true bugs)

Corixidae (water boatman). The herbivorous nymphs live near the bottom, and the adults also live under water feeding on aquatic plants and algae. The adults have to surface to breathe, they trap a small bubble of air under the wings before diving down and feeding near the bottom – their third pair of legs are shaped like oars to push them through the water. The adults often emerge at night, flying to lights in large numbers.


Corixidae nymph	Final nymphal stage	Water boatman adult	Bubble of air under wings

Notonectidae (backswimmers). Both nymphs and adults are aquatic predators, the nymphs feed near the bottom while the adults swim upside down near the surface. They trap air in pockets in the abdomen and can stay submerged for up to 6 hours. They prefer still lakes, ponds and marshes – the adults fly well and disperse easily to new habitats.


Nymph	Adult at surface	Adult on land

Nepidae (water scorpion). Nymphs and adults are predators capturing prey with raptorial forelegs. The nymphs breathe through abdominal spiracles, while the adult hangs from the surface by a long caudal breathing siphon – diving down to catch prey swimming in the water. They feed mostly on invertebrates, but will occasionally take tadpoles and small fish.


Nymphs and adult	Hanging from surface		Catching prey	Adult water scorpion

Gerridae (pond skaters, water striders, Jesus bugs). The nymphs live near the bottom and are carnivorous or omnivorous, feeding on what is available. The adults live on the surface of the water, they have long legs to spread their body weight to be able to run on top of the water surface. They are not good fliers, and feed on insects and zooplankton just under the surface – the piercing mouthparts will pierce the prey and suck out the body fluids. The adults live in large groups, but are very territorial within the group. A few species have adapted to the marine environment and are often seen skittering along the surface of tidal pools.


Nymphs feeding on bottom		Adult on surface	Adults mating

Megaloptera (alderflies, dobsonflies, fishflies)

The carnivorous larvae feed mostly on other insects in the water, they grow slowly taking several years to reach the last stage when they crawl out onto land to pupate. The adults are terrestrial, short-lived and many do not feed at all.


Larva on bottom	Larva	Adult	Adult

Neuroptera (lacewings)

Sisyridae (spongeflies). Most neuropterans are terrestrial, but one family, the spongeflies, have aquatic larvae. The larvae feed on freshwater sponges and bryozoans, inserting their mouthparts into the host and sucking out the cell contents. The final larval stage spins a net-like cocoon wherein it pupates. The terrestrial adults are omnivores and scavengers.


Larvae	Larva	Pupa in net	Spongefly adults

Trichoptera (caddis-flies)

Larvae live in a wide variety of aquatic environments; many species make protective cases of silk decorated with gravel, sand, twigs, bits of leaf, etc. They prefer clean water and are usually present in large numbers – the group include predators, herbivores and algal grazers. The nocturnal adults are weak fliers, they are short-lived and do not feed.


Caddis-fly larvae and examples of protective cases


Larvae in cases		Caddis-fly adults

Coleoptera (beetles)

Gyrinidae (whirligig beetles). Both larvae and adults are active pelagic predators. The adults live on the surface where they usually group in large numbers, swimming rapidly in circles when alarmed. They have divided eyes which can see both above and below the surface – they trap a bubble of air beneath the abdomen when diving to feed, and can stay under water for a long time.


Gyrinid larva	Adult	Adults on surface of pond.

Dytiscidae (diving beetles, water tigers). The larvae are known as water tigers and are ferocious predators of tadpoles, worms, etc. The adults are also predators, hovering just beneath the surface and diving down to catch their prey – they trap a bubble of air under their carapace when diving.


Larva	Larva	Adult at surface	Adult catching prey	Swimming

Diptera (true flies)

Chironomidae (chironomids, midges, muggies). This is a large family of over 10,000 species, occurring in a wide variety of aquatic or semi-aquatic environments. Many species prefer degraded or low biodiversity ecosystems, and some can live in virtually anoxic conditions and are dominant in polluted waters. In some species the larvae have a haemoglobin derivative in their body fluids which extracts oxygen from the water. They feed on bacteria, diatoms, algae, often burrowing into the mud and sand of the bottom of lakes and ponds. Many of the terrestrial adults do not feed, but those that do, feed on nectar, pollen, etc. The adults swarm in large numbers and clouds of them are often seen near water bodies.


Chironomid larvae		Chironomid adults

Simuliidae (black flies). The aquatic larvae prefer running water, anchoring themselves to the substrate with tiny hooks on the end of the abdomen, also using silk threads to hold them in place. They filter organic particles, algae and bacteria out of the water with fans around their mouthparts. They are found in large numbers in irrigation canals and have spread to many areas by way of these canals. They pupate under water, the flying adult emerging in a bubble of air. While nectar feeders, the females need a blood meal for their eggs to develop and they can range up to 60km from the aquatic breeding sites in search of a blood meal. They do spread diseases like river blindness.


Larvae attached to bottom - note fans around mouthparts.		Blackfly adults females feeding on blood.

Culicidae (mosquitoes). The larvae inhabit a wide variety of aquatic habitats, but they prefer standing water in salt marshes, lakes, puddles, plants, discarded bottles or tins, etc. They normally hang from the surface by a breathing siphon, either filtering plankton out of the water or are predators of other small insects. The adult females need a blood meal for their eggs to develop, and a few species are vectors of serious infectious diseases, like malaria, affecting millions of people a year.


Mosquito larvae.		Adult female feeding	Adult mosquito.

Compiled by Niel van Wyk from various sources