ARMI Ecology Notes

Biology and Ecology of the ARMI 8

Caddisflies or Sedges (Trichoptera)

Adult caddisflies are moth-like but have hairy wings rather than scales. They hold their wings like a tent or roof over their bodies when at rest.

Life cycle

There are four stages in the life cycle: egg, larva, pupa, adult. They complete their life cycle within one year.

Cased caddis. The larvae of most caddisflies live in cases which they build out of their surroundings, usually either from plant material or stones. The material used for the case or the way the case is constructed often provides a good clue to which family of cased caddis the larva belongs. Some cling fast to large stones in the fast-flowing sections of rivers, while others are confined to the slower flowing or vegetated parts of the river. The larvae of many species browse algae from the surface of stones and plants, and others shred vegetation. Larvae of cased caddis are extremely or moderately sensitive to pollution depending on family.

Caseless caddis. The larvae of some families of caddisfly do not build hard cases. Instead some are free-living predators, while others construct silk webs which they use to filter out organic matter or prey items from the current, which they then consume. Some consume rotting wood, in which they make galleries. Most live in the moderate to fast-flowing stretches of rivers where they are found under stones or amongst vegetation. They are moderately sensitive to pollution.

Up-winged Flies (Ephemeroptera)

So-called because the adults hold their wings vertically above their bodies when at rest.

Life cycle

Up-winged flies are unique amongst insects in having two winged stages (the subimago and the imago).

There are three stages in the life cycle: the egg, the larva (which includes the subimago) and the adult. There is no pupal stage. Adult Up-winged flies have no mouthparts and so are unable to feed. For this reason they rarely live for more than a few days as adults.

The larvae browse algae and bacteria from the surface of stones and plants.

 

Mayfly (Ephemeridae) is the largest of the British Up-winged flies. The larvae usually take 2 years to complete development. They live partially buried in fine silts and sand in slower flowing stretches of the river. They are sensitive to pollution.

Blue-winged olive (Ephemerellidae) is a small up-winged fly that is usually found amongst vegetation in the faster flowing sections of rivers. One year is needed to complete development. During the autumn and winter they almost all overwinter in the egg stage and so do not usually occur in kick samples. Very few overwinter as larvae. It is very sensitive to pollution and siltation.

                                                                                                             Figure 1 Lifecycle of the Mayfly. Original image: Peter Hallam

Flat-bodied up-wing (Heptageniidae) has moderate to large larvae that occur in fast-flowing stretches of rivers and can be found clinging to large stones. They take one year to complete their development. They are extremely sensitive to pollution.

Olive (Baetidae) has small to moderately sized larvae that occur in moderate to fast-flowing stretches of the river, often amongst vegetation or under large stones. Some species can complete development within a few months, but some take one year, and adults may occur year round. Most species are highly sensitive to pollution, but some of the common species are only moderately sensitive to pollution.

Stoneflies (Plecoptera)

So called because the larvae are usually found crawling over stones on the river bed.

Life cycle

There are three stages to the life cycle: egg, larva, adult.

The larvae are moderate to large. The smaller species are usually found in slow to moderately fast rivers especially in the lowlands and southern England, while the large larvae occur in fast flowing rivers, especially in the uplands and northern Britain. The small species take one year to complete development, whereas the large larvae may take several years to complete development. The larvae of the large species are carnivorous, whereas the smaller species browse algae and bacteria from the surface of plants and stones, and some shred vegetation. They are mostly extremely sensitive to pollution. There are some that are highly tolerant of acidic and metal rich mine waters.

Freshwater shrimp (Gammarus)

These are crustaceans with a three stage life cycle: egg, larvae, adult. They live in moderate to fast-flowing water. They shred plant matter and can be extremely abundant amongst leaf litter and woody debris. They are moderately sensitive to pollution but are very intolerant of pesticides.

ARMI Eight: Sensitivities, Habitat and Diet

Cased and caseless caddises have been omitted from the table below as their sensitivities are highly variable between species.

✔ - tolerant     ▶ - medium tolerant   ❌ - intolerant

 

Slow Flow

Nutrient Enrichment

Sedimentation

Acidification

Mayfly

Blue Winged Olive

Flat Bodied Up Wing

Olive

Stonefly

Gammarus

 

 

 

 

 

 

 

 

 

 

 

 

Riverfly

Habitat

Cased Caddis

Depending on species- Still and slow flowing lakes, rivers and streams, ponds and ditches

Caseless

Depending on species- Fast flowing rivers and streams, lakes ponds rivers and streams

Mayfly

Lakes to fast flowing rivers. Burrows into substrate

Blue Winged Olive

Fast flowing streams and rivers

Flat Bodied Up Wing

Fast flowing water

Olive

Running and still water

Stonefly

Running water or still waters

Gammarus

Most water bodies but rarely in acid waters

 

 

 

 

 

 

 

 

 

 

Riverfly

Diet

Cased Caddis

Depending on species- Plant matter, algae, sometimes small invertebrates- herbivorous and detritus

Caseless

Depending on species-  omnivores, predatory detritus or herbivorous

Mayfly

Filter feeder

Blue Winged Olive

Algae and detritus

Flat Bodied Up Wing

Algal grazer

Olive

Algae and detritus

Stonefly

Predator or detritus

Gammarus

Omnivores

 

 

 

 

 

 

 

 

 

 

Understanding your ARMI Sample Further

Gammarus and chironomids tend to increase when organic matter increases. They are often found in greater numbers above dams for example, where organic matter is trapped.    

Figure 2 Chironomid larvae. Photo :Tom Koerner/USFWS

Heavy rainfall can increase flow and wash away riverflies temporarily. It can also introduce riverflies from much further upstream- introducing riverflies that might not normally be encountered.

Blue-Winged Olives are rarely found in Autumn- Winter. They over winter in the egg stage, so you will often see an increase in size and abundance in Spring- Summer.

 

A decline in Gammarus can indicate acidification. If a decline in Gammarus is accompanied with an increase in BWO then the decline is more likely to be due to a decrease in organic matter, and indicates that the river is returning to more favourable conditions.

 

Figure 3 2 Gammarus infected with Pomphorynchus Photo: David Gardner

Gammarus- observed with a yellow or orange mass can be an indication of the parasites Polymorphus or Pomphorhyncus.

 

Figure 4 Left:  Male and female gammarus in precopulatory amplexus. Photo: Dr David W Sutcliffe.  Right:  Male and female hoglouse. Photo: Neil Phillips.

Gammarus and hoglice  both engage in ‘precopulatory amplexus’ as part of their mating. This is where the male will carry the female for a number of days before she moults and mating can occur. During this stage they will swim and eat together, the male is the larger of the two.