Fort River Macroinvertebrate Information
Classification
Phylum
Arthopoda (crustaceans, insects and spiders)
Mollusca (mollusks)
Class
Insecta (flies)
Bivalvia (mussels)
Malacostraca (crayfish, pill bugs, shrimp)
Gastropoda (slugs and freshwater snails)
Ephemeroptera (mayflies)
Unionoida (mussels)
Odonata (dragonflies and damselflies)
Hemiptera (true bugs)
Plecoptera (stoneflies)
Coleoptera (beetles)
Diptera (two-winged or true flies)
Trichoptera (caddisflies)
Order
Megaloptera (alderflies, dobsonflies, fishflies)
Water Quality Indicators
Benthic macroinvertebrates (benthic = bottom) can be good indicators of water quality because they live in
habitats that they can tolerate. Some macroinvertebrates can tolerate pollution better than others; when
these known pollution-tolerant critters are found in the stream, while pollution-intolerant ones are not
present, then it is likely the stream is impaired. Other reasons why macroinvertebrates are studied:





Easy to collect
Easy to identify
They don’t move far in their habitat
They live in areas that are suitable for survival
They each have different tolerant levels for pollution
Feeding Groups





Predators – feed on other invertebrates (dragonflies, water striders)
Omnivore – generalist feeders able to feed on both dead and living organic matter (crayfish)
Shredder – eats coarse organic matter like leaves and woody material, which helps to convert this
matter into finer particles eaten by collectors (sowbugs, crayfish)
Collector/Filter feeder– consume small pieces of organic matter (pieces of leaves). These gatherers can
collect their food by filtering it (mussels) or by moving around to ‘gather’ it (caddisfly larvae, mayfly
nymphs, worms)
Scraper/Grazer – feed on algae or small pieces of detritus along streambed (snails, mayfly larvae)
1
What will we see in the river?
Many macroinvertebrates are flying insects in their adult stages. When surveying streams for
macoinvertebrates, we find them in their earlier stages: larvae and nymphs. The following two images depict
the two different types of metamorphosis that flies undergo.
…As you can see, flies that undergo Incomplete Metamorphosis, never go through the larvae stage.
2
Order: Ephemeroptera
Mayflies
Pollution tolerance level: Sensitive
The nymphs are collectors and live primarily in streams under rocks, decaying vegetation, or in the sediment.
The nymph stage can last from several months to several years. The lifespan of the adult mayfly is much
shorter – they usually live from 30 minutes up to one day! During this time adult mayflies lay their eggs on the
surface of streams or lakes, which then sink to the bottom to begin the cycle again. Mayflies undergo
incomplete metamorphosis.
Figure 1: Mayfly adult
Figure 2: Mayfly nymph – Look for the distinctive three long tails
3
Order: Odonata
Dragonflies and Damselflies
Pollution tolerance level: Somewhat sensitive
Eggs are laid on the water surface, wet places or on vegetation near water bodies. When they hatch as
nymphs they live off the nutrients provided in the egg. Some dragonfly and damselfly nymphs can live in the
water up to six years. During this time they are voracious predators and will even eat small fish or tadpoles.
Figure 3: Dragonfly adult
Figure 4: Dragonfly nymph – Look for the thick abdomen and two short spikey tails
4
Order: Plecoptera
Stoneflies
Pollution tolerance level: Sensitive
An adult female carries a ball of hundreds to thousands of eggs around her abdomen and deposits them into
the water. When hatched, the nymphs are collectors and eat leaves and algae matter. They can live up to four
years in this form. As adults, their lifespan is a few weeks.
Figure 5: Stonefly adult
Figure 6: Stonefly nymph – Look for the two long tails
5
Order: Diptera (“True Flies”)
Black flies
Pollution tolerance level: Tolerant
Blackfly adults prefer rapidly moving water, like rivers, to deposit their eggs in. The eggs hatch larvae which
dwell just underneath the water’s surface. The larvae create basket-like cacoons to pupate, which are
attached to logs or rocks slightly submerged in the river. The larvae are filter-feeders. Black fly larvae are also
a good food source for fish. After several days to weeks, adults emerge from the pupal case.
Figure 7: Blackfly adult
Figure 8: Blackfly larva
6
Order: Tricoptera
Caddisflies
Pollution tolerance level: Sensitive
Closely related to butterflies and moths, they use silk to build cases (AKA: periwinkle) where they pupate
during their larval stage. The larval stage spans the fall and winter months before emerging as adults in the
summer and fall. The larvae are collector feeders that eat small pieces of organic matter. They live as adults
for 1-2 weeks.
Figure 9: Caddisfly adult
Figure 10: Caddisly larva – Look for them in their small periwinkles attached to rocks and other objects
7
Order: Megaloptera
Dobsonflies
Pollution tolerance level: Sensitive
Females deposit a single mass of 100-1,000 eggs hanging on vegetation over water. This egg mass is white and
looks like bird droppings, which may protect them from predators. Once the larvae hatch, they fall into the
stream, where they will borrow under rocks to avoid predators like fish. However, the larvae are predators
themselves and eat the larvae of dragonflies, stoneflies, etc. Dobsonflies spend two to three years as larvae.
Like other flies, they build a cocoon to pupate. As adults they only live for a few hours, in which they spend
their energy mating, not eating.
Figure 11: Dobsonfly adult – large
mandibles on an adult male. These are not used for biting.
Figure 12: Dobsonfly larva
8
Order: Unionoida
Freshwater mussels
Pollution tolerance level: Somewhat sensitive
The eastern pearlshell and dwarf wedge mussel) burrow into the streambed of fast flowing freshwater rivers
and streams, such as the Fort River. They are filter feeders and help purify the water. They can live very long
(the eastern pearlshell can live up to 150 years!) Mussels use host fish to disperse themselves upstream. Adult
mussels release their larvae or glochidia, which attach themselves to the gills of fish. Although this is a form of
parasitism, the fish are not harmed. They remain on the fish for several weeks to months before falling off and
burrowing into the sediment. The dwarf wedge mussel is listed as a federally endangered species.
Figure 13: Eastern pearlshell
Figure 14: Dwarf wedge mussel
9
Order: Coleoptera
Riffle Beetles
Pollution tolerance level: Sensitive -Somewhat sensitive
As their name implies, the riffle beetle is found crawling on stones and woody debris in the riffle area of
streams. They are considered scrapers/grazers because they eat dead plant material. They undergo Complete
metamorphosis.
Figure 15: Riffle Beetle adult
Figure 16: Riffle Beetle larva
10
Order: Hemiptera
Common water striders (True bugs)
Pollution tolerance level: Tolerant
Common water striders live on still water. They get their name from the way they stride on top of the water’s
surface. Common water striders eat living and dead insects, both aquatic (ie. mosquitos) and terrestrial (ie.
Butterflies). The nymphs are hatched on the water’s edge, usually on plant stems, and grow for over a month
before striding on the water as an adult. Adult water striders can overwinter in plant stems. They move quickly
through the water to avoid predation.
Figure 17: Common water strider adults (The young resemble the adults)
11
Food web of a riparian ecosystem
Image borrowed from: http://lakewhatcom.wsu.edu/
12