Functional Groups | CSKT | Division of Fish, Wildlife, Recreation, & Conservation

Insect Habitat: Functional Groups

Aquatic insects are often specialized in the way they feed and have evolved specialized mouth parts and other structures. Even very different species can have similar structures if they make their living the same way. For example, one functional group, called scrapers, graze algae and diatoms from rocks and wood. To do this, they need mouth parts that can scrape algae. Certain caddisflies, mayflies, beetles, and midges are scrapers, and because of that, their mouth parts are similar as in the example of the beetle and caddisfly below. Explore aquatic insect functional groups by clicking on the buttons below.

Members of a Functional Group Often Share Similar Structures

An example of convergent evolution, the mandibles of these two very different species (both scrapers) are remarkably similar (mandible illustration after Merritt and Cummins 1996) because they have the same function. Both have a blade-like inner margin that, when moved across a rock or piece of wood, scrapes the algae off. Brushes at the base of this blade help to catch the detached algae and other debris and move it to the insect's mouth. Insects within a given functional group share similar feeding strategies and structures.

Functional Group Foodwebs

Shredders, which include craneflies, stoneflies, and caddisflies, consume leaves that have fallen into the stream. Scrapers, on the other hand, use special mouth parts to scrape algae from rocks and wood. Meanwhile, collectors like net-spinning caddisflies and blackflies gather and eat the small fragments of leaves and feces generated by the shredders and scrapers. Predators, in turn, prey upon all three of these functional groups.

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Shredders

Shredders make their living by feeding on both living and decompos-ing plant tissues, which they chew, mine, or gouge with their mouth parts. Leaf fall from the forest canopy is used by shredders. So are living rooted aquatic plants and pieces of wood that fall into the stream. Unless they are eating live plants, shredders prefer leaves and wood that have been softened and pre-conditioned by fungi and bacteria and often leave only leaf veins and other finer material behind. Indeed, it is thought that shredders get nutrition primarily from the fungi and bacteria that colonize the leaf or wood surface. Certain stoneflies, craneflies, caddisflies, and aquatic sow bugs are important members of the shredder functional group.

Shredder Foods

The Work of Shredders

The Shredder Foodweb

Leaves fall from the riparian area and land in the stream. Pieces of wood and mosses also fall into the stream. This material is "attacked" by microorganisms and partially decomposed. The shredders, which include certain stoneflies, craneflies, caddisflies, and aquatic sow bugs, take over, chewing the leaves, getting nutrition primarily from the fungi and bacteria growing on the leaf surface. The shredders in turn are eaten by predators while their wastes are consumed by collectors.

The role of shredders in the food web of a small stream.

Scrapers

Scrapers make their living by eating algae, diatoms, and other microorganisms that they scrape off rocks, wood, and other hard sur-faces. Some have suction disks on their abdomens and many have flat bodies to reduce their profile, both adaptations prevent these insects from being swept downstream in swift currents.

Scrapper Food

Scrappers at Work

The Scraper Foodweb

Sunshine stimulates the growth of algae on rocks and wood in the stream. Scrapers graze this abundant resource. Certain mayflies, beetles, and case-building caddisflies have mouth parts adapted to feeding on the algae, scraping it from the hard surfaces where it grows. They in turn are eaten by predators while their wastes are consumed by collectors.

The role of shredders in the food web of a small stream.

Collectors

Collectors feed on small organic particles that are usually less than a millimeter in diameter. Some of this material is debris and fecal matter left from other organisms like shredders, scrapers, and predators. Collectors also eat algae and fragments of animals and plants. Blackfly larvae are filtering collectors, capturing particles by using a fan. Some free-living caddis are collectors. They spin nets made of a sticky substance. Gathering collectors include mayfly nymphs and beetle larvae. They burrow and live in lower sediments.

Brachycentrus Caddis larvae

Net-spinning Caddis

Blackfly Larvae Filtering Food

The Collector Foodweb

Streams are like conveyor belts carrying all kinds of very small particles of food downstream, and there are lots of insects, called collectors, that filter them out, eating the soup of small fragments of leaves, mosses, and wood as well as feces from shredders and scrapers and fish. Netspinning caddisflies and blackflies are examples of collectors. Collectors are then eaten by predators while their wastes might be consumed by other collectors.

The role of collectors in the food web of a small stream.

Predators

Predators eat live animals either by engulfing and eating all or parts of other insects or by piercing prey and sucking body fluids. As predators, they hunt their prey. Imagine what it would be like to be a mayfly larvae and have a locomotive-size predator (like a dragon-fly larvae) suddenly appear out of nowhere to eat you. Most headwater species are largely invertivores (insect eaters). But piscivorous (fish-eating) and invertivorous species characterize midsized rivers, with the fish eaters eating fry or very small fish.

Dragonfly Larvae with an insect it has captured (Pen and Ink by Ken Wildman) (left) Stonefly Nymph Hesperoperla spp. Hesperoperla or golden stoneflies are engulfers that eat midges, caddisflies and mayfly larvae (right).

The Stonefly Claassenia Subulosa

Claassenia Eat Caddisflies, Mayflies, Midges, and Blackflies.

The Predator Foodweb

Shredders, scrapers, and collectors are food for predators. Predators consume all or parts of other invertebrates (these are called engulfers) or by piercing prey and sucking body fluids (piercers).

The role of predators in the food web of a small stream.

Insect Habitat: Functional Groups

Members of a Functional Group Often Share Similar Structures

Functional Group Foodwebs

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scroll down to explore funcitonal groups

Shredders

Shredder Foods

The Work of Shredders

The Shredder Foodweb

The role of shredders in the food web of a small stream.

Scrapers

Scrapper Food

Scrappers at Work

The Scraper Foodweb

The role of shredders in the food web of a small stream.

Collectors

Brachycentrus Caddis larvae

Net-spinning Caddis

Blackfly Larvae Filtering Food

The Collector Foodweb

The role of collectors in the food web of a small stream.

Predators

The Stonefly Claassenia Subulosa

Claassenia Eat Caddisflies, Mayflies, Midges, and Blackflies.

The Predator Foodweb

Shredders, scrapers, and collectors are food for predators. Predators consume all or parts of other invertebrates (these are called engulfers) or by piercing prey and sucking body fluids (piercers).