Below is a dead caddis fly larvae coated in oil. This little guy came from the bed of Trout Brook in the City of Brockton, just below the Avon town line. It was one of many that we found there.
Aquatic insects are very vulnerable to contaminants in the water. Like fish, many of these insects breath through gills, which allow them to remove and use life giving oxygen from the water.
When contaminants such as the oil above enters the water their gills become coated with it and cease to function. They die by suffocation.
As an example look at the mayfly nymph below. The brown feathery structures along its abdomen are its gills.
Most threats to these aquatic insects and their habitats are subtler than the example above. It can be something as simple as a clogged culvert or poorly placed street drain.
Fertilizer runoff from a yard, housing development or horse stable could foul the water with enough nutrients to reduce oxygen levels. When dissolved oxygen levels drop to low, our aquatic insect friends suffocate. This is a serious problem on many of our streams. This sort of pollution often goes unseen. There are no oil slicks or rainbows on the surface of the water to point a finger at. Yet our aquatic friends are just as dead.