Rivers and streams meander, fall, swirl, pool and flow through landscapes based on how their channel has been shaped and filled. Moving water, referred to as a current, goes in different directions and speeds as it flows. Currents are slower on the water’s surface, sides, and bottom due to friction with air, soil and rock. The fastest current is just below the surface where nothing impedes its progress, and the slowest is where the water pushes in all directions due to obstacles in its path.
Aquatic insects living in these turbulent waters employ a variety of methods to anchor themselves in place to breathe and eat. Most aquatic insect species are in their larval stage and will leave their aquatic environment when they become adults. Some are benthic species living attached to the bottom; some are swimmers commonly found in slower moving water; some live in the swift-moving current.
Water Penny by Cliff White; Stonefly larva by Jim Rathert, Riffle beetle by Cliff White, all images courtesy of Missouri Department of Conservation
Most insects must be able to securely anchor themselves to breathe and eat. The water penny, a flow-adapted insect, uses its whole body, a flattened oval disc, to adhere itself to a rock using suction. With its smooth back and with no protrusions, water easily flows over and around this species. Where the current flows around rocks on the bottom, an area of quiet water forms just behind each rock. The caddisfly builds a case around itself from rocks, sticks, and mud and remains in this quieter area. Stoneflies and riffle beetles use sharp hooks at the ends of their legs to hang onto the bottom substrate even while walking about in the flowing water. Where water pools and currents slow after a sandbar, mayflies, crane flies, and midges live in burrows in the silty bottom.
For animals with lungs, muscles expand the lungs pulling in air where oxygen can be absorbed into the blood stream and transported to cells in the body. Insects do not have lungs but use air tubes attached to spiracles, openings on the outside of their bodies that can be opened or closed. A fine network of air tubes allow oxygen to be absorbed directly by each cell. Damselfly, mayfly, and stonefly use gills to extract oxygen from flowing water and pump it into their air tubes. Mosquitos and water scorpions use snorkeling, where a breathing tube connected to a spiracle breaks the water’s surface. Water beetle adults trap a bubble of air under their wing covers when they dive. It is held in place by hairs on their bodies and covers the spiracles located along the sides of the abdomen.
In addition to oxygen, water currents bring a steady supply of food downriver and carry away waste. In forested streams, vegetation and decaying animals falling into the water provide a variety of food. Midges are able to collect tiny particles of plants and animals found in slow-moving water. Some species of stonefly and caddisfly are shredders, feeding on vegetation moving past them by chewing it into small bits. Filter feeders, including black fly and riffle beetles, use nets and fans to trap particles from faster-moving currents.
In areas exposed to lots of sunlight, algae and aquatic insects are the main food sources. Mayfly, caddisfly, and water pennys scrape algae from rocky surfaces. Other species of stonefly and riffle beetles are predators and use their hooked feet to position themselves in steady currents where they are able to snag other species being swept along. Another predator, the alderfly, burrows its bottom into the streambed to catch prey swimming nearby. Damselfly larva may anchor themselves to the bottom or fill a bladder at their rear end with water and expel it with enough force to propel themselves through the current to snag prey.
Whether the current is fast or slow, flowing through clear areas or obstacles, streams provide insects with numerous places to live. On you next walk along a waterway, consider all that is happening just below the surface.