Insect adaptations to stream systems provide key bioindicators of these riparian environments. Stream ecosystems are one of the most disturbed ecosystems on earth, and therefore much research has been devoted to their study in recent years. In particular, stream macro-invertebrates have been an area of interest for study. Stream insects are extremely sensitive to changes in the stream system, and are therefore important bioindicators.
Stream insect species are particularly useful in the study of stream ecology due to their world-wide uniformity. In response to the stream environment, rheophilous insect species have developed a diversity of morphological and behavioral adaptations, including flattening and/or streamlining of the body, hooks or suckers to enable adherence to stones, and a tendency to creep under stones for shelter from the current. Several taxa are limited entirely to stream habitats, having lost the capacity for active respiration, depending instead on the swift, cool, oxygen-rich current, being confined to high current, high oxygen content, and low temperature. For more information about aquatic insects, see: Common aquatic insects and Insecta (Aquatic).
One of the most important factors influencing stream organisms is current, and therefore, the primary concern for stream insects is tolerating, or even utilizing swift stream currents. Several species of stream insects have adapted to stream current conditions, particularly those inhabiting stony, particularly fast moving waters around riffles, have developed flattened bodies. The flattened body shape is most suitable to large animals, and smaller insects must adapt in other ways. However, flattened insects are still extremely common in stream environments. Perhaps the most extreme example of this can be seen in the water pennies, the larvae of Psephenid beetles. Water pennies are composed of a series of flat, flexible plates capable of adhering to the shape of whatever surface they are on. Stonefly and a few species of lotic dragonfly larvae also have highly flattened bodies, though not to the extreme of the water pennies. Several of the flattened stream insects are also adapted primarily to life underneath rocks and boulders rather than to endure stream currents. Although it is much more common in much larger stream fauna, such as fish, a few stream insects also take on a streamlined shape, most notably species of mayfly in the family Baetidia. Several stream species also have reduced, filamental gills that further streamline the body.
For insects living on the surface of rocks in benthic stream environments, a form of attachment to the substrate is important in order to keep insects from becoming dislodged. There are several methods of attachment in various stream insects. The most efficient method of attachment is the presence of one or several suckers. However, this is extremely rare in stream insects, with only a single family of dipterans, the Blepharoceridae, bearing these structures. These larva are highly specialized, living only in extremely swift currents where they tightly attach themselves to the substrate. They have flattened bodies, and six ventral suckers, one on each body segment. These animals are capable of moving against currents up to and exceeding 240 cm/sec.
Much more common, however, are insects with flattened bodies that have a structure around the perimeter of the body to increase frictional resistance with the substrate and help to keep the animal fixed. This is the strategy seen in the water pennies, which have a dense fringe of short hairs around the outer edge of the plates. Combining these hairs with their extreme flattened body shapes, water pennies are able to bind themselves to the substrate so tightly that they can be difficult to remove from rocks even with the use of forceps and other tools. In several species of mayfly and other orders, the gills themselves are arranged under the body in such a way that it provides a similar effect, increasing friction with the substrate and allowing the insects to avoid becoming dislodged. Some species also have rough, dense hairs for the purpose of attachment. The majority of stream insects that live on the surface of rocks are able to hold to the substrate with well-developed claws. This can be seen in several dipteran species as well as a few beetles. Other insects, such as the caddisflies use silk secretions to hold themselves in place.
Several species of stream insects are able to avoid the harsh stream currents and other difficulties through behavioral adaptations that reduce direct contact with the environment. Several groups, particularly the few dragonfly species that live in stream environments live in dense growths of plants and mosses where they are completely sheltered from the current. These insects often have claws adapted for grasping plants, but overall have a greater resemblance to still-water insects. Many of the large, flat-bodied species mentioned previously are adapted not for life in running water, but for life underneath rocks where they mostly avoid the harsh currents. These animals also respond negatively to sunlight, and use this as a measure to avoid swift currents. Still other groups such as the hellgrammite larvae of the megaloptera burrow into soft, silty areas of stream environments where they are likewise unaffected by changes in current. Even the insects that live on stones in high-current areas show high selectivity in habitat, seeking out areas optimal for their own morphology.
One of the most important limiting factors of stream inhabitants is the availability of food. Primary productivity is low and almost all energy in streams comes from detrital matter entering from the outside. Stream insects have developed several different feeding strategies. Several species survive as scrapers, feeding on the sunlight stimulated growth of algae that accumulates on rock faces. Still others are shredders or collectors, feeding on living plant tissues or detritus they catch flowing downstream. Others are predators.
The most important feeding method in stream environments is filter feeding. Certain caddisflies use silk to create nets that passively filter water. Some species of mayfly use their gills, catching nutritious particles as well as oxygen as water moves over them. Several stream dwelling Dipteran larvae have large “cephalic fans” or posterior hooks for feeding in high-current zones.
Stream environments, unlike most other freshwater ecosystems, are highly oxygenated, often to saturation point. This allows relaxed, simplified methods of gas exchange in most stream insects. Most freshwater arthropods have some manner of respiratory movement. Most either wave their gills, undulate their bodies, or fan themselves with their limbs in order to create some sort of current to allow oxygen exchange. This is not necessary in stream insects, and in several groups, particularly the stoneflies, the gills are incapable of movement, and therefore no respiratory movement takes place. These insects require oxygenated water to be constantly moving over their body. Several caddisfly species arrange their silk tubes in a form that most efficiently channels water flow through them and over the larvae.
Riffle beetles (family Elmidae) are the only beetles that are able to survive in lotic environments as adults. The adults breathe by means of a collection of densely packed microscopic hairs called a plastron that holds a water bubble in place. Unlike other aquatic beetles, this plastron provides a surface for gas exchange, and serves as a gill, requiring no visits to the surface. The hairs also provide a mechanism for depth sensing, allowing the beetles to avoid going to a depth that will cause the plastron bubble to collapse. Riffle beetles, therefore, are the only beetle species that do not require any contact with the surface as adults. This requires high amounts of oxygen, and these beetles, therefore, can only be found in cold, swift areas of streams.
References and Further Reading
- Cummins, Kenneth W. and Merritt, Richard W. 1978. An introduction to the Aquatic Insects of North America. Kendal/Hunt. Iowa. ISBN: 0787232408
- Huryn, Alexander D. and Wallace, Bruce J. 2000. Life history and production of stream insects. Annual Review of Entomology, 45:83-110.
- Hynes, H.B.N. 1970. The Ecology of Running Waters. University of Toronto Press. Toronto.
- Hynes, H.B.N. 1970. The ecology of stream insects. Annual Review of Entomology, 15:25-42.
- Macan, T.T. 1962. Ecology of aquatic insects. Annual Review of Entomology, 7:261-288.
- Merritt, Richard W., Wallace, Bruce J. 1980. Filter-feeding ecology of aquatic insects. Annual Review of Entomology, 25:103-132.
- Voshell, Reese Jr. 2002. A Guide to Common Freshwater Invertebrates of North America. McDonald & Woodward Publishing Company. Virginia.
This article was partially researched by a student at Texas Tech University participating in the Encyclopedia of Earth's Student Science Communication Project. The project encourages students in undergraduate and graduate programs to analyze timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.