Benthic Macroinvertebrate Index

Group II Taxa: Pollution-Intermediate Organisms

GROUP TWO TAXA: Pollution-Intermediate Organisms. These organisms are somewhat less sensitive to pollution, but they cannot live in very polluted water. Each group of these animals which is found is given a score of 2 toward the overall index value. The presence of these animals does not indicate that the water is either polluted or clean; they can live under a broad range of conditions.
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Dragonflies and Damselflies:

These animals belong to the Order Odonata. The group is divided into two distinct suborders, the Anisoptera (true dragonflies) and the Zygoptera (damselflies). They are medium to large insects with two pairs of long equal sized wings. The body is long and slender, with short antennae. Immature stages are aquatic and development occurs in three stages (egg, nymph, adult).



[Suborder Zygoptera]

Damselflies are smaller, more slender, and more delicate than the dragonflies. They have three long, very distinct feathery tails, and long slender legs. They are commonly gray, greenish, or brown to black in color. The eating habits are similar to those of dragonfly nymphs, and they emerge from the water as adults in the same manner. The adult damselflies are slow and seem uncertain in flight. Wings are commonly black or clear, and the body is often brilliantly colored. When at rest, they perch on vegetation with their wings closed upright.




[Suborder Anisoptera]

Dragonflies are also known as darning needles. The nymphal stage of dragonflies are grotesque creatures, robust and stoutly elongated. They do not have long "tails". They are commonly gray, greenish, or brown to black in color. They are medium to large aquatic insects size ranging from 15 to 45 mm. The legs are short and used for perching. They are often found on submerged vegetation and at the bottom of streams in the shallows. They are rarely found in polluted waters or torrential streams. Food consists of other aquatic insects, annelids, small crustacea and molluscs. Transformation occurs when the nymph crawls out of the water, usually onto vegetation. There it splits its skin and emerges prepared for flight. The adult dragonfly is a strong flier, capable of great speed and maneuverability. When at rest, the wings remain open and out to the sides of the body. Dragonflies eat small insects, mainly mosquitoes, while in flight.




[Order Diptera, Family Tipulidae]

Cranefly larvae are easily identified by the spiracular (star shaped) disc at the posterior (rear) end. They are wormlike: The body is fleshy, segmented, and elongate, but the head is black. Color ranges from pale grey, cream, to dirty white, and the outer skin appears transparent. Most species are found in the bottom sediments or in waterlogged wood. Their food varies, for there are both carnivorous and herbivorous Cranefly larvae (the majority are herbivores). The adult cranefly looks like a giant mosquito, and is often seen clambering around in the corner of a ceiling or hanging by its long legs.



[Order Coleoptera]

Beetles are by far the largest order of insects, having over 250,000 known species. With over 250 different families, only a few are aquatic in the adult or larval stages. Most are terrestrial (land dwellers). Five of the most common families of aquatic beetles/larvae in Northeast Ohio are: Haliplidae (Crawling Water Beetle), Dytiscidae (Predaceous Diving Beetles), Gyrinidae (Whirligig Beetles), Hydrophilidae (Water Scavenger Beetle), Psephenidae (Water Penny), and Elmidae (Riffle Beetle).

The food habits of a group as large and varied as the beetles vary tremendously. Most are carnivorous, but many beetles are herbivorous as well. The mouth parts are well developed for biting, chewing or sucking. Larger beetle larvae ranging from 20 to 40 mm in length are often of the Dytiscidae Family (Predaceous Diving Beetle) or Hydrophilidae (Water Scavenger Beetle). These larvae are quite nasty and will often violently attack other specimens within a collection container.



[Order Decapoda, Family Astacidae]

Caryfish are also called "crawdads" and "crawfish", depending on the local culture. They resemble small lobsters. They have two claws, four large pairs of walking legs, long antennae, large compound eyes, six abdominal segments and a flat tail. Their size varies greatly from 10 to 150 mm. Often a crayfish will loose an appendage, but in time it will grow back. Body color can range from gray to black, brown, green, and orange to red. Crayfish typically walk along the bottom of a stream, but when alarmed they can dart rapidly into safe hiding places such as under rocks. Despite their appearance crayfish are omnivores (scavengers) and seldom attack living creatures for food.



[Order Amphipoda, Suborder Gammaroidea]

Scuds are often called "fresh-water shrimp." They look very much like miniature shrimp, but they are not closely related to marine shrimp. They are about 5 to 20 mm in length. The body is often slightly curled, and there are seven pairs of legs. Common species show a wide variety of color ranging from translucent to white, cream, light brown, green and gray. The scud moves very fast and appears to be moving in a backwards direction. Scuds are found in unpolluted streams, usually under vegetation and rocks, for they do not care for light. Scuds are omnivorous (scavenger), and they eat a variety of dead plant and animal materials.



[Order Pelecypoda, Families Unionidae and Sphaeriidae]

The bivalve mollusks are aquatic creatures and found in all types of fresh water environments, and are most abundant in large rivers. The size of mussels and clams vary from 2-250 mm in length. The two sides of the shell are firmly held together by the soft animal that lives inside. Clams move along the bottom of a stream bed by opening slightly to allow the foot to make contact with the substrate. Through a series of contractions it is able to hunch forward, moving a little at a time. Clam color ranges from light yellow, light green, dark green, brown, gray and black. The diet of clams consists of zooplankton, phytoplankton, and organic detritus. Clams that are found opened indicate that the creature has died.



[Order Isopoda]

These creatures look very much like the terrestrial sowbug (potato bug/ pill bug), except they are extremely flattened. The body is segmented. American species have their last four segments fused into a large shield-like region. There are seven pairs of walking legs. The body length varies from 5 and 20 mm. Coloration is typically gray, but can be whitish, creamy, black, brown or reddish. Sowbugs are scavengers and eat dead and decaying plants and animals. They are seldom found in open waters, but rather hide in waters less than a meter deep under rocks, debris and vegetation. They prefer unpolluted waters but can be found under a wide range of conditions.


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