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Stream Biomonitoring Unit Descriptions


Resembling their marine relatives the lobsters, crayfish are small crustaceans with ten legs (five pair), including the enlarged front pair that are used for food-gathering and defense. Mature adults range in size from two to six inches long, not including the antennae, which may be nearly as long. Crayfish possess a hard exoskeleton, which they shed several times as they grow. Coloration ranges from blackish through brown, red, orange, green and sometimes blue. Despite their formidable appearance, crayfish are seldom predaceous, mostly preferring to eat aquatic vegetation and decaying organic matter. They are nocturnal, staying hidden during the daytime under rocks or in burrows that they construct. Their normal life span is less than 2 years. They may be found in streams, lakes, and wetlands. Crayfish are found in a variety of locations, in which water quality ranges from pristine to severely polluted.


The abdomen is very long, parallel sided, round and slender compared to that of the more robust dragonfly larva. Nymphs measure three quarters to a little over an inch not including tails. The nymphs of damselflies are also distinguished from dragonfly larvae by possessing leaflike gills at the tip of the abdomen.

The wings of adult damselflies are held back along the body when at rest, while those of dragonflies are held horizontally. Some adults are beautifully iridescent. Like dragonflies, the nymphs are predaceous, feeding on small insects, worms, and crustaceans. Identification of the nymphs, even to genera can be difficult.


Unlike damselfly larvae that have parallel-sided abdominal segments, the abdomens of dragonflies widens from the base and narrow posteriorly. The abdomen ends in three short pointed "tails". The nymphs of most species are found in quieter waters, burrowing in soft sediments or clinging to vegetation. They are predators, using their hinged, spoon-like mouthpart, the labium, to scoop up or seize nearby prey. Size ranges from one half inch to almost two inches. Many dragonfly nymphs are sensitive to pollution. The life cycle of most dragonflies is one year, although in some species the nymphs require up to four years to reach adulthood.

Water strider

Water striders are members of the Hemiptera or true bugs. They are long-legged forms that often are seen skating across the surface film of a pond or slow-moving portion of a stream. As with other true bugs, the mouthparts are designed for piercing and sucking. Their long middle and rear pairs of legs are specialized for skating across the water, equipped with non-wettable hairs for riding on the water's surface tension. The front legs are short and raptorial, enabling them to grasp small prey organisms. Water striders can move quickly across the water surface to avoid being captured, as many a youth will attest to. Adult water striders that emerge in the summer are usually wingless; winged forms that emerge in the fall seek out terrestrial environs in which to hibernate.

Backswimmer & Water Boatman (water bugs)

These water bugs look very similar and both use their hind legs with swimming hairs like oars to propel them through the water. They feed on a variety of organisms, including larger species such as tadpoles and small fish.

Aquatic Sowbug

These are the aquatic relatives of the more familiar terrestrial sowbugs, or pillbugs. They are very flattened and usually measure from one third to three quarters of an inch. They have 7 pairs of walking legs with the first pair modified for grasping. Common coloration is gray-brown. The abdominal segments are fused into a relatively short region. They are chiefly found in streams under stones or among vegetation or organic debris, which they feed on. Some species are adapted to living in caves or groundwater. Other sowbugs are associated with organic material, and are used as indicators of the recovery zone of streams with sewage pollution.


These crustaceans, also called scuds, side swimmers, or freshwater shrimp are only distant relatives of true shrimp. The body is flattened from side to side, they have seven pairs of legs and the majority of species are one third to approximately one inch long. Eyes if present are usually well developed. Coloration can vary from light brown, or greenish to gray-blue. Preserved specimens bleach out to a whitish, gray or cream color. When they swim they often roll over and swim on their sides. Amphipods are omnivorous and will feed on all types of plants and animals but they usually will not consume living organisms. In freshwater habitats, they are often associated with moss or other vegetation in streams and lakes.


A pupa is a stage in the development of insects with complete metamorphosis and refers to the time when the larva transforms from the immature to adult form. A familiar pupal stage is the chrysalis or cocoon formed by a caterpillar before emerging as a butterfly or moth. Most pupae are inactive, but some caddisfly species have pupae that swim. Not all aquatic insects have a pupal stage. Mayflies and stoneflies do not have a true pupal stage. Caddis larvae and fly larvae do and are occasionally collected in kick net samples. Pupae come in many sizes and colors but most can be recognized by the presence of developing wings on their thoracic area.


Flatworms are sometimes confused with leeches because they are somewhat similar in shape and color. Most flatworms are, as the name implies, very flat or at least ventrally flattened. They are brownish-black, yellow, olive or brown and may be lightly to heavily mottled. Dark eyes are usually present near the anterior end. Lengths range form about a tenth of an inch to just over an inch. They can be collected in many habitats ranging from ditches to streams and ponds. Flatworms feed on living or dead animal matter. Identification of flatworms to genus is very difficult and is best done with live material.

Proboscis worms

These worms are not collected often in kick samples and rarely in any abundance. They prefer weedy ponds or masses of filamentous algae or on the underside of lily pads. Color is variable from whitish or pale to yellowish-red, orange or red. They are usually no greater than about a tenth of an inch but some can be up to an inch in length. Species collected in New York are generally about a quarter of an inch. Three pairs of eye spots are usually present but can be difficult to see. The proboscis is very long, sometimes two or three times the length of the body, but is often retracted into the body so it can not be seen.


These worms usually live on crayfish, attaching to the gills, and so are not often collected in kick samples. They look very similar to leeches, having a head, trunk, and a muscular caudal sucker. Occasionally the posterior portion of the body is swollen. Body length ranges form less than one tenth of an inch to about a half an inch. With high magnification a pair of jaws can be seen in the mouth.


These worms are primarily marine but one species is commonly collected in freshwater of New York. Manayunkia speciosa is less than a quarter of an inch in length. It builds a tube of sand, mud, or mucus, and has about 18 long tentacles used for feeding. These tentacles are about one quarter the length of the worm and can be seen when magnified about 30 times. A pair of eyes are usually distinct on the head.


Also know as "bloodsuckers", they are found in ponds, marshes, lakes and streams. Leeches are flattened, with an anterior mouth surrounded by an oral sucker and also have a more noticeable disk-shaped ventral posterior sucker. Length can range from one quarter to three inches although a few species can obtain much greater lengths. They can be highly patterned with spots or stripes, but color fades quickly in preserved specimens. Small leeches only live in very clean waters but some are very tolerant of pollution. Most are scavengers or predators, feeding on dead animal matter or very small macroinvertebrates, and do not feed on the blood of warm-blooded animals.

Aquatic Earthworms

These segmented worms look very similar to the ordinary garden worm but are usually much smaller, commonly being from one twentieth of an inch to an inch and a half long. A few are almost the size of the common night crawler. The head has a mouth which is difficult to see, and may have a proboscis. Identification is often based on the shape of chaetae, which are microscopic in size and require special techniques to identify them can thrive in clean or heavily polluted waters and some species can withstand the almost complete lack of oxygen.

Mollusca (clams and snails)

Freshwater clams and mussels (order Pelecypoda) have two hinged two shells that completely surround the animal inside them. Pelecypoda can be broken down into two large groups. Members of the family Sphaeriidae are small and thin shelled and known as fingernail or pea clams. They are usually one tenth to approximately three quarters of an inch in length. The families Unionidae and Margariteferidae (referred to as mussels or clams) are larger than the fingernail clams, commonly one and a half and up to seven inches, and usually have thicker shells.

Snails (order Gastropoda) have a shell that is either spiraled or coiled. They can range in size from one tenth of an inch to two and one half inches. Part of the head sometimes protrudes from the shell and a pair of tentacles may be seen.


Known as springtails, they are not truly aquatic, but are occasionally seen on the surface film of slow areas of streams. They are usually less than one tenth of an inch long. On land these organisms have the ability to jump several inches in the air, powered by a spring-like structure near their last abdominal segment.

Lepidoptera (aquatic caterpillars)

The larvae are very similar to terrestrial caterpillars with distinct heads, short thoracic legs and ventral prolegs on some abdominal segments. Some species have long paired gills down the length of the abdomen. Wing pads are not present. Aquatic caterpillars are similar to some beetle larva but beetle larvae lack abdominal prolegs. Aquatic fly larvae may also be confused with caterpillars but lack the three pairs of thoracic legs.

Polydesmida (Millipede)

These are occasionally collected in kick samples although it is not known if they are truly aquatic. They seem to be found in areas with large amounts of rotting leaves or other organic matter. They have two pairs of legs per body segment. Specimens collected in New York State are about an inch long.

Spongillaflies (spongilla flies-order Neuroptera)

These members of the family Sisyridae have larvae that live in the cavities and on the surface of freshwater sponges, on which they feed. The mature larvae are generally a little less than a quarter of an inch long with many long stiff bristles coming from their body. The mouthparts are modified into a very distinctive long needlelike tube. This character should prevent confusing the spongilla larvae with those of some beetle larvae. Wing pads are absent. These are not commonly collected in our kick samples. The pale green or yellowish coloration helps to camouflage the larvae.

Acariformes (water mites)

Water mites look like minute spiders with a globular body and eight pairs of legs. They have two pigmented eyes, body coloration is bright red, green, blue, yellow, tan or brown. The body is often soft but some have leathery plates. The largest water mites are about one tenth of an inch long but many species are much smaller.

Megaloptera (Fishflies, Dobsonflies, and Alderflies)

Larvae are soft-bodied, long, and slightly flattened, and range in length from one half to three and a half inches. The head has well-developed mandibles. Long lateral filaments are paired on abdominal segments 1-7 or 1-8. The abdomen ends in either a single filament or in a pair of very short legs with two claws per leg. Some water beetle larvae are very similar to Megaloptera larvae, but never have a single terminal filament or a pair of short posterior legs with two hooks. Megaloptera larvae are carnivorous and feed on other insects.


Adult beetles look similar to terrestrial beetles and are one-twentieth to one and a half inches long. They are hard bodied, and the forewings are modified into hardened covers. Beetle larvae take many forms and may be mistaken for caddis or dobsonfly larvae. They range in size from one tenth to two and one half inches long. The head is distinct with chewing mouthparts. Thoracic legs are usually present and the abdomen is made up of eight to ten segments.

Ostracoda (seed shrimp)

These are small laterally flattened crustaceans that are no more than one tenth of an inch long. The body is covered in a shell-like covering and are not often collected in our kick net samples. They prefer silty bottoms and rooted aquatic vegetation. Seed shrimp are similar in appearance and size to water fleas (Cladocera). Water fleas often have a spine at the end of the body and the anterior end as head.

Diptera (fly) larvae

Fly larvae are soft bodied, usually maggot like forms that measure one twentieth of an inch to four inches in length. The thorax may possess one or two fleshy prolegs, but never three pairs of segmented legs. The abdomen has eight to ten segments and may also have very short prolegs or lobe like legs.

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