The True Flies

The "true flies" include not only the flies that you are most likely familiar with, like houseflies and fruitflies, they also include mosquitoes, midges, and craneflies. Some bite, and some don't. When we think of flies, we usually think of the adult forms. Flies metamorphose, however. They begin their lives as eggs, hatch into larvae, and then become adults with wings. Fly larvae are often found in streams. On this page you will find photos of some of the larvae you might see.

no-see-um (Ceratopogonidae)
"no-see-um" (Ceratopogonidae)
cranefly (Tipulidae)
cranefly (Tipulidae)
The infamous and irritating "no-see-um" is in the Ceratopogonidae family. The cranefly, often called a mosquito hawk, looks like a giant mosquito, but is completely harmless. It is in the Tipulidae family.

Some true flies live only on land, like houseflies and fruitflies, and we won't concern ourselves with them. Some, however, spend nearly their whole lives in water, and contribute to the ecology of streams.

Why do we call them true flies? Because they have only one pair of wings. Other insects like mayflies, dobsonflies, and dragonflies have two pairs of wings. If you look very closely at an ordinary housefly, you will see two tiny, white knobs just behind its wings. These are all that remains of its original second pair of wings. It uses the knobs, called halters, for balance while flying.

True flies are in the order Diptera, and are one of the most diverse orders of the class Insecta, with about 120,000 species worldwide. They live everywhere on earth except Antarctica and deserts where there is no running water. They can be found in hot springs, stagnant water, rain barrels, tree holes, groundwater zones, and--of course--streams and rivers, and even waterfalls.

True flies come in some forms very familiar to us, like mosquitoes. There are also midges-- some that bite and some that don't. And black flies, many of which bite. Black flies and mosquitoes can both serve as vectors; that is, they can spread parasites by biting people and animals. Tropical areas of the world have a terrible problem with "river blindness," caused by these animals. Most people have heard of sleeping sickness, malaria, dengue fever, and yellow fever. Besides biting, though, flies serve an important role in cleaning water and breaking down decaying material, and they are a vital food source for many of the animals living in and around streams.

The true flies that we find in streams are mostly in larval form. You can also find their pupae, because they go through complete metamorphosis. Most of them are free-living--that is, they can crawl around. Although none of the true fly larvae have the six, jointed legs we see on the other insects in the stream, they sometimes have strange little almost-legs--prolegs--to move around with. Others may move somewhat like worms do, and some--the ones who live in waterfalls and rapids--have a row 6 suction discs that they use to move much like a caterpillar does.

blackfly (Simuliidae) midge (Chironomidae)
Many, like the blackfly larva (Simuliidae) here, use silk pads and hooks at the ends of their abdomens to hold them fast to smooth rock surfaces. Midge larvae, in the Family Chironomidae, can be found in large numbers in highly polluted streams and lakes. The bloodworm is simply a type of midge larva that is often grown commercially, dried, packaged, and sold as fish food. Bloodworms are red because they are a special kind of chironomid (midge) that uses hemoglobin in its blood, like people do. The hemoglobin allows it to live in environments with very little oxygen: in other words, in very polluted or muddy bottoms. "Bloodworms" are fly larvae, whereas "Tubifex" worms--also used as fish food--are true worms that are also red and use hemoglobin in their blood.

True fly larvae may live in a variety of places within a stream; buried in sediments, attached to rocks, beneath stones, in saturated wood or moss, or in silken tubes, attached to the stream bottom. Some even live below the river bottom. Many can take oxygen in through their skin, although some have to have access to air to breathe.

True fly larvae may eat almost anything, depending on their species. Those with brushes on their heads, like the one above, use them to strain food out of the water that passes through. Others may eat algae, detritus, plants, and even other fly larvae.

The larval stage is the longest part of the true fly's life cycle. It may remain an underwater larva anywhere from a week to 5 years. The colder the environment, the longer it will take to mature. It pupates and emerges, then, and becomes a winged adult. The adult may live 4 months--or it may only live for a few days. While it is reproducing, it will often eat aphid honeydew or plant nectar for the energy it needs to make its eggs. Mating sometimes takes place in aerial swarms. The eggs are deposited back in the stream; some females will crawl along the stream bottom, losing their wings, to search for the perfect place to put their eggs. Once they have laid them, they will die.

Some true flies have to have a "blood meal" before they can reproduce, and it is during this time of their lives that people--and animals--find themselves getting bitten by tiny airborne insects. In a cold environment, this may only happen once a year. In warm places, where the insects grow very fast, it may happen more than 16 times a year.

To learn more about macroinvertebrates, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).

Go back to Aquatic Invertebrates

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Trophic Groups

Of the trophic groups that R. W. Merritt and K. W. Cummins (1978) have identified for aquatic insects, only 5 are likely to be found in a stream using typical collection and sorting methods. These are listed below.

Aquatic Insect Trophic Groups

Shredders Collectors Scrapers Piercers Predators
Have strong, sharp mouthparts that allow them to shred and chew live plants or decomposing fragments. These are common among true flies, caddisflies, and stoneflies. Gather the very finest suspended matter in the water. To do this, they often sieve the water through rows of tiny hairs. These sieves of hairs may be displayed in fans on their heads (blackfly larvae) or on their forelegs (some mayflies). Some caddisflies and midges spin nets, and catch their food in them as the water flows through. Scrape the algae and diatoms off of surfaces of rocks and debris, using their mouthparts. Many mayflies, caddisflies, and true flies eat this way. These herbivores pierce plant tissues or cells and suck the fluids out. Some caddisflies do this, as well as many true bugs, or Hemiptera. You may have watched a weevil sucking plant juices through a tubular mouthpiece--a weevil is a true bug. Predators eat other living creatures. Some of these are engulfers; that is, they eat their prey whole or in parts. This is very common in stoneflies and dragonflies, as well as caddisflies. Others are piercers, which are like the herbivorous piercers except that they are eating live animal tissues.

Conditions Influence Growth Rates

How fast many animals grow, mature, and reproduce, and even how short their lives are, depends on how favorable are the conditions in which they live. This is true of insects. In general, when there is more food, an insect will hatch sooner, grow faster, reach sexual maturity sooner, and die younger. Often, where it is warmer on the planet, there is also more sunlight, and sunlight leads to more food being present in the stream, thanks to photosynthesis. Streams in the southern United States are often much richer in nutrients than streams in Alaska. In temperate climates, some larvae or nymphs will delay turning to adults until the spring, spending the winter eating and slowly growing. In these climates, often an aquatic insect population will have only one or two generations in a year. In the South, insects might have 16 generations in a year!