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True flies are the most common insect order in aquatic environments throughout the world.
  • True flies account for 40% of all freshwater insects, and there are approximately 200,000 species all over the world.
  • At present, however, we have names for only about half of them.
  • There are 16,914 species of diptera in North America.
The feeding strategies of true flies are just as diverse as their other attributes.
  • Some families are shredders, breaking up and feeding on aquatic plants, algae, leaf litter, and submerged wood by chewing and boring into it.
  • Other families are adapted for filter-feeding through the use of specialized 'brush-like' appendages that work as filters, collecting organic matter from the water passing through them.
  • Still other families are known as 'scrapers' because they consume algae, microbes, and detritus from the surface of substrates.
  • Finally, some families are predatory, and they feed on other species of macroinvertebrates.
  • Adult females of many species require a blood meal to obtain enough protein for the production of eggs.

                                                                                                                       Source: Iowa State University Entomology

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The larvae of true flies have invaded nearly every aquatic habitat on Earth.
  • They can be found in rushing streams, stagnant pools, hot springs, and frozen sediments.
  • The only exception is in the open ocean.
  • Dependent on species, true flies can tolerate low dissolved oxygen, highly saline conditions, heavily polluted water bodies, and some can even survive in thermal springs.
  • These organisms can be classified as clingers, sprawlers, planktonic swimmers, burrowers, climbers, and miners; this range of feeding strategies allows them to survive just about everywhere underwater.
  • The specific habitat preference of each species is dependent on its oxygen needs; some diptera species, such as black flies, have larvae that need fast-flowing water rushing over their bodies in order to obtain sufficient oxygen.
  • On the other hand, other species may have developed snorkel-like air tubes that can be extended up to the surface for air.

Flies go through complete metamorphosis, which means that they pass through four complete life stages. These are the egg, larval, pupal, and adult stages.

  • Eggs may either be deposited on the surface of the water or cemented to a submerged substrate.
  • They hatch within a few days to several weeks after being laid.
  • The larvae progress through three or four stages of development, known as 'instars'.
  • In the case of black flies and a few other species, however, there may be additional stages.
  • In general, the first instar has the shortest lifespan, while the last instar has the longest lifespan.
  • This is because the larva needs to acquire greater amounts of energy as it grows in preparation for entering the pupal stage.
  • The pupal stage lasts approximately two weeks, except in species that overwinter as pupae.
  • In this case, the pupal stage can last several months.
  • There are four different types of pupae. In the first type, the cocoon is anchored to the stream substrate and the adult must swim to the surface after it emerges from the cocoon.
  • In the second, the pupa cocoon floats to the surface of the water when the adult is ready to emerge.
  • The third type of pupae involves the larvae crawling onto the shoreline and digging into the substrate to transform into its pupal form.
  • Finally, the four type occurs when the body of the final larval instar acts as the pupal case.
  • Different species have varying adult life cycles.
  • In general, however, the male usually dies after mating, while the female searches for a blood-meal in order to allow her eggs to develop.
  • Upon laying one to several batches of eggs, the female will also die.
  • Most Flies are univoltine (having only one generation per year), but under favourable conditions some may complete more generations in a year. In colder climates, it can take two to three years for a fly larva to develop into an adult.

True flies are very important in the aquatic food web. They are often abundant and provide energy for many different species, such as fish and waterfowl.

  • The scientific name Diptera comes from the words 'di', which means two, and 'ptera', which means wing. This is because as adults, true flies have only two wings.
  • True fly larvae don't have any legs; they move around by using different appendages, including creeping welts, prolegs, or suctorial discs.
  • Adults flies have a significant impact on human civilization. They can be carriers of different types of pathogens, sometimes causing diseases such as malaria, filariasis, and yellow fever.
  • There are several other benthic invertebrates that are referred to as 'flies', including mayflies, dragonflies, stoneflies, caddisflies, alderflies, and fishflies. These, however, are not true flies, and do not belong to the group Diptera.
  • The common names of members in the group Diptera have the word 'fly' as a separate word. For example; Black fly instead of 'blackfly'.

Canada's Aquatic Environments (2002). Ephemeroptera. Available here.
Iowa State University Entomology (2010). Order Diptera-Flies. Available here.
Soil & Water Conservation Society of Metro Halifax (2004). Family Simuliidae. Available here.
Univesity of Alberta, Department of Biological Sciences (2010). Diptera, True Flies. Available here.
Universty of Minnesota; Guide to Aquatic Invertebrates of the Upper Midwest (2004). Diptera. Available here.