The insect order Diptera, which includes flies, exhibits remarkable diversity in reproductive habits. While common house flies avoid standing water, preferring decaying organic matter for their offspring, a significant number of other fly species are obligate aquatic breeders. For these specialized groups, water is a prerequisite for their initial life stages. These insects must deposit their eggs in or near a water source to complete their development.
Not All Flies are the Same
The nearly 160,000 known species of flies are broadly categorized by their preferred breeding grounds. Flies commonly encountered around human dwellings, known as synanthropic species, typically select sites rich in decomposing materials, such as manure, garbage, or decaying food. These species, including house flies (Muscidae) and blow flies (Calliphoridae), require moist, nutrient-dense substrates for their larvae and avoid open bodies of water.
In contrast, a vast array of other fly families are truly aquatic, spending a substantial portion of their life cycle submerged. Their eggs, larvae, and sometimes pupae are specifically designed to live in standing or flowing water, ranging from temporary puddles to fast-moving rivers. The adult flies of these aquatic species are generally short-lived, focusing primarily on mating and returning to the water to restart the cycle.
Life Cycle of Water-Borne Flies
Flies that reproduce in water undergo complete metamorphosis, involving four distinct stages: egg, larva, pupa, and adult. The adult female deposits her eggs directly onto the water surface, on emergent vegetation, or on damp substrate near the water’s edge. Depending on the species, eggs may be laid singly or grouped together to form a floating structure, such as the boat-shaped rafts created by many mosquito species.
The larval stage is spent entirely underwater and requires physiological modifications for aquatic existence. Since water holds less dissolved oxygen than air, aquatic fly larvae have developed specialized respiratory mechanisms. Some, like the “rat-tailed maggot” larvae of drone flies, possess an extendable siphon that functions like a snorkel, allowing them to draw atmospheric air from the surface while submerged.
Other species extract oxygen directly from the water or adapt to low-oxygen conditions. The larvae of certain midges, often called “bloodworms,” synthesize high concentrations of hemoglobin. This respiratory pigment gives them a reddish color and allows them to survive in extremely anoxic or polluted sediments. Larvae living in flowing water, such as black fly larvae, have structural adaptations like hooks and suction disks to anchor themselves firmly to rocks and prevent being swept away.
Common Flies That Breed in Water
One widely recognized aquatic fly group is the mosquito (Culicidae), which requires stagnant water for its immature stages. Female mosquitoes lay eggs in virtually any collection of standing water, including temporary rain pools, clogged gutters, and discarded containers. The larvae develop there until they emerge as adults. The water’s stillness and permanence over the short developmental period are more important than its depth or quality.
Non-biting midges (Chironomidae) are another aquatic group whose larvae are the bloodworms mentioned previously. These flies are often found near lakes and ponds, emerging from the benthic zone where the larvae live embedded in the mud and sediment. Their presence can indicate a high level of organic matter in the waterbody, which serves as their food source.
Drain flies, also known as moth flies (Psychodidae), are smaller, fuzzy-winged insects that thrive in the moist, film-coated environments of household plumbing. Their larvae feed on the gelatinous organic sludge that builds up inside drains, overflow pipes, and septic systems. Black flies (Simuliidae) restrict their larvae to clean, fast-moving water where they filter organic particles from the flow. Horse flies and deer flies (Tabanidae) lay their eggs on vegetation overhanging water. Their larvae burrow into the damp mud or sand along the edges of marshes and wetlands, where they live as predators.
Controlling Aquatic Fly Populations
Managing aquatic fly populations primarily involves controlling or eliminating the water sources required for their reproduction. For homeowners, the most effective preventative measure is to consistently remove all sources of standing water around the property. This includes regularly emptying bird baths, flower pot saucers, old tires, and any containers that collect rainwater, as even a small amount of water can support mosquito larvae development.
Targeted actions are required to address specific pest sources, such as meticulously cleaning and maintaining plumbing to eliminate drain flies. Pouring boiling water or a biological drain cleaner down infrequently used drains breaks down the organic film that serves as the larval food source. For larger water bodies like ornamental ponds, control strategies involve introducing natural predators, such as mosquito fish, or improving water quality through aeration to disrupt the low-oxygen conditions favored by midge larvae. Larvicides, which target the water-dwelling immature stages, can be applied to water that cannot be drained, preventing the insects from reaching the adult flying stage.