Megaselia scalaris Loew, 1866

Phorid fly

Tiny humpbacked flies that run instead of flying, and can breed in almost anything moist and decaying.

Order: Diptera
Family: Phoridae
Group: Flies
Ontario status: Established, common

Why it matters

Nuisance and contamination indicator. Their presence after drain cleaning means the real breeding site is hidden, a broken pipe, a wall void, a long-lost spill.

Origin

Probably tropical in origin; now worldwide wherever humans leave moist organic matter.

Behavior

Runs in short, jerky bursts instead of flying, the 'scuttle fly' habit. Tiny (2-4 mm). Does not bite.

Living conditions

The ultimate opportunist: drains, garbage, overwatered plant soil, carrion, famously called 'coffin flies' because they can reach buried remains.

Signs of presence

Tiny humpbacked flies running (not flying) on counters and walls
Flies persisting after drains are cleaned, the source is elsewhere
Emergence near garbage areas, broken sewer lines, or dead animals

Weaknesses

Every breeding site is a moist organic material, find it and remove it
They reveal the source: adults stay close to where they breed
Drying and sanitation end them; they have no resistant stage in homes
Often the hidden source is under an appliance or inside a wall void, inspection beats spraying

Myths and facts

Myth They're just baby house flies.
Reality They are a completely different family, full-grown at 2-4 mm. Their humpback and running habit are the giveaways.

Research notes

Morphology

Adult 2-4 mm; thorax strongly arched (humpbacked), the instant field mark. Wing venation reduced, with heavy veins crowded along the leading edge (costa). Runs in short bursts; reluctant flier.

Diagnostics

Confirm by: 2-4 mm, humpbacked profile, reduced wing venation, scuttling run. No other tiny indoor fly combines the hump with the running habit.

Easily confused with

Fruit fly, Drosophila melanogaster Fruit flies hover and fly readily with red eyes; phorids run, are humpbacked, and rarely sustain flight.
Drain fly, Psychoda spp. Drain flies are moth-like and furry; phorids are smooth, dark, and humpbacked.

Life cycle

Egg: Laid on moist decaying matter of astonishing variety
Nymph or larva: Maggots feed on decaying organic matter, fungi, carrion; 3 instars
Adult: Lives 1-2 weeks; runs more than it flies
Egg to adult: Approximately 2-3 weeks at 25 °C
Generations per year: Continuous where breeding media persist

Development

Complete metamorphosis: egg → larva (3 instars) → pupa → adult.

Reproduction

Females exploit an extraordinary range of substrates, the species' success is its lack of pickiness.

Distribution

Native range: Probably pantropical
Ontario: Common indoors and around structures
Global: Cosmopolitan

Ecology

Breeding media: drains, garbage, carrion, fungi, plant material, animal waste. Forensically notable: adults can reach buried remains (the 'coffin fly' reputation). Poor dispersers, indoor populations mark a nearby source.

Resistance

Unknown, managed by source elimination; no resistance documented.

IPM implications

The biology makes this an inspection pest: adults' poor dispersal means the breeding site is within metres, under appliances, in wall voids from plumbing leaks, broken sewer lines, forgotten waste. Strategy is systematic source-hunting (often with drain cameras and moisture meters), then removal. Adult control without source removal is futile by life-cycle arithmetic.

Chemical ecology

Generalist attraction to decay volatiles; oviposition accepts an exceptionally broad substrate range, the chemical ecology is breadth, not specificity.

Genetics and DNA

No reference genome published as of 2026 (transcriptomic datasets exist).

Taxonomy

Diptera: Phoridae. Megaselia scalaris (Loew, 1866), one of the most ecologically flexible flies known; the genus Megaselia is enormous and taxonomically difficult.

Open questions

True species boundaries within the M. scalaris complex (likely cryptic species); the chemical cues behind its extreme substrate breadth.

References

Disney (1994), Scuttle Flies: The Phoridae (Chapman & Hall), the family monograph
Byrd & Castner (2010), Forensic Entomology (CRC Press), coffin-fly cases
UF/IFAS Extension, phorid fly biology