Diachlorus ferrugatus
In 3 days ~20% in a week ~40%
Diachlorus ferrugatus, commonly known as the yellow fly or yellow fly of the dismal swamp in the United States or doctor fly in Belize, is a species of highly aggressive biting horse-fly of the family Tabanidae native to North and Central America to Costa Rica.
The yellow fly was described as Chrysops ferrugatus by Johan Christian Fabricius in 1805. Adult yellow flies are around 1 centimetre (0.39 in) long with yellow bodies, mid-legs, and hind-legs, and black fore-legs. The eyes are blue-green with purple bands. They fly with little sound, and the first sign of their presence noticed by humans is usually their bite.
The larvae are thin white grubs with fine yellow fur, each segment having three pairs of pseudopodia. The yellow fly ranges from the southeastern United States, where it is found from New Jersey to Texas, to Costa Rica. It is most common near bodies of water, along with trees nearby to hide in. Spanish moss (Tillandsia usneoides) is a popular hiding spot.
It is notorious for its propensity to inflict painful bites on people, having been described as the most aggressive fly species in... Wikipedia →
| # | Park | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Sian Ka'an Biosphere Reserve | 🇲🇽 Mexico | |
| 2 | Big Cypress National Preserve | 🇺🇸 United States | |
| 3 | Everglades National Park | 🇺🇸 United States |
| # | City | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Orlando | 🇺🇸 United States |
With luck
About 10% of wildlife watchers report a Yellow Fly Of The Dismal Swamp sighting in Big Cypress National Preserve on a given day, based on 5 verified sightings from GBIF citizen-science data from 2019 to 2026. It is the 220th most-seen of the 262 insects we can rate there. Over three days the chance rises to about 20%, and over a week to about 40%, if each day is a fresh chance.
March–May and July
In those months about 10% of wildlife watchers report a Yellow Fly Of The Dismal Swamp in Big Cypress National Preserve on a given day, against under 5% in June and August–February, the lowest months.
Between noon and 3 pm
Across all the parks we cover, that three-hour window holds the most Yellow Fly Of The Dismal Swamp sightings with a recorded time, and about 8 in 10 of them come between 6 am and 6 pm: it is a daytime animal.
Among the best
Of the 3 parks where we can rate Yellow Fly Of The Dismal Swamp sightings, Big Cypress National Preserve ranks 2nd at about 10%, level with the leaders once rounded. The best odds are in Sian Ka'an Biosphere Reserve (Mexico, about 10%).
Same family: Tabanidae
The closest relatives of the Yellow Fly Of The Dismal Swamp that you can see in Big Cypress National Preserve (family Tabanidae, Horse and Deer Flies) are Green Horse Fly (about 10%) and Black Horse Fly (about 10%), with each one's daily chance of a sighting.
GBIF citizen-science records
It is estimated from how often the Yellow Fly Of The Dismal Swamp is recorded against all the insects logged in Big Cypress National Preserve (5 verified sightings from 2019 to 2026), calibrated against known sighting rates. Figures are rounded to the nearest 10% because citizen-science data supports honest approximations, not decimals, and months with records from fewer than five people show "—". How it works.
